Loud exhaust
Description of problem: There is a loud exhaust noise which may be coming from either the front or rear of the vehicle.
Probable Causes:
1. Muffler or exhaust pipe worn out.
2. Exhaust manifold worn out.
Gray smoke from the exhaust
Description of problem: You notice a grayish smoke coming from the exhaust when you start your car. The smoke may still be there after the car is warmed, but it may be less noticeable. The smoke may have a bluish tint to it. This problem normally develops over time, and the amount of smoke indicates the seriousness of the problem.
Probable Causes:
1. Worn piston rings.
2. Worn valve guides.
3. Damaged or worn valve guides.
White smoke or water vapor from the exhaust
Description of problem: You notice a white smoke coming from the exhaust when you start your car. This may be normal if the weather is cold. However, if the smoke continues after the engine is warmed up, there is a problem. This problem normally develops over time, and the amount of smoke indicates the seriousness of the problem.
Probable Causes:
1. Automatic transmission fluid may be entering the intake manifold through vacuum connections.
2. The engine's cylinder head gasket may be bad.
3. The engine's cylinder head may be warped or cracked.
4. The engine's block may be cracked.
Black smoke from the exhaust
Description of problem: You notice black smoke coming from the exhaust when you start your car. The smoke may still be there after the car is warmed, but it may be less noticeable. The smoke may be accompanied by engine idling problems. This problem normally develops over time, and the amount of smoke indicates the seriousness of the problem.
Probable Causes:
1. If you have a carburetor, the carburetor choke may be stuck closed.
2. The fuel injectors may be leaking.
3. The air filter may be clogged.
4. There may be an ignition problem.
Popping noise from exhaust
Description of problem: Whenever you press on the gas pedal, you hear a popping from the exhaust. For the most part, the engine seems to run fine. However, you have noticed your gas mileage has gotten worse. The louder the popping noise, the worse the problem.
Probable Causes:
1. You have a vacuum leak.
2. One or more of your fuel injectors are leaking.
3. There is a hole or leak in the car's exhaust.
The car uses more oil than normal, and there is some smoke from the exhaust
Description of problem: You notice that the oil level is low between oil changes. This did not happen before. It appears that the oil is being burned by the engine because of the smoke in the exhaust. You may also have noticed that the car doesn't have the same amount of power as it once did. This type of problem seems to get worse once it develops.
Probable Causes:
1. The PCV system is not working properly.
2. The engine may have mechanical problems.
3. The engine's valve seals may be worn.
4. The engine's piston rings may be worn.
There is a rotten egg smell coming from the exhaust
Description of problem: Whenever you run the engine and are not moving, you notice an awful smell from the exhaust. The smell is like that of rotten eggs. Not only do you notice this, so does everyone around your car. You may also have noticed that your gas mileage has been worse lately.
Probable Causes:
1. There is a problem with your electronic engine control system.
2. You have an ignition problem.
3. Your fuel pressure regulator is bad.
4. The engine may have mechanical problems.
5. The engine is running too hot.
There is a strong gas smell coming from the exhaust
Description of problem: Whenever you run the engine and are not moving, you notice the smell of gas from the exhaust. The smell can be strong enough to make you think you have a gas leak. Not only do you notice the smell, so does everyone around your car. You may also notice that your gas mileage has been worse lately.
Probable Causes:
1. There is a problem with your electronic engine control system.
2. You have an ignition problem.
3. Your fuel injectors are clogged or dirty.
4. There is an engine mechanical problem.
5. You have a vacuum leak.
6. If you have a carburetor, the choke may be stuck closed.
The car uses more oil than normal, but there is no trace of smoke from the exhaust
Description of problem: You notice that the oil level is low between oil changes. This did not happen before. It doesn't appear that the oil is being burned by the engine because there is not a trace of smoke in the exhaust. You may have also noticed that the car doesn't have the same amount of power it once did. This type of problem seems to get worse once it develops.
Probable Causes:
1. The PCV system is not working properly.
2. The engine may have mechanical problems.
3. The engine's valve seals may be worn.
4. There may be a small oil leak.
The car uses more fuel than normal, and there is a strong gas odor coming from the exhaust
Description of problem: You notice a tank of gas doesn't last as long as it used to. You also smell raw gasoline, especially when you stop the engine. You don't find obvious signs of a gas leak such as puddles under the car. You may have also noticed that the car doesn't have the same amount of power it once did. This type of problem seems to get worse once it develops and can lead to other serious problems such as not starting.
Probable Causes:
1. The fuel lines may have a leak.
2. The engine may have mechanical problems..
3. The fuel pressure regulator may be operating at too high a pressure.
4. The fuel injectors may leak.
5. The gas cap may be faulty.
Thursday, January 31, 2008
Engine Problems Troubleshooting (Part 1)
Engine hesitates
Description of problem: Whenever you push on the accelerator (gas pedal), the car doesn't seem to move like it should or like it did before. You sense a general lack of power and know something is not right. This problem may appear suddenly or get worse over time. You may notice the problem when the engine is hot or cold or when you are low on fuel. A description of when the problem occurs will help to identify the exact cause of the problem.
Probable Causes:
1. You may have a dirty air filter.
2. The spark plugs may be dirty or worn.
3. The spark wires may be bad.
4. There may be some other type of ignition problem.
5. The fuel filter may be clogged.
6. You may have water in the gasoline.
7. If you have a carburetor, you may have a bad accelerator pump or power circuit.
8. Your catalytic converter may be clogged.
The engine surges or misfires while moving
Description of problem: The engine seems to start fine and will normally accelerate fine. However, once you try to keep a steady speed, the engine sputters and runs rough. You may notice that the engine runs differently when it is cold or warm. This type of problem normally is slow to develop but gets worse the longer you drive the car.
Probable Causes:
1. If you have a carburetor, the choke may not be set properly, or the choke may not be working correctly.
2. The engine may be running too hot.
3. The fuel pressure regulator may be operating at too low of a pressure.
4. The ignition timing may be set wrong.
5. There may be some type of ignition problem.
6. There may be a fault in the computerized engine control system.
7. The fuel filter may be partially clogged.
8. The torque converter in the transmission may not be locking at the right time, or it may be slipping.
9. There may be a vacuum leak.
10. The engine may have mechanical problems.
11. The EGR valve may be stuck open.
12. The drive axles may be loose or worn.
13. The fuel injectors may be dirty.
A hissing sound is heard from the engine
Description of problem: The engine may or may not seem to run fine. Normally, the hissing noise becomes apparent soon after the driver notices that the engine is not running properly. This problem can occur suddenly.
Probable Causes:
1. The engine is overheating.
2. The exhaust system and/or catalytic converter is plugged.
3. A vacuum line is disconnected.
4. A vacuum device is leaking
Whirring from the engine that gets worse as engine speed increases
Description of problem: Although the whirring noise is evident at all engine speeds, it is the change in volume as engine speed increases that makes the noise most noticeable. This type of problem can be caused by many things. Some causes gradually develop, while others occur suddenly.
Probable Causes:
1. Low power steering fluid
2. The alternator bearings are bad.
3. A bad water pump.
4. A bad power steering pump.
5. A bad air conditioning compressor.
Smoke is coming from under the hood
Description of problem: Sometimes you will only see the smoke when you start your car or when you stop at a traffic light or stop sign. The smoke may be accompanied by engine idling problems. This is a problem that should not be ignored as it may be an indication of a serious and dangerous condition. The cause of the problem is best identified by the color, smell, and amount of smoke. Pay close attention to this. This problem may develop over time and the amount of smoke indicates the seriousness of the problem.
Probable Causes:
1. If the smoke has an oily smell, there is an oil leak.
2. If the smoke is white, there is probably an anti-freeze (engine coolant) leak.
3. If the smoke is blue or black and has a strong acrid smell, there is probably an electrical problem, and wires are burning.
Engine backfires when you press on the gas pedal
Description of problem: Basically, your engine is a mess. Every time you press on the gas pedal, the engine pops back at you. Sometimes the noise is loud, other times it is rather soft. This noise may result in an under hood fire, so don't ignore it.
Probable Causes:
1. Your camshaft timing belt or chain may have slipped
2. Your ignition timing needs adjusting.
3. There is a serious engine problem.
4. Your spark plug wires are placed on the wrong spark plugs.
Engine hesitates, and a popping is heard from the engine
Description of problem: Whenever you push on the accelerator (gas pedal), the car doesn't seem to move like it should or like it did before. You sense a general lack of power and know something is not right. The popping noise convinces you that something really isn't right. The noise only happens when you press on the gas pedal. This problem may appear suddenly or get worse over time. You may notice the problem when the engine is hot or cold or when you are low on fuel. A description of when the problem occurs will help to identify the exact cause of the problem.
Probable Causes:
1. You may have a dirty air filter.
2. The spark wires may be bad.
3. There may be some other type of ignition problem.
4. There may be a mechanical problem in the engine.
Engine makes a clicking noise when idling
Description of problem: As the engine is idling, you hear a clicking noise coming from the engine. You don't notice the click when you are moving or when you increase the speed of the engine. The problem seems to get worse (noisier) when the engine is warm. The clicking is also getting more noticeable every day.
Probable Causes:
1. Your valves need adjusting.
2. The engine is low on oil.
3. The engine's oil pressure is low.
Engine makes a ticking noise
Description of problem: As soon as you start the engine, you hear a ticking noise from the engine. It sounds like a loud clock. The speed of the noise increases with an increase in engine speed. In fact, when you reach a particular speed, the noise is occurring so fast it seems that it is gone.
Probable Causes:
1. The valves in your engine need to be adjusted.
2. There is a lot of sludge in your engine which is stopping oil from circulating properly.
3. The engine's valve lifters are collapsed.
4. One or more of the engine's valves are stuck.
There is a rattling noise from the engine when you accelerate
Description of problem: Your car seems to run fine at all times. You notice nothing unusual except when you press on the gas pedal to accelerate or to go up a hill. Then the engine rattles like something is loose inside of it. Normally, this problem begins slowly and gets worse.
Probable Causes:
1. Your ignition timing needs adjusting.
2. The engine is overheating.
3. You have a loose vacuum hose on the engine.
4. You bought low-octane fuel even though your owner's manual says to only use high-octane fuel.
5. There is an excessive amount of carbon built up in your engine.
6. You have a problem with the electronic engine control system.
The engine surges or misfires while moving
Description of problem: The engine seems to start fine and will normally accelerate fine. However, once you try to keep a steady speed, the engine sputters and runs rough. You may notice that the engine runs differently when it is cold or warm. This type of problem normally is slow to develop but gets worse the longer you drive the car.
Probable Causes:
1. If you have a carburetor, the choke may not be set properly, or the choke may not be working correctly.
2. The engine may be running too hot.
3. The fuel pressure regulator may be operating at too low of a pressure.
4. The ignition timing may be set wrong.
5. There may be some type of ignition problem.
6. There may be a fault in the computerized engine control system.
7. The fuel filter may be partially clogged.
8. The torque converter in the transmission may not be locking at the right time, or it may be slipping.
9. There may be a vacuum leak.
10. The engine may have mechanical problems.
11. The EGR valve may be stuck open.
12. The drive axles may be loose or worn.
13. The fuel injectors may be dirty.
Clunking from the engine that worsens when engine speed increases
Description of problem: When you press on the gas pedal, the engine makes a clunking noise. The noise increases as you press harder on the gas pedal. The noise is there whether you are in gear or in neutral. Sometimes the noise is not noticeable when you are letting the engine idle but occurs as soon as you press on the gas pedal. Normally, the problem begins gradually, but the noise may go unnoticed. As the problem worsens, the noise gets louder.
Probable Causes:
1. Worn engine bearings.
2. Broken engine parts.
3. Loose or missing flywheel mounting bolts.
The car uses more oil than normal, and there are oil puddles under the car after it has been parked
Description of problem: You notice that the oil level is low between oil changes. You also notice puddles of oil under the car. It seems obvious that the loss of oil is due to oil leaks. Sometimes when you stop at a light or stop sign, smoke comes from under the hood. To identify the exact cause of the problem, it is best to look at the source of the oil leak. In addition to having the oil leaks repaired, you should make sure the engine always has the proper oil level. You may have also noticed that the car doesn't have the same amount of power as it once did. This type of problem seems to get worse once it develops.
Probable Causes:
1. The PCV system is not working properly.
2. The engine may have mechanical problems.
3. The engine's seals may be damaged.
4. The oil filter may not be tightened properly.
The engine quickly overheats
Description of problem: The engine seems to run fine but gets very hot shortly after you start it. This problem normally occurs after only five minutes of running or after traveling about a mile. You may also notice steam coming from the hood or smell something hot. This type of problem is slow to develop but gets worse the longer you drive. Once the engine begins to get too hot, turn it off or further damage will occur.
Probable Causes:
1. The engine's coolant level may be too low.
2. The engine's drive belts may be broken or slipping.
3. The electric cooling fan may not be coming on.
4. The ignition timing may be set wrong.
5. There may be a vacuum leak.
6. The engine may have mechanical problems.
7. The engine's thermostat may be stuck closed.
8. There may be a leak in the cooling system.
9. The engine's head gasket may be leaking.
The engine overheats
Description of problem: The engine seems to run fine but gets very hot after it has been driven. This problem normally occurs after driving some distance or while pulling a heavy load. You may also notice steam coming from the hood or smell something hot. This type of problem is slow to develop but gets worse the longer you drive it. Once the engine begins to get too hot, turn it off or further damage will occur.
Probable Causes:
1. The engine's coolant level may be too low.
2. The engine's drive belts may be broken or slipping.
3. The electric cooling fan may not be coming on.
4. The ignition timing may be set wrong.
5. There may be a vacuum leak.
6. The engine may have mechanical problems.
7. You have been pushing the car too hard and making it work too hard.
8. There may be a leak in the cooling system.
9. The engine's head gasket may be leaking.
10. The radiator may be clogged.
Your engine or oil light comes on while driving
Description of problem: The oil light may be marked engine. If this is the case, this light and warning system monitors the water temperature of the engine in addition to the oil. If this light stays on regardless of how fast you run the engine, there is a serious problem. Sometimes the light will come on when the engine is idling and go out when the engine's speed is increased. In most cases, this problem becomes more evident as the problem gets worse.
Probable Causes:
1. The engine has lost oil pressure or has low oil pressure (this is a serious problem that should be repaired at once).
2. The oil pressure sending unit is bad.
3. The engine is extremely low on oil.
4. The engine is overheating.
Your check engine or service engine light comes on or stays on
Description of problem: This is an area of much confusion since most manufacturers, until recently, have called this light by different names. This light also adds to confusion because the manufacturers have different systems that are monitored by this light circuit. In most cases, this warning light is part of the electronic engine control circuit. When the light comes on, it means that the car's computer has detected something wrong in the control system. The lamp remains lit until the problem is corrected. However on some systems, this light simply means there is a problem. The check engine or service engine light may suddenly come on and remain on, or it may come on and go out after a period of time.
Probable Causes:
1. The engine's computer has detected a problem in the system.
2. The engine has a serious problem that should be repaired at once.
3. The engine's oil pressure is exteremely low.
4. The engine is overheating.
Engine doesn't want to increase its speed
Description of problem: Whenever you push on the accelerator (gas pedal), the engine seems to slow down and sometimes it stalls. In order to increase the speed of the engine, you must very slowly press down on the gas pedal. Even then, the car doesn't seem to move like it should, and you sense a general lack of power. This problem may appear suddenly or get worse over time. You may notice the problem when the engine is hot or cold or when you are low on fuel. A description of when the problem occurs will help to identify the exact cause of the problem.
Probable Causes:
1. You may have a dirty air filter.
2. The fuel filter may be clogged with dirt.
3. The fuel pump may be worn.
4. The ignition timing may be wrong.
5. You may have water in the gasoline.
6. Your catalytic converter may be plugged.
Engine doesn't have its normal amount of power
Description of problem: Whenever you push on the accelerator (gas pedal), the car doesn't seem to move like it should or like it did before. You sense a general lack of power and know something is not right. There are no unusual noises or vibrations, but the engine just isn't performing well. The problem seems to be getting increasingly worse over time, although you did not notice when it first started.
Probable Causes:
1. You may have a dirty air filter.
2. The spark plugs and/or wires may be bad.
3. There may be some other type of ignition problem.
4. There may be a mechanical problem in the engine.
5. The catalytic converter may be plugged.
6. The exhaust system may be plugged due to dirt or damage.
7. The fuel filter may be clogged.
The engine will not idle smoothly, or it stalls during idle when the engine is cold.
Description of problem: When the engine is cold and you take your foot off the gas pedal, the engine runs very rough and may even stall. When you run the engine at higher speeds, it seems to run fine. This problem may get worse over time, or it may appear suddenly.
Probable Causes:
1. If you have a carburetor, the choke may not be set properly, or the choke is not working correctly.
2. There may be a vacuum leak in the intake system.
3. The idle speed may be set wrong.
4. There may be some type of ignition problem.
5. The ignition timing may be set wrong.
6. There may be a fault in the computerized engine control system.
7. The EGR valve may be defective.
8. There may be a mechanical problem in the engine.
9. The fuel injectors may be dirty.
The engine will not idle smoothly, or it stalls during idle when the engine is warm
Description of problem: When the engine is warm or hot and you take your foot off the gas pedal, the engine runs very rough and may even stall. When you run the engine at higher speeds, it seems to run fine. This problem may get worse over time or it may suddenly appear.
Probable Causes:
1. If you have a carburetor, the choke may not be set properly, or the choke is not working correctly.
2. There may be a vacuum leak in the intake system.
3. The fuel pressure regulator may be operating at too low of a pressure.
4. The idle speed may be set wrong.
5. There may be some type of ignition problem.
6. There may be a fault in the computerized engine control system.
7. The EGR valve may be defective.
8. There may be a mechanical problem in the engine.
9. The fuel injectors may be dirty.
The engine idles too fast
Description of problem: After you start the engine and it has run long enough to be warm, the engine seems to be racing. You especially notice this when you stop at a light or a stop sign and must push hard on the brake pedal to keep the car from moving. This problem normally happens without warning.
Probable Causes:
1. If you have a carburetor, the choke may not be set properly, or the choke is not working correctly.
2. The engine may be running too hot.
3. The fuel pressure regulator may be operating at too low of a pressure.
4. The ignition timing may be set wrong.
5. There may be some type of ignition problem.
6. There may be a fault in the computerized engine control system.
7. The alternator may not be working properly.
8. There is an air leak in the intake system.
9. You have a bad idle speed control unit.
Car stalls when stopped quickly
Description of problem: You are driving down the road, and everything is fine until you let off the gas pedal and apply the brakes. At this point the engine shakes badly and may even quit running. This is very dangerous as you lose power steering when the engine is not running. This type of problem may occur suddenly.
Probable Causes:
1. An intake system gasket is leaking.
2. The throttle linkage or mechanism needs to be repaired or replaced.
3. There is a problem with the electronic engine control system.
Engine surges while driving at a steady speed.
Description of problem: While driving at a constant speed on a highway, you notice the car seems to buck or jerk. If you press on the gas pedal, the engine smooths out. You only notice this problem when the road is flat and you are trying to keep a steady speed. This problem may occur suddenly and get worse over time.
Probable Causes:
1. Your fuel filter is dirty.
2. Your fuel pump is worn out.
3. The fuel pressure regulator isn't working properly
4. The last tank of fuel you bought was bad gas.
Engine continues to run when you try to turn it off
Description of problem: The car seems to run fine except that it doesn't want to turn right off. After you have driven awhile, you turn the key to stop the engine and it continues to run for a few seconds. At first you notice that this happens once in awhile, and now it happens quite often.
Probable Causes:
1. One or more of your fuel injectors are leaking.
2. The engine's idle is set too high.
3. The ignition timing is set wrong.
Description of problem: Whenever you push on the accelerator (gas pedal), the car doesn't seem to move like it should or like it did before. You sense a general lack of power and know something is not right. This problem may appear suddenly or get worse over time. You may notice the problem when the engine is hot or cold or when you are low on fuel. A description of when the problem occurs will help to identify the exact cause of the problem.
Probable Causes:
1. You may have a dirty air filter.
2. The spark plugs may be dirty or worn.
3. The spark wires may be bad.
4. There may be some other type of ignition problem.
5. The fuel filter may be clogged.
6. You may have water in the gasoline.
7. If you have a carburetor, you may have a bad accelerator pump or power circuit.
8. Your catalytic converter may be clogged.
The engine surges or misfires while moving
Description of problem: The engine seems to start fine and will normally accelerate fine. However, once you try to keep a steady speed, the engine sputters and runs rough. You may notice that the engine runs differently when it is cold or warm. This type of problem normally is slow to develop but gets worse the longer you drive the car.
Probable Causes:
1. If you have a carburetor, the choke may not be set properly, or the choke may not be working correctly.
2. The engine may be running too hot.
3. The fuel pressure regulator may be operating at too low of a pressure.
4. The ignition timing may be set wrong.
5. There may be some type of ignition problem.
6. There may be a fault in the computerized engine control system.
7. The fuel filter may be partially clogged.
8. The torque converter in the transmission may not be locking at the right time, or it may be slipping.
9. There may be a vacuum leak.
10. The engine may have mechanical problems.
11. The EGR valve may be stuck open.
12. The drive axles may be loose or worn.
13. The fuel injectors may be dirty.
A hissing sound is heard from the engine
Description of problem: The engine may or may not seem to run fine. Normally, the hissing noise becomes apparent soon after the driver notices that the engine is not running properly. This problem can occur suddenly.
Probable Causes:
1. The engine is overheating.
2. The exhaust system and/or catalytic converter is plugged.
3. A vacuum line is disconnected.
4. A vacuum device is leaking
Whirring from the engine that gets worse as engine speed increases
Description of problem: Although the whirring noise is evident at all engine speeds, it is the change in volume as engine speed increases that makes the noise most noticeable. This type of problem can be caused by many things. Some causes gradually develop, while others occur suddenly.
Probable Causes:
1. Low power steering fluid
2. The alternator bearings are bad.
3. A bad water pump.
4. A bad power steering pump.
5. A bad air conditioning compressor.
Smoke is coming from under the hood
Description of problem: Sometimes you will only see the smoke when you start your car or when you stop at a traffic light or stop sign. The smoke may be accompanied by engine idling problems. This is a problem that should not be ignored as it may be an indication of a serious and dangerous condition. The cause of the problem is best identified by the color, smell, and amount of smoke. Pay close attention to this. This problem may develop over time and the amount of smoke indicates the seriousness of the problem.
Probable Causes:
1. If the smoke has an oily smell, there is an oil leak.
2. If the smoke is white, there is probably an anti-freeze (engine coolant) leak.
3. If the smoke is blue or black and has a strong acrid smell, there is probably an electrical problem, and wires are burning.
Engine backfires when you press on the gas pedal
Description of problem: Basically, your engine is a mess. Every time you press on the gas pedal, the engine pops back at you. Sometimes the noise is loud, other times it is rather soft. This noise may result in an under hood fire, so don't ignore it.
Probable Causes:
1. Your camshaft timing belt or chain may have slipped
2. Your ignition timing needs adjusting.
3. There is a serious engine problem.
4. Your spark plug wires are placed on the wrong spark plugs.
Engine hesitates, and a popping is heard from the engine
Description of problem: Whenever you push on the accelerator (gas pedal), the car doesn't seem to move like it should or like it did before. You sense a general lack of power and know something is not right. The popping noise convinces you that something really isn't right. The noise only happens when you press on the gas pedal. This problem may appear suddenly or get worse over time. You may notice the problem when the engine is hot or cold or when you are low on fuel. A description of when the problem occurs will help to identify the exact cause of the problem.
Probable Causes:
1. You may have a dirty air filter.
2. The spark wires may be bad.
3. There may be some other type of ignition problem.
4. There may be a mechanical problem in the engine.
Engine makes a clicking noise when idling
Description of problem: As the engine is idling, you hear a clicking noise coming from the engine. You don't notice the click when you are moving or when you increase the speed of the engine. The problem seems to get worse (noisier) when the engine is warm. The clicking is also getting more noticeable every day.
Probable Causes:
1. Your valves need adjusting.
2. The engine is low on oil.
3. The engine's oil pressure is low.
Engine makes a ticking noise
Description of problem: As soon as you start the engine, you hear a ticking noise from the engine. It sounds like a loud clock. The speed of the noise increases with an increase in engine speed. In fact, when you reach a particular speed, the noise is occurring so fast it seems that it is gone.
Probable Causes:
1. The valves in your engine need to be adjusted.
2. There is a lot of sludge in your engine which is stopping oil from circulating properly.
3. The engine's valve lifters are collapsed.
4. One or more of the engine's valves are stuck.
There is a rattling noise from the engine when you accelerate
Description of problem: Your car seems to run fine at all times. You notice nothing unusual except when you press on the gas pedal to accelerate or to go up a hill. Then the engine rattles like something is loose inside of it. Normally, this problem begins slowly and gets worse.
Probable Causes:
1. Your ignition timing needs adjusting.
2. The engine is overheating.
3. You have a loose vacuum hose on the engine.
4. You bought low-octane fuel even though your owner's manual says to only use high-octane fuel.
5. There is an excessive amount of carbon built up in your engine.
6. You have a problem with the electronic engine control system.
The engine surges or misfires while moving
Description of problem: The engine seems to start fine and will normally accelerate fine. However, once you try to keep a steady speed, the engine sputters and runs rough. You may notice that the engine runs differently when it is cold or warm. This type of problem normally is slow to develop but gets worse the longer you drive the car.
Probable Causes:
1. If you have a carburetor, the choke may not be set properly, or the choke may not be working correctly.
2. The engine may be running too hot.
3. The fuel pressure regulator may be operating at too low of a pressure.
4. The ignition timing may be set wrong.
5. There may be some type of ignition problem.
6. There may be a fault in the computerized engine control system.
7. The fuel filter may be partially clogged.
8. The torque converter in the transmission may not be locking at the right time, or it may be slipping.
9. There may be a vacuum leak.
10. The engine may have mechanical problems.
11. The EGR valve may be stuck open.
12. The drive axles may be loose or worn.
13. The fuel injectors may be dirty.
Clunking from the engine that worsens when engine speed increases
Description of problem: When you press on the gas pedal, the engine makes a clunking noise. The noise increases as you press harder on the gas pedal. The noise is there whether you are in gear or in neutral. Sometimes the noise is not noticeable when you are letting the engine idle but occurs as soon as you press on the gas pedal. Normally, the problem begins gradually, but the noise may go unnoticed. As the problem worsens, the noise gets louder.
Probable Causes:
1. Worn engine bearings.
2. Broken engine parts.
3. Loose or missing flywheel mounting bolts.
The car uses more oil than normal, and there are oil puddles under the car after it has been parked
Description of problem: You notice that the oil level is low between oil changes. You also notice puddles of oil under the car. It seems obvious that the loss of oil is due to oil leaks. Sometimes when you stop at a light or stop sign, smoke comes from under the hood. To identify the exact cause of the problem, it is best to look at the source of the oil leak. In addition to having the oil leaks repaired, you should make sure the engine always has the proper oil level. You may have also noticed that the car doesn't have the same amount of power as it once did. This type of problem seems to get worse once it develops.
Probable Causes:
1. The PCV system is not working properly.
2. The engine may have mechanical problems.
3. The engine's seals may be damaged.
4. The oil filter may not be tightened properly.
The engine quickly overheats
Description of problem: The engine seems to run fine but gets very hot shortly after you start it. This problem normally occurs after only five minutes of running or after traveling about a mile. You may also notice steam coming from the hood or smell something hot. This type of problem is slow to develop but gets worse the longer you drive. Once the engine begins to get too hot, turn it off or further damage will occur.
Probable Causes:
1. The engine's coolant level may be too low.
2. The engine's drive belts may be broken or slipping.
3. The electric cooling fan may not be coming on.
4. The ignition timing may be set wrong.
5. There may be a vacuum leak.
6. The engine may have mechanical problems.
7. The engine's thermostat may be stuck closed.
8. There may be a leak in the cooling system.
9. The engine's head gasket may be leaking.
The engine overheats
Description of problem: The engine seems to run fine but gets very hot after it has been driven. This problem normally occurs after driving some distance or while pulling a heavy load. You may also notice steam coming from the hood or smell something hot. This type of problem is slow to develop but gets worse the longer you drive it. Once the engine begins to get too hot, turn it off or further damage will occur.
Probable Causes:
1. The engine's coolant level may be too low.
2. The engine's drive belts may be broken or slipping.
3. The electric cooling fan may not be coming on.
4. The ignition timing may be set wrong.
5. There may be a vacuum leak.
6. The engine may have mechanical problems.
7. You have been pushing the car too hard and making it work too hard.
8. There may be a leak in the cooling system.
9. The engine's head gasket may be leaking.
10. The radiator may be clogged.
Your engine or oil light comes on while driving
Description of problem: The oil light may be marked engine. If this is the case, this light and warning system monitors the water temperature of the engine in addition to the oil. If this light stays on regardless of how fast you run the engine, there is a serious problem. Sometimes the light will come on when the engine is idling and go out when the engine's speed is increased. In most cases, this problem becomes more evident as the problem gets worse.
Probable Causes:
1. The engine has lost oil pressure or has low oil pressure (this is a serious problem that should be repaired at once).
2. The oil pressure sending unit is bad.
3. The engine is extremely low on oil.
4. The engine is overheating.
Your check engine or service engine light comes on or stays on
Description of problem: This is an area of much confusion since most manufacturers, until recently, have called this light by different names. This light also adds to confusion because the manufacturers have different systems that are monitored by this light circuit. In most cases, this warning light is part of the electronic engine control circuit. When the light comes on, it means that the car's computer has detected something wrong in the control system. The lamp remains lit until the problem is corrected. However on some systems, this light simply means there is a problem. The check engine or service engine light may suddenly come on and remain on, or it may come on and go out after a period of time.
Probable Causes:
1. The engine's computer has detected a problem in the system.
2. The engine has a serious problem that should be repaired at once.
3. The engine's oil pressure is exteremely low.
4. The engine is overheating.
Engine doesn't want to increase its speed
Description of problem: Whenever you push on the accelerator (gas pedal), the engine seems to slow down and sometimes it stalls. In order to increase the speed of the engine, you must very slowly press down on the gas pedal. Even then, the car doesn't seem to move like it should, and you sense a general lack of power. This problem may appear suddenly or get worse over time. You may notice the problem when the engine is hot or cold or when you are low on fuel. A description of when the problem occurs will help to identify the exact cause of the problem.
Probable Causes:
1. You may have a dirty air filter.
2. The fuel filter may be clogged with dirt.
3. The fuel pump may be worn.
4. The ignition timing may be wrong.
5. You may have water in the gasoline.
6. Your catalytic converter may be plugged.
Engine doesn't have its normal amount of power
Description of problem: Whenever you push on the accelerator (gas pedal), the car doesn't seem to move like it should or like it did before. You sense a general lack of power and know something is not right. There are no unusual noises or vibrations, but the engine just isn't performing well. The problem seems to be getting increasingly worse over time, although you did not notice when it first started.
Probable Causes:
1. You may have a dirty air filter.
2. The spark plugs and/or wires may be bad.
3. There may be some other type of ignition problem.
4. There may be a mechanical problem in the engine.
5. The catalytic converter may be plugged.
6. The exhaust system may be plugged due to dirt or damage.
7. The fuel filter may be clogged.
The engine will not idle smoothly, or it stalls during idle when the engine is cold.
Description of problem: When the engine is cold and you take your foot off the gas pedal, the engine runs very rough and may even stall. When you run the engine at higher speeds, it seems to run fine. This problem may get worse over time, or it may appear suddenly.
Probable Causes:
1. If you have a carburetor, the choke may not be set properly, or the choke is not working correctly.
2. There may be a vacuum leak in the intake system.
3. The idle speed may be set wrong.
4. There may be some type of ignition problem.
5. The ignition timing may be set wrong.
6. There may be a fault in the computerized engine control system.
7. The EGR valve may be defective.
8. There may be a mechanical problem in the engine.
9. The fuel injectors may be dirty.
The engine will not idle smoothly, or it stalls during idle when the engine is warm
Description of problem: When the engine is warm or hot and you take your foot off the gas pedal, the engine runs very rough and may even stall. When you run the engine at higher speeds, it seems to run fine. This problem may get worse over time or it may suddenly appear.
Probable Causes:
1. If you have a carburetor, the choke may not be set properly, or the choke is not working correctly.
2. There may be a vacuum leak in the intake system.
3. The fuel pressure regulator may be operating at too low of a pressure.
4. The idle speed may be set wrong.
5. There may be some type of ignition problem.
6. There may be a fault in the computerized engine control system.
7. The EGR valve may be defective.
8. There may be a mechanical problem in the engine.
9. The fuel injectors may be dirty.
The engine idles too fast
Description of problem: After you start the engine and it has run long enough to be warm, the engine seems to be racing. You especially notice this when you stop at a light or a stop sign and must push hard on the brake pedal to keep the car from moving. This problem normally happens without warning.
Probable Causes:
1. If you have a carburetor, the choke may not be set properly, or the choke is not working correctly.
2. The engine may be running too hot.
3. The fuel pressure regulator may be operating at too low of a pressure.
4. The ignition timing may be set wrong.
5. There may be some type of ignition problem.
6. There may be a fault in the computerized engine control system.
7. The alternator may not be working properly.
8. There is an air leak in the intake system.
9. You have a bad idle speed control unit.
Car stalls when stopped quickly
Description of problem: You are driving down the road, and everything is fine until you let off the gas pedal and apply the brakes. At this point the engine shakes badly and may even quit running. This is very dangerous as you lose power steering when the engine is not running. This type of problem may occur suddenly.
Probable Causes:
1. An intake system gasket is leaking.
2. The throttle linkage or mechanism needs to be repaired or replaced.
3. There is a problem with the electronic engine control system.
Engine surges while driving at a steady speed.
Description of problem: While driving at a constant speed on a highway, you notice the car seems to buck or jerk. If you press on the gas pedal, the engine smooths out. You only notice this problem when the road is flat and you are trying to keep a steady speed. This problem may occur suddenly and get worse over time.
Probable Causes:
1. Your fuel filter is dirty.
2. Your fuel pump is worn out.
3. The fuel pressure regulator isn't working properly
4. The last tank of fuel you bought was bad gas.
Engine continues to run when you try to turn it off
Description of problem: The car seems to run fine except that it doesn't want to turn right off. After you have driven awhile, you turn the key to stop the engine and it continues to run for a few seconds. At first you notice that this happens once in awhile, and now it happens quite often.
Probable Causes:
1. One or more of your fuel injectors are leaking.
2. The engine's idle is set too high.
3. The ignition timing is set wrong.
Tire talk: Tire Care and Replacement
There are some easy things you can do to prolong the life of your tires and improve your vehicle's safety.
Keep your tires properly inflated -- correct air pressure is required for good handling and traction, good fuel economy and even wear. The only way to determine proper tire pressure is to use an accurate gauge. Tire pressure should be checked and corrected only when the tires are cold; even a short drive can make your tires too hot for accurate pressure readings. Don?t inflate tires to the maximum pressure printed on the tire -- use the tire pressure recommended in your vehicle?s owners manual or tire information sticker (located in the glove box, on the door post, or inside the fuel door). Remember to check the pressure in your spare tire
Checking Your Tire Pressure
The main reason you should care about tire pressure is car performance. Cars are easier to handle when the tire pressure is correct. Properly maintained tires also last longer, and improve your gas mileage.
The best way to maintain your tires is to buy an inexpensive tire pressure gauge. The correct tire pressure is printed on the sidewalls -- or the outside, non-tread part -- of your tires. It's also listed in your manual, and is often listed on a sticker in the glove compartment or on the door jamb. The pressure is listed in pounds per square inch, or PSI.
Here is how to check your tire pressure:
Changing a Flat Tire
Changing a flat can be a miserable experience for anyone. But if you have a jack, a lug wrench and a spare tire, you are half way there.
1) First Steps
2) Remove the hubcap and loosen the lug nuts
3) Jack Up the Car
4) Change the Tire
Car Tire Replacement Advice
When your tires wear out, you have to decide how you?re going to replace them. Often it's not just a simple matter of buying the exact tires that came with the car -- they may have been discontinued; may cost a lot more than a comparable brand; or may not fit your driving style. Don't skimp on your tire purchase if you care about your car's ride and handling. Conversely, if you only drive sedately and your car's expensive low-profile performance tires have worn out, don't break the bank to replace them if a lesser tire will fit. Determine if you want to stay with the same kind of tire that came with the car, or upgrade to something better (and more expensive...). Price out similar tires made by a few different manufacturers so you can find the best deal.
Tire prices vary considerably. Dealerships charge the most for tires. Service stations and auto parts stores are also expensive. Tire stores are generally expensive, but can have some good deals. Department stores have good prices, especially when they have sales. Wholesale stores and shopping clubs have even better prices. Low tire prices, and a large selection, can be found through mail order suppliers -- even after shipping charges are figured into the price.
No matter where you buy tires, buy a name brand. Low quality is the reason why unknown brands remain unknown. The major brands are -- Bridgestone, Dayton, Firestone, Continental, Cooper, General, Goodyear, Kelly, B.F. Goodrich, Michelin, Uniroyal, Armstrong, Pirelli, Centennial, Dunlop, Remington, Sumitomo, Toyo and Yokohama. Sears department stores also sell major manufacturer?s tires under the Sears brand name.
Three different charges are incurred when buying new tires. The first and the most expensive is the basic cost of the tire. Then there is a fee to mount and balance your tires. (Shop around, these fees vary widely.) There is also a nominal charge for new valve stems. Many large retail stores mount and balance tires and provide lifetime rotations and road hazard insurance for one surprisingly low fee.
Any warranty is better than no warranty, but don?t make a tire purchase based on this criteria alone. A tire warranted to go 70,000 miles might be a bad choice. Its hard rubber tread won't wear out quickly, but won't provide good traction either. Also, basic tire warranties only cover defects in workmanship and materials. It is difficult to prove that your driving style and lack of maintenance weren?t to blame for early wear-out.
Modern tires are usually not defective and do not often go flat -- "road hazard" or tire insurance is not necessary unless your car is rolling on some very expensive rubber.
Keep your tires properly inflated -- correct air pressure is required for good handling and traction, good fuel economy and even wear. The only way to determine proper tire pressure is to use an accurate gauge. Tire pressure should be checked and corrected only when the tires are cold; even a short drive can make your tires too hot for accurate pressure readings. Don?t inflate tires to the maximum pressure printed on the tire -- use the tire pressure recommended in your vehicle?s owners manual or tire information sticker (located in the glove box, on the door post, or inside the fuel door). Remember to check the pressure in your spare tire
Checking Your Tire Pressure
The main reason you should care about tire pressure is car performance. Cars are easier to handle when the tire pressure is correct. Properly maintained tires also last longer, and improve your gas mileage.
The best way to maintain your tires is to buy an inexpensive tire pressure gauge. The correct tire pressure is printed on the sidewalls -- or the outside, non-tread part -- of your tires. It's also listed in your manual, and is often listed on a sticker in the glove compartment or on the door jamb. The pressure is listed in pounds per square inch, or PSI.
Here is how to check your tire pressure:
- Find an air pump at a gas station and park so that the air pump hose can reach your tire comfortably. It's best to check tires when they are cold -- that is, when you haven't been driving on them for very long.
- Remove the tiny black valve cap on the valve that comes out of your tire, near the hubcap.
- Press the round part of the tire gauge firmly onto the valve. Try to press it so that the hissing sound of air escaping from the tire stops while you're pressing. When it does, you'll get an accurate reading..
- Read the gauge like a thermometer. The highest number you see closest to the stem of the gauge is the PSI. That number should match the recommended PSI for your tire
- If the gauge reading is higher than it should be, use your finger, or the notch on the opposite side of most tire gauges, to release a bit of air by pressing it on the pin inside the tire valve.
- If the gauge reading is lower than it should be, use the pump to add more air. On some pumps, you'll have to take the hose completely off the hose cradle to activate the pump. Press the head of the air hose firmly onto the tire just like you did with the tire gauge.
- Check your tire pressure with the gauge again, repeating your steps until you get the PSI right.
- Don't forget to replace the valve cap.
- If the gauge reading is higher than it should be, use your finger, or the notch on the opposite side of most tire gauges, to release a bit of air by pressing it on the pin inside the tire valve.
Changing a Flat Tire
Changing a flat can be a miserable experience for anyone. But if you have a jack, a lug wrench and a spare tire, you are half way there.
1) First Steps
- When you're driving and feel the rumble of a flat tire, slow down, turn on your hazard lights and try to park the car on level ground as quickly as possible.
- Put the automatic transmission into park and put the emergency brake on. If you have a manual transmission, leave it in first gear and pull the emergency brake.
- If you have to park on even a slight incline, try to find a heavy object to wedge up against the good tires. This will help to keep the car from rolling when you have it jacked up.
Once you've parked, take out the lug wrench, jack and the spare tire from the trunk. - Make sure the spare tire has enough air in it.
2) Remove the hubcap and loosen the lug nuts
- Pry off the hubcap with a screwdriver. Sometimes the lug wrench has a screw driver at the end of it. If it does, use that. Some cars don't have hubcaps at all.
- Now use the lug wrench to loosen the lug nuts, which are the hexagonal bolts under the hubcap. If the lug nut has an L on it, turn clockwise. If it has an R or doesn't have anything on it, turn counterclockwise. Try to loosen the nuts an equal amount.
- Very important: Don't remove the lug nuts yet. Just loosen them
3) Jack Up the Car
- Put the jack on the ground near the flat tire, under the car frame. Make sure it is under something structural that can support the weight of the car.
- Start pumping the jack, so that the top of it reaches the bottom of the car. When it does, keep going until the flat tire lifts off the ground. If the car seems unstable, lower the car, reposition the jack and try again.
- Very important: Never get under the car when it is jacked up.
4) Change the Tire
- Now that the flat tire is in the air, remove the lug nuts and place them in the upturned hub cap, or someplace easy to reach later.
- With all the lug nuts removed, pull the tire off by pulling it toward you. It will be heavy, so be careful it doesn't fall on you
- Put the spare tire on, positioning it so that the holes line up with the lug bolts.
- Replace the lug nets and tighten them, turning the opposite way you did when you removed them
- Put the spare tire on, positioning it so that the holes line up with the lug bolts.
- Replace the lug nets and tighten them, turning the opposite way you did when you removed them
- Then lower the jack even further and remove it.
- Put the flat tire, hubcap, jack and the lug wrench back in the trunk.
- Don't forget to remove the wheel blocks.
- Get your original tire fixed as soon as you can. Your spare may be only good for short distances at low speeds.
Car Tire Replacement Advice
When your tires wear out, you have to decide how you?re going to replace them. Often it's not just a simple matter of buying the exact tires that came with the car -- they may have been discontinued; may cost a lot more than a comparable brand; or may not fit your driving style. Don't skimp on your tire purchase if you care about your car's ride and handling. Conversely, if you only drive sedately and your car's expensive low-profile performance tires have worn out, don't break the bank to replace them if a lesser tire will fit. Determine if you want to stay with the same kind of tire that came with the car, or upgrade to something better (and more expensive...). Price out similar tires made by a few different manufacturers so you can find the best deal.
Tire prices vary considerably. Dealerships charge the most for tires. Service stations and auto parts stores are also expensive. Tire stores are generally expensive, but can have some good deals. Department stores have good prices, especially when they have sales. Wholesale stores and shopping clubs have even better prices. Low tire prices, and a large selection, can be found through mail order suppliers -- even after shipping charges are figured into the price.
No matter where you buy tires, buy a name brand. Low quality is the reason why unknown brands remain unknown. The major brands are -- Bridgestone, Dayton, Firestone, Continental, Cooper, General, Goodyear, Kelly, B.F. Goodrich, Michelin, Uniroyal, Armstrong, Pirelli, Centennial, Dunlop, Remington, Sumitomo, Toyo and Yokohama. Sears department stores also sell major manufacturer?s tires under the Sears brand name.
Three different charges are incurred when buying new tires. The first and the most expensive is the basic cost of the tire. Then there is a fee to mount and balance your tires. (Shop around, these fees vary widely.) There is also a nominal charge for new valve stems. Many large retail stores mount and balance tires and provide lifetime rotations and road hazard insurance for one surprisingly low fee.
Any warranty is better than no warranty, but don?t make a tire purchase based on this criteria alone. A tire warranted to go 70,000 miles might be a bad choice. Its hard rubber tread won't wear out quickly, but won't provide good traction either. Also, basic tire warranties only cover defects in workmanship and materials. It is difficult to prove that your driving style and lack of maintenance weren?t to blame for early wear-out.
Modern tires are usually not defective and do not often go flat -- "road hazard" or tire insurance is not necessary unless your car is rolling on some very expensive rubber.
Tire talk: Reading Specifications
Tire Specifications
When it's time to replace your tires, you have to know what brand and type you want, as well as their size. This information is printed on the sidewall. Brand name and tire name are easy enough to find sometimes they're even printed in raised white letters.
Tire size is measured in a combination of millimeters, letter codes and inches. The size of the tire pictured above is: P205/60SR15. The first letter is "P" for passenger tire. The first number is the tire?s width in millimeters -- in this case 205mm. The second number is its aspect ratio -- the ratio of sidewall height to width (also known as "profile"). In this case the sidewall height is 60 per cent of 205mm -- or 123mm. This tire is speed-rated, so the second letter is the speed rating -- in this case it?s "S" (112 mph).
Speed ratings give a general idea of a tire's overall performance characteristics -- a family sedan needs no more than an "S" rated tire, while a Ferrari will use a "Z" rated tire. Tires with high speed ratings are more expensive and shorter-lived than tires with low speed ratings. Speed ratings use the following letter codes:
Speed rating
A letter imprinted on the tire sidewall which indicates the maximum speed that a tire is designed to withstand if it is properly inflated and not overloaded, for short periods of time. The speed rating appears in one of two forms depending on the marking system used on the tire. If it is present, the rating will appear as a letter preceding the construction type designator (as in P175/70HR13 or 225/50VR15) or it will appear following the load index (as in 195/60R14 85T).
All European tires carry a speed rating, ranging from A5 (a maximum of 15 mph for forklift tires) to Z (speeds above 149 mph).
Some U.S.-produced tires carry speed ratings, some do not because in this country the only requirement is that any new tire must be capable of 85 mph. The most common speed ratings are: S (112 mph), T (118 mph), H (130 mph), V (149 mph), W (167 mph) and Z (higher than 149 mph).
The ability to withstand higher sustained speeds is only one characteristic of speed-rated tires. Speed ratings are not so much a measure of speed as they are a measure of performance and quality. Generally, the higher a tire's speed rating, the better its resistance to head build-up. It will provide better wet and dry traction and stability and thus will have enhanced ability to corner, brake and accelerate. The next letter is "R" for radial construction -- a superior design to the bias ply tires of old. The last number designates the wheel diameter -- this tire mounts on a 15-inch wheel
Sometimes a load index and a speed rating are printed together following the size designation. This tire's size is: P205/60R15 85S. "85S" means that this tire?s load index is 1135 lbs. and it has a speed rating of "S." This means that four tires can safely carry a maximum weight of 4540 lbs. (4 tires x 1135 lbs.) at 112 mph. This is something most drivers never have to worry about, but here?s a sampling of some load ratings:
Some light truck tires use a different sizing system.
This tire's size is LT 31X10.5R15. The first two letters stand for "light truck." The first number is the tire?s diameter in inches -- in this case, 31-inches. The second number is its width in inches, 10.5-inches. The "R" stands for radial. The 15 designates wheel diameter -- this tire is made to fit on a 15-inch wheel.
The very small type on the tire's sidewall contains the following information:
Uniform Tire Quality Grades are also printed on the sidewall. These grades are a result of government mandated tests that measure tread wear, traction and temperature resistance. The actual testing and grading is done by the manufacturer, so take these ratings with a grain of salt.
Tread wear measures how long the tread should last compared with a reference standard of 100. A tread wear rating of 400 means that the tread wears four times as well as the standard. This grade is only accurate for comparing tires within a certain brand.
Traction is a measurement of a tire's ability to stop in a straight line on a wet road. The highest grade is AA; A is good; B is intermediate; and C is the worst.
Temperature measures a tire's ability to withstand the heat build up caused by prolonged high speed driving, under inflation, or overloading. The highest grade is A; B is intermediate; and C is the worst.
M + S: Means the tire has the minimum required mud and snow traction.
Maximum Load: Maximum weight that the individual tire can support -- shown in pounds or kilograms.
Maximum Inflation Pressure: Shown in psi (pounds per square inch) or kPA (kilopascals). Never inflate your tires over the maximum inflation pressure.
D.O.T. Serial Number: Shows compliance with Department of Transportation regulations along with the coded name of the tire manufacturer and the place and date of manufacture. The date of manufacture is shown by the last three digits of the serial number -- Because rubber can dry out and deteriorate, tires that are extremely old can be more prone to failure than newer tires.
Tire Construction: Shows the number and type of plies (interwoven belts) which make up the tire's tread and sidewall.
When it's time to replace your tires, you have to know what brand and type you want, as well as their size. This information is printed on the sidewall. Brand name and tire name are easy enough to find sometimes they're even printed in raised white letters.
Tire size is measured in a combination of millimeters, letter codes and inches. The size of the tire pictured above is: P205/60SR15. The first letter is "P" for passenger tire. The first number is the tire?s width in millimeters -- in this case 205mm. The second number is its aspect ratio -- the ratio of sidewall height to width (also known as "profile"). In this case the sidewall height is 60 per cent of 205mm -- or 123mm. This tire is speed-rated, so the second letter is the speed rating -- in this case it?s "S" (112 mph).
Speed ratings give a general idea of a tire's overall performance characteristics -- a family sedan needs no more than an "S" rated tire, while a Ferrari will use a "Z" rated tire. Tires with high speed ratings are more expensive and shorter-lived than tires with low speed ratings. Speed ratings use the following letter codes:
| Q | 99 mph |
| S | 112 mph |
| T | 118 mph |
| U | 124 mph |
| H | 130 mph |
| V | 149 mph |
| Z | 149 mph & over |
Speed rating
A letter imprinted on the tire sidewall which indicates the maximum speed that a tire is designed to withstand if it is properly inflated and not overloaded, for short periods of time. The speed rating appears in one of two forms depending on the marking system used on the tire. If it is present, the rating will appear as a letter preceding the construction type designator (as in P175/70HR13 or 225/50VR15) or it will appear following the load index (as in 195/60R14 85T).
All European tires carry a speed rating, ranging from A5 (a maximum of 15 mph for forklift tires) to Z (speeds above 149 mph).
Some U.S.-produced tires carry speed ratings, some do not because in this country the only requirement is that any new tire must be capable of 85 mph. The most common speed ratings are: S (112 mph), T (118 mph), H (130 mph), V (149 mph), W (167 mph) and Z (higher than 149 mph).
The ability to withstand higher sustained speeds is only one characteristic of speed-rated tires. Speed ratings are not so much a measure of speed as they are a measure of performance and quality. Generally, the higher a tire's speed rating, the better its resistance to head build-up. It will provide better wet and dry traction and stability and thus will have enhanced ability to corner, brake and accelerate. The next letter is "R" for radial construction -- a superior design to the bias ply tires of old. The last number designates the wheel diameter -- this tire mounts on a 15-inch wheel
Sometimes a load index and a speed rating are printed together following the size designation. This tire's size is: P205/60R15 85S. "85S" means that this tire?s load index is 1135 lbs. and it has a speed rating of "S." This means that four tires can safely carry a maximum weight of 4540 lbs. (4 tires x 1135 lbs.) at 112 mph. This is something most drivers never have to worry about, but here?s a sampling of some load ratings:
| 75 | 853 lbs. |
| 85 | 1135 lbs. |
| 88 | 1235 lbs. |
| 91 | 1356 lbs. |
| 93 | 1433 lbs. |
| 105 | 2039 lbs. |
Some light truck tires use a different sizing system.
This tire's size is LT 31X10.5R15. The first two letters stand for "light truck." The first number is the tire?s diameter in inches -- in this case, 31-inches. The second number is its width in inches, 10.5-inches. The "R" stands for radial. The 15 designates wheel diameter -- this tire is made to fit on a 15-inch wheel.
The very small type on the tire's sidewall contains the following information:
Uniform Tire Quality Grades are also printed on the sidewall. These grades are a result of government mandated tests that measure tread wear, traction and temperature resistance. The actual testing and grading is done by the manufacturer, so take these ratings with a grain of salt.
Tread wear measures how long the tread should last compared with a reference standard of 100. A tread wear rating of 400 means that the tread wears four times as well as the standard. This grade is only accurate for comparing tires within a certain brand.
Traction is a measurement of a tire's ability to stop in a straight line on a wet road. The highest grade is AA; A is good; B is intermediate; and C is the worst.
Temperature measures a tire's ability to withstand the heat build up caused by prolonged high speed driving, under inflation, or overloading. The highest grade is A; B is intermediate; and C is the worst.
M + S: Means the tire has the minimum required mud and snow traction.
Maximum Load: Maximum weight that the individual tire can support -- shown in pounds or kilograms.
Maximum Inflation Pressure: Shown in psi (pounds per square inch) or kPA (kilopascals). Never inflate your tires over the maximum inflation pressure.
D.O.T. Serial Number: Shows compliance with Department of Transportation regulations along with the coded name of the tire manufacturer and the place and date of manufacture. The date of manufacture is shown by the last three digits of the serial number -- Because rubber can dry out and deteriorate, tires that are extremely old can be more prone to failure than newer tires.
Tire Construction: Shows the number and type of plies (interwoven belts) which make up the tire's tread and sidewall.
Tire talk: Kinds of Tires
No tire can handle every road condition and driving style perfectly. Positive attributes are always offset by negative factors, as the following list of tire types shows:
All-Season Tires: The Jack-of-all-trades of the tire world, and, as a result, they're the most compromised. They provide only adequate traction and handling, but they have long tread life and a smooth, quiet ride. They're also relatively affordable.
Touring All-Season Tires: These tires combine good handling with a civilized ride. Their performance oriented construction means that they?re somewhat noisier and harsher than regular all-season tires. They're also more expensive than regular all-season radials, but last just as long. Some manufacturer?s arbitrarily add "touring" to a tire?s name as a selling point.
Performance Tires: Wider tread and lower profiles combine good looks with good grip for precise, high-speed driving. Performance tires tend to have a harsh, noisy ride, relatively poor wet traction, bad snow traction, and they wear out faster than all-season radials. They?re also much more expensive. The price of ultra-high performance tires can cause your jaw to drop.
Conventional Snow Tires: Have chunky, aggressive treads that dig down to pavement covered by snow and ice. They?re noisy and handle poorly on dry roads. They're more expensive than all-season radials. They should last a long time, especially since they're only on the car for one season each year. Studded snow tires have tiny metal studs embedded in the tread for even better traction. (These days snow tire use is less common than in past decades. If you live in a place where it snows, and you drive a rear-wheel drive car, invest in a set of snow tires.)
"High-Tech" Snow Tires: Have precision engineered tread patterns and state-of-the-art multi-cell compounds which lend to good ice/snow traction and stopping ability. They can be used all year, but they?re noisy and somewhat clumsy on dry pavement. They're expensive and wear out quickly.
Light Truck Tires: Specifically designed for trucks and sport-utility vehicles, yet they are as diverse as passenger car tires. "Highway ribbed," on-road tires emphasize civilized ride and handling, while aggressive "off-road" or "mud tires" have a loud, harsh ride and sloppy handling on pavement. Light truck tires are more expensive than passenger car tires due to their larger sizes, higher load ratings and heavy-duty construction. Deep treads mean that they'll last a relatively long time.
There are a variety of specialty tires:
Rain Tires: Have a drainage channel in the tread that directs water away from the tire's surface more efficiently than conventional drainage grooves.
High Flotation Tires: Big, wide tires that people put on 4x4 trucks and sport-utilities so they can drive on the sand without sinking. These tires have poor traction in the ice and snow, so put those skinny, un-cool tires back on the truck for the winter.
Directional Tires: Have a "one-way" tread pattern optimized for the direction the tires rotate on the car.
Asymmetrical Tires: Combine multiple tread patterns in order to make a more well-rounded performance tire.
Self-Sealing Tires: Have a flexible inner-lining that seals around an object if punctured, stopping air loss.
"Twin" Tires: This setup employs two thin, "half-width" tires which are mounted on a special wheel. If one tire goes flat, the other "half" can still support the car.
"Run-flat" Tires: Use special rubber compounds and reinforced sidewalls that can support the car even when deflated -- allowing limited travel.
"Lifetime" Tires: Last for many years, as the name suggests. These tires wear out very slowly while delivering adequate traction.
All-Season Tires: The Jack-of-all-trades of the tire world, and, as a result, they're the most compromised. They provide only adequate traction and handling, but they have long tread life and a smooth, quiet ride. They're also relatively affordable.
Touring All-Season Tires: These tires combine good handling with a civilized ride. Their performance oriented construction means that they?re somewhat noisier and harsher than regular all-season tires. They're also more expensive than regular all-season radials, but last just as long. Some manufacturer?s arbitrarily add "touring" to a tire?s name as a selling point.
Performance Tires: Wider tread and lower profiles combine good looks with good grip for precise, high-speed driving. Performance tires tend to have a harsh, noisy ride, relatively poor wet traction, bad snow traction, and they wear out faster than all-season radials. They?re also much more expensive. The price of ultra-high performance tires can cause your jaw to drop.
Conventional Snow Tires: Have chunky, aggressive treads that dig down to pavement covered by snow and ice. They?re noisy and handle poorly on dry roads. They're more expensive than all-season radials. They should last a long time, especially since they're only on the car for one season each year. Studded snow tires have tiny metal studs embedded in the tread for even better traction. (These days snow tire use is less common than in past decades. If you live in a place where it snows, and you drive a rear-wheel drive car, invest in a set of snow tires.)
"High-Tech" Snow Tires: Have precision engineered tread patterns and state-of-the-art multi-cell compounds which lend to good ice/snow traction and stopping ability. They can be used all year, but they?re noisy and somewhat clumsy on dry pavement. They're expensive and wear out quickly.
Light Truck Tires: Specifically designed for trucks and sport-utility vehicles, yet they are as diverse as passenger car tires. "Highway ribbed," on-road tires emphasize civilized ride and handling, while aggressive "off-road" or "mud tires" have a loud, harsh ride and sloppy handling on pavement. Light truck tires are more expensive than passenger car tires due to their larger sizes, higher load ratings and heavy-duty construction. Deep treads mean that they'll last a relatively long time.
There are a variety of specialty tires:
Rain Tires: Have a drainage channel in the tread that directs water away from the tire's surface more efficiently than conventional drainage grooves.
High Flotation Tires: Big, wide tires that people put on 4x4 trucks and sport-utilities so they can drive on the sand without sinking. These tires have poor traction in the ice and snow, so put those skinny, un-cool tires back on the truck for the winter.
Directional Tires: Have a "one-way" tread pattern optimized for the direction the tires rotate on the car.
Asymmetrical Tires: Combine multiple tread patterns in order to make a more well-rounded performance tire.
Self-Sealing Tires: Have a flexible inner-lining that seals around an object if punctured, stopping air loss.
"Twin" Tires: This setup employs two thin, "half-width" tires which are mounted on a special wheel. If one tire goes flat, the other "half" can still support the car.
"Run-flat" Tires: Use special rubber compounds and reinforced sidewalls that can support the car even when deflated -- allowing limited travel.
"Lifetime" Tires: Last for many years, as the name suggests. These tires wear out very slowly while delivering adequate traction.
Repair Guide: Power-Assisted Brakes
A power-assist brake system is used on most cars to reduce the braking effort required by the driver. There are two general types of power-assist brakes. The most common type uses an intake manifold vacuum acting on a diaphragm to provide the power assist. The other type uses hydraulic power developed by a pump to provide the assist. The vacuum power-assist brake uses a large diaphragm in a canister behind the master cylinder. The canister is connected by a hose to the engine intake manifold. The push rod connected from the brake pedal goes through the power-assist unit on its way to the master cylinder. Inside the canister is a large diaphragm. This diaphragm is connected to the push rod. Engine intake manifold vacuum is applied to both sides of the diaphragm. When the brakes are applied, valving inside the canister is operated by the push rod. Atmospheric pressure is admitted to one side of the diaphragm. With vacuum on one side and atmospheric pressure on the other, the diaphragm moves toward the master cylinder. Because the push rod is connected to the diaphragm, it also moves. The large diaphragm has enough force to take much of the effort of applying the brakes away from the driver.
Hydraulic boosters require a hydraulic pump to provide the power assist. In many systems, the power steering pump is used to provide power for both the power steering and power brakes. As shown below, fluid under pressure goes from the power steering pump to a hydraulic boost unit behind the master cylinder. This unit contains a power piston and valving. When the brakes are applied, hydraulic pressure pushes on the boost piston, which in turn pushes on the brake push rod. The hydraulic power takes much of the effort out of brake application.
The hydraulic booster normally has an accumulator, which is a device that is spring loaded or charged with pressurized gas. This device provides emergency pressure to apply the brakes should the power steering flow be interrupted for any reason. The system is good for one to three stops, depending on the type of accumulator. Dash warning lights indicate when the system has failed.
CAUTION: Accumulators are under very high pressure. Never disassemble an accumulator without specific instructions to do so in a shop service manual. Incorrect disassembly procedures could cause the accumulator to fly apart and injure you.
Hydraulic boosters require a hydraulic pump to provide the power assist. In many systems, the power steering pump is used to provide power for both the power steering and power brakes. As shown below, fluid under pressure goes from the power steering pump to a hydraulic boost unit behind the master cylinder. This unit contains a power piston and valving. When the brakes are applied, hydraulic pressure pushes on the boost piston, which in turn pushes on the brake push rod. The hydraulic power takes much of the effort out of brake application.
The hydraulic booster normally has an accumulator, which is a device that is spring loaded or charged with pressurized gas. This device provides emergency pressure to apply the brakes should the power steering flow be interrupted for any reason. The system is good for one to three stops, depending on the type of accumulator. Dash warning lights indicate when the system has failed.
CAUTION: Accumulators are under very high pressure. Never disassemble an accumulator without specific instructions to do so in a shop service manual. Incorrect disassembly procedures could cause the accumulator to fly apart and injure you.
Repair Guide: Parking Brakes
The parking brake assembly is designed to apply the brakes mechanically to prevent the car from rolling when parked, or to stop the car in the event of a complete hydraulic failure. Most parking brakes operate on the two rear brakes. Some vehicles with front wheel drive have front wheel parking brakes. In these cases, in an emergency stop, most of the stopping power would be required on the front of the car.
The parking brake may be activated by a hand lever or a foot pedal. In either style, pushing the pedal or pulling the lever causes a cable connected to the rear brakes to be pulled. The cable has an equalizer so that the cable pulls the same amount on both left and right rear brakes.
The parking brake lever is connected to the secondary brake shoe. The lever is mounted on the back of the shoe and is connected to it by a pivot pin located in the upper end of the lever. The pivot pin is retained in the shoe by a washer and clip. The parking brake cable is attached to the lower end of the lever. A strut, located below the lever pivot pin, connects the lever to the primary brake shoe. The strut is notched at each end and is fitted into accommodating notches in the lever and primary brake shoe. An oval-shaped spring, installed on the primary shoe end of the strut, is used to position the strut.
When the parking brake is applied, the cable pulls the lower end of the parking brake lever forward, causing the connecting strut to push the primary brake shoe forward. At the same time, the upper end of the lever pushes the secondary brake shoe rearward. The combined action of the lever and strut expands the brake shoes, forcing them against the drum to develop brake action.
The parking brake may be activated by a hand lever or a foot pedal. In either style, pushing the pedal or pulling the lever causes a cable connected to the rear brakes to be pulled. The cable has an equalizer so that the cable pulls the same amount on both left and right rear brakes.
The parking brake lever is connected to the secondary brake shoe. The lever is mounted on the back of the shoe and is connected to it by a pivot pin located in the upper end of the lever. The pivot pin is retained in the shoe by a washer and clip. The parking brake cable is attached to the lower end of the lever. A strut, located below the lever pivot pin, connects the lever to the primary brake shoe. The strut is notched at each end and is fitted into accommodating notches in the lever and primary brake shoe. An oval-shaped spring, installed on the primary shoe end of the strut, is used to position the strut.
When the parking brake is applied, the cable pulls the lower end of the parking brake lever forward, causing the connecting strut to push the primary brake shoe forward. At the same time, the upper end of the lever pushes the secondary brake shoe rearward. The combined action of the lever and strut expands the brake shoes, forcing them against the drum to develop brake action.
Repair Guide: Master Cylinder, Brake fluid, Bleeding
The master cylinder is a type of hydraulic pump operated by the driver through the pedal and push rod. The master cylinder push rod is connected to a piston inside the cylinder. There is hydraulic fluid in front of the piston.
When the pedal is depressed, the master cylinder piston is pushed forward. The fluid in the master cylinder and the entire system, being incompressible, transmits the force exerted by the master cylinder piston to all the inner surfaces of the system. At this point, only the pistons in the wheel cylinders or caliper are free to move, and because the hydraulic fluid is not compressible, the pistons move outward to force the brake shoes against the brake drums or rotors.
There are two basic advantages of using a hydraulic system to operate the brakes. First, fluid lines are easy to route from the master cylinder to each of the wheel brake units. Second, hydraulics allow us to multiply the force used to apply the brake shoes. When you use a hydraulic jack to lift a car, you are multiplying your effort using the principles of hydraulics. This multiplying effect is done using the principles of pressure and force.
Pressure can be defined as the amount of force applied to a specific area. Suppose a hydraulic pressure of 10 psi (pounds per square inch) were applied to an object with a surface area of 16 square inches. The total applied force would equal 160 pounds (10 psi pressure times an area of 16 square inches). If the same 10 PSI were applied to an object with a surface area of 2 square inches, a force of 20 pounds would be applied. The relative size of the master cylinder piston and the pistons used at the wheel brake units allow the driver's brake pedal effort to be multiplied hydraulically.
Master Cylinder Construction and Operation
The master cylinder is constructed of two main parts: a reservoir and a master cylinder body. The reservoir provides a supply of brake fluid for cylinder operation. All current reservoirs are split designs. This means they provide two separate storage areas for two separate piston assemblies. The split design allows for separating the front and rear, or one front and one rear system, from each other in case of hydraulic failure.
The reservoir may be cast as one piece with the cylinder body or it may be a separate plastic container. All reservoirs have a removable cover or caps so that brake fluid can be added to the system. A flexible rubber diaphragm at the top of the master cylinder reservoir seals the hydraulic system from possible entrance of contamination while permitting expansion or contraction of the fluid within the reservoirs without direct venting. There are two holes, or ports, at the bottom of each reservoir section. One is called the replenishing port, the other a vent port. These ports permit passage of fluid between each pressure chamber and its fluid reservoir during certain operating conditions.
The body is a long aluminum or cast iron cylinder positioned under the reservoir. Inside the cylinder are two spool-shaped pistons. The pistons are fitted with rubber seals used to prevent fluid from leaking around the pistons. One piston is called the primary, the other the secondary. Each piston provides a separate hydraulic system for the front and rear brakes or on the diagonal system between one set of front and rear brakes. Springs in the cylinder return the pistons into position after braking. Two outlet holes provide the connection for the hydraulic lines. A snap ring holds the components inside the cylinder and a boot fits around the rear of the cylinder and push rod to prevent dirt from entering the cylinder.
The operation of the primary and secondary piston is the same. We examine how one piston works when the brakes are applied. When the brake pedal is depressed, force is transferred through the push rod to the master cylinder piston, which moves forward. After the primary seal covers the replenishing port, the master cylinder chamber is closed to the reservoir so that further piston travel builds up pressure. Fluid is then forced through the outlet into the lines leading to the wheel cylinders. When the brakes are released, return springs in the drum brake mechanisms pull the shoes away from the drums. This movement pushes the wheel cylinder pistons inward, forcing fluid back through the lines to the master cylinder. However, the master cylinder piston returns to the released position faster than fluid can fill the chamber, thus creating a momentary vacuum. To compensate for the vacuum, fluid flows from the reservoir, through the vent port, through the vent holes in the piston, and around the primary seal.
The dual master cylinder operates in the same manner as the single unit just described, except that it provides two independent systems, one for the front brakes and one for the rear, or one for each diagonal set of front and rear brakes. Under normal conditions, when the brakes are applied, the primary piston moves forward. At the same time, a combination of hydraulic pressure and the force of the primary piston spring moves the secondary piston forward. When the pistons have moved forward so that their primary seals cover the replenishing ports, hydraulic pressure is built up and transmitted to the front and the rear wheels.
In case of a hydraulic failure in the rear brake system, the primary piston will move forward when the brakes are applied, but will not build up hydraulic pressure. Only a small force is transferred to the secondary piston through the primary piston spring until the piston extension screw comes in contact with the secondary piston. Then, push rod force is transmitted directly to the secondary piston and enough pressure is built up to operate the front brakes.
If there is a hydraulic failure in the front brake system, both pistons will move forward when the brakes are applied, just like normal. Due to the front system failure, however, there is nothing to resist secondary piston travel except the secondary piston spring. This permits the primary piston to build up only negligible pressure until the secondary piston bottoms in the cylinder bore. Then, enough hydraulic pressure will be built up to operate the rear brakes.
Brake Master Cylinder Fluids
The brake system uses hydraulic power generated by a master cylinder to activate the four wheel brake assemblies. A fluid reservoir is located on top of the master cylinder. The fluid level in the brake master cylinder must be checked regularly at intervals specified by the manufacturer.
Brake fluid is a specially formulated liquid that must meet Society of Automotive Engineers and federal standards. These specifications list the necessary qualities of a brake fluid. The following are the most important:
Brake fluid is rated by the Department of Transportation (DOT). The brake fluid is then assigned a number. The common ratings are DOT 3, DOT 4, and DOT 5. The higher the number, the higher the fluid's boiling point. The DOT rating is found on the can of brake fluid. The shop and owner's manual specify what rating is correct for the car. Do not use a brake with a lower DOT rating than specified by the manufacturer. The lower rated fluid could boil and cause a loss of brake effectiveness.
Most brake fluid is glycol based. The word 'glycol' is usually not shown on the front of the container. The word 'silicone' is shown next to the name of the brake fluid if it has a silicone base. Always use the correct type of fluid specified in the shop or owner's manual. Do not mix the types of fluids. If you use the incorrect type of fluid you might cause a loss of brake efficiency.
WARNING: Brake fluid must always be stored in clean, dry containers. Brake fluid is hygroscopic; that is, it will attract moisture and must be kept away from dampness in a tightly sealed container. When water enters brake fluid, it causes its boiling point to lower. Fluid should be protected from contamination, especially oil, grease, or other petroleum products. Never reuse brake fluid.
CAUTION: Never use gasoline, kerosene, motor oil, transmission fluid, or any fluid containing mineral oil to clean brake system components. These fluids will cause the rubber cups and seals in the master or caliper units to soften, swell, and distort, resulting in brake system failure.
When the pedal is depressed, the master cylinder piston is pushed forward. The fluid in the master cylinder and the entire system, being incompressible, transmits the force exerted by the master cylinder piston to all the inner surfaces of the system. At this point, only the pistons in the wheel cylinders or caliper are free to move, and because the hydraulic fluid is not compressible, the pistons move outward to force the brake shoes against the brake drums or rotors.
There are two basic advantages of using a hydraulic system to operate the brakes. First, fluid lines are easy to route from the master cylinder to each of the wheel brake units. Second, hydraulics allow us to multiply the force used to apply the brake shoes. When you use a hydraulic jack to lift a car, you are multiplying your effort using the principles of hydraulics. This multiplying effect is done using the principles of pressure and force.
Pressure can be defined as the amount of force applied to a specific area. Suppose a hydraulic pressure of 10 psi (pounds per square inch) were applied to an object with a surface area of 16 square inches. The total applied force would equal 160 pounds (10 psi pressure times an area of 16 square inches). If the same 10 PSI were applied to an object with a surface area of 2 square inches, a force of 20 pounds would be applied. The relative size of the master cylinder piston and the pistons used at the wheel brake units allow the driver's brake pedal effort to be multiplied hydraulically.
Master Cylinder Construction and Operation
The master cylinder is constructed of two main parts: a reservoir and a master cylinder body. The reservoir provides a supply of brake fluid for cylinder operation. All current reservoirs are split designs. This means they provide two separate storage areas for two separate piston assemblies. The split design allows for separating the front and rear, or one front and one rear system, from each other in case of hydraulic failure.
The reservoir may be cast as one piece with the cylinder body or it may be a separate plastic container. All reservoirs have a removable cover or caps so that brake fluid can be added to the system. A flexible rubber diaphragm at the top of the master cylinder reservoir seals the hydraulic system from possible entrance of contamination while permitting expansion or contraction of the fluid within the reservoirs without direct venting. There are two holes, or ports, at the bottom of each reservoir section. One is called the replenishing port, the other a vent port. These ports permit passage of fluid between each pressure chamber and its fluid reservoir during certain operating conditions.
The body is a long aluminum or cast iron cylinder positioned under the reservoir. Inside the cylinder are two spool-shaped pistons. The pistons are fitted with rubber seals used to prevent fluid from leaking around the pistons. One piston is called the primary, the other the secondary. Each piston provides a separate hydraulic system for the front and rear brakes or on the diagonal system between one set of front and rear brakes. Springs in the cylinder return the pistons into position after braking. Two outlet holes provide the connection for the hydraulic lines. A snap ring holds the components inside the cylinder and a boot fits around the rear of the cylinder and push rod to prevent dirt from entering the cylinder.
The operation of the primary and secondary piston is the same. We examine how one piston works when the brakes are applied. When the brake pedal is depressed, force is transferred through the push rod to the master cylinder piston, which moves forward. After the primary seal covers the replenishing port, the master cylinder chamber is closed to the reservoir so that further piston travel builds up pressure. Fluid is then forced through the outlet into the lines leading to the wheel cylinders. When the brakes are released, return springs in the drum brake mechanisms pull the shoes away from the drums. This movement pushes the wheel cylinder pistons inward, forcing fluid back through the lines to the master cylinder. However, the master cylinder piston returns to the released position faster than fluid can fill the chamber, thus creating a momentary vacuum. To compensate for the vacuum, fluid flows from the reservoir, through the vent port, through the vent holes in the piston, and around the primary seal.
The dual master cylinder operates in the same manner as the single unit just described, except that it provides two independent systems, one for the front brakes and one for the rear, or one for each diagonal set of front and rear brakes. Under normal conditions, when the brakes are applied, the primary piston moves forward. At the same time, a combination of hydraulic pressure and the force of the primary piston spring moves the secondary piston forward. When the pistons have moved forward so that their primary seals cover the replenishing ports, hydraulic pressure is built up and transmitted to the front and the rear wheels.
In case of a hydraulic failure in the rear brake system, the primary piston will move forward when the brakes are applied, but will not build up hydraulic pressure. Only a small force is transferred to the secondary piston through the primary piston spring until the piston extension screw comes in contact with the secondary piston. Then, push rod force is transmitted directly to the secondary piston and enough pressure is built up to operate the front brakes.
If there is a hydraulic failure in the front brake system, both pistons will move forward when the brakes are applied, just like normal. Due to the front system failure, however, there is nothing to resist secondary piston travel except the secondary piston spring. This permits the primary piston to build up only negligible pressure until the secondary piston bottoms in the cylinder bore. Then, enough hydraulic pressure will be built up to operate the rear brakes.
Brake Master Cylinder Fluids
The brake system uses hydraulic power generated by a master cylinder to activate the four wheel brake assemblies. A fluid reservoir is located on top of the master cylinder. The fluid level in the brake master cylinder must be checked regularly at intervals specified by the manufacturer.
Brake fluid is a specially formulated liquid that must meet Society of Automotive Engineers and federal standards. These specifications list the necessary qualities of a brake fluid. The following are the most important:
- Must flow freely at low and high temperatures.
- Must have a high boiling point (over 400F).
- Must not deteriorate metal or rubber brake parts.
- Must lubricate metal and rubber parts.
- Must be able to absorb moisture that enters the hydraulic system.
Brake fluid is rated by the Department of Transportation (DOT). The brake fluid is then assigned a number. The common ratings are DOT 3, DOT 4, and DOT 5. The higher the number, the higher the fluid's boiling point. The DOT rating is found on the can of brake fluid. The shop and owner's manual specify what rating is correct for the car. Do not use a brake with a lower DOT rating than specified by the manufacturer. The lower rated fluid could boil and cause a loss of brake effectiveness.
Most brake fluid is glycol based. The word 'glycol' is usually not shown on the front of the container. The word 'silicone' is shown next to the name of the brake fluid if it has a silicone base. Always use the correct type of fluid specified in the shop or owner's manual. Do not mix the types of fluids. If you use the incorrect type of fluid you might cause a loss of brake efficiency.
WARNING: Brake fluid must always be stored in clean, dry containers. Brake fluid is hygroscopic; that is, it will attract moisture and must be kept away from dampness in a tightly sealed container. When water enters brake fluid, it causes its boiling point to lower. Fluid should be protected from contamination, especially oil, grease, or other petroleum products. Never reuse brake fluid.
CAUTION: Never use gasoline, kerosene, motor oil, transmission fluid, or any fluid containing mineral oil to clean brake system components. These fluids will cause the rubber cups and seals in the master or caliper units to soften, swell, and distort, resulting in brake system failure.
Repair Guide: Hydraulics
Hydraulic lines made of tubes and hoses are used to transmit fluid under pressure between the master cylinder and each of the wheel brake units. Several valves may be used in the system. A warning light pressure switch is used on all brake systems. A combination valve is used on some front disc brake-equipped cars to improve brake balance between the front disc brakes and rear drum brakes. A stop light switch is used to signal other drivers during a stop.
Hydraulic tubes are used to direct fluid between stationary brake parts. Most hydraulic tubes used in the brake system are double wall, welded steel tubes, coated to resist rust. The tube ends are double flared or have a chamber-type flare to guard against leakage. Threaded fittings are used to connect the tubes to brake parts.
The wheel brake units move up and down with the suspension. The master cylinder and steel brake tubes are mounted to the stationary body and frame. Flexible hoses are used to connect stationary brake components with moving components. Hoses must be able to withstand high fluid pressures without expansion and must be free to flex during spring deflection and wheel turns without damaging the hose. Hoses come in different sizes and lengths, with a variety of end fittings to accommodate different vehicle requirements.
The stop light switch is a spring-loaded electrical switch installed in the vehicle's stop light circuit. In some installations, the switch is operated by hydraulic pressure. In others, the switch is mechanically operated through contact with the brake pedal. With the brakes released, the circuit through the switch is open. When brakes are applied, the switch closes to complete the circuit to the stop lights.
Most late-model cars use a combination switch that contains a warning light switch, a metering valve, and a proportioning valve. The warning light switch is designed to light a brake warning lamp on the instrument panel if there is a hydraulic failure in either side of the split system. The switch works by sensing a pressure difference between two hydraulic circuits.
The switch body has connections for the hydraulic lines from the master cylinder. Outlet connections go to the two separate systems. This can be the front and rear wheels. On the diagonal style, a left front and right rear is paired together. The right front and left rear are separate systems.
A small switch piston is positioned between the two hydraulic systems. A spring positions the switch piston in the center of the switch. When the pressure is equal in both the front and rear hydraulic systems, the switch piston remains centered and does not contact the switch terminal in the bore of the switch.
If pressure fails in one of the hydraulic systems, hydraulic pressure moves the piston toward the inoperative side. The piston moves off-center to push up a switch pin. The switch pin activates the contacts inside the electrical terminal. This provides a ground for the warning lamp circuit and lights the warning lamp.
Most cars that use front disc brakes and rear drum brakes use a metering and proportioning valve in the system to achieve balanced braking between the front and rear wheels. The purpose of the metering valve is to improve braking balance, particularly during light brake application.
The metering valve keeps the front discs from operating until the rear drums have started to work. The valve is needed because disc brakes are fast acting; drum brakes have spring tensions and play in linkages to overcome first. The valve is located in the line to the front brakes. It works on fluid pressure and is normally closed. When the brake pedal is depressed, the fluid flows first to the rear brakes. As they begin to take hold, the system pressure builds up enough to open the metering valve, admitting pressure to the front brakes. Once open, the valve has no effect. The effect of the metering valve is felt during the beginning stages of all brake applications and throughout the duration of light brake applications. The proportioning valve is installed in the hydraulic circuit going to the rear drum brakes. Its function is to maintain the correct proportion between line pressures to the front and rear brakes and, therefore, provide a balanced vehicle braking system.
The proportioning valve reduces the hydraulic pressure at the rear drum brakes when high pressure is required at the front disc brakes. The valve helps to prevent premature rear wheel lock-up and skidding during heavy brake applications and provides better braking balance.
The Modern Hydraulic System
When you think about it, it's pretty hard to believe that a teeny tiny column of fluid can transmit enough force to stop a ton or two of hurtling automobile, but that's the magic of hydraulics. Early cars used mechanical apply set-ups -- cams, cables, and levers. No matter how clever the design, however, they were almost impossible to equalize perfectly, and required constant adjustment, so the idea of using hydraulics to do the job intrigued engineers from about 1897 onward. But it took many years to develop reasonably dependable systems, so the first domestic car with fluid pressure-actuated brakes was the '21 Dusenberg, and Chrysler followed in '24.
With refinements, hydraulic systems essentially the same as those originals got us through four decades. But in the mid-60's the changes and complications started. First there were discs and dual circuits with metering, proportioning, and a warning light, then came combo valves, diagonally-split systems, low-drag calipers, Quick Take-Up/step-bore masters, load-sensitive proportioning, etc. (ABS is another whole subject I won't tackle here except for maintenance).
A firm grasp of the modern hydraulic system and the service procedures it requires is about as important to anybody doing brake work as remembering to breathe. Unfortunately, I've found that lots of you out there still have some fuzzy areas in that essential understanding and also harbor a few misconceptions and prejudices.
Fail safe
A case in point is the dual, split, or tandem master cylinder, which has been used on every car sold in this country since 1967 (although Cadillac had it as far back as '62). Plenty of people still aren't comfortably familiar with its construction and operation.
A typical modern specimen will be of the composite variety (in other words, aluminum with a plastic reservoir), but iron one-piece units are still around in abundance. Two pistons ride in the bore, the rear piston being the primary, and the front the secondary.
Each piston has a primary cup at its front and a secondary at its rear, so you'll be hearing such combinations as primary piston secondary seal, secondary piston secondary seal, etc. The primary seals are the most important because they trap the fluid that's about to be squeezed into the lines. The primary piston's secondary seal keeps fluid from escaping out of the back of the cylinder, and the secondary piston's secondary seal acts as a barrier to make two essentially separate cylinders out of one. In normal braking, the pushrod from the pedal or booster forces the primary piston forward. No pressure is created until the primary seal covers the compensating or vent port from the reservoir, but once it does fluid is trapped in the chamber between the pistons and becomes, for all intents and purposes, a solid column. Pressure is routed from this chamber to two wheels. A combination of the trapped fluid and the primary piston coil spring bears on the secondary piston, moving it forward also and creating pressure in the chamber ahead of the secondary piston's primary seal, to which the line to the other two wheels is attached.
When the pedal is released, a partial vacuum occurs in both pressure chambers because the fluid is too lazy to return from the lines fast enough. So, in order to re-arm the brakes instantaneously, the primary seals are designed to allow fluid to flow one way (forward) from behind each seal into the pressure chambers.
The replenishing ports allow fluid to move freely between the chambers behind both pistons' primary cups and the reservoir according to demand and expansion and contraction from temperature changes.
Blow out
If a hose lets go or a saboteur has sawed through one of the brake lines so there's a catastrophic loss of fluid in half the system, the other half will still provide a means of decelerating the vehicle, albeit with a lower pedal and reduced stopping power. Both pistons have extensions which project out in front of their primary seals. A failure in the circuit that's connected to the primary piston's pressure chamber will allow the piston to move forward enough so the extension will bear on the secondary piston, push it ahead, and generate pressure in the other circuit. If, on the other hand, the circuit that gets its juice from the secondary chamber blows, the extension on the secondary piston will bottom out on the front of the cylinder and the fluid trapped between the pistons will operate the alternate set of brakes.
The residual pressure valve was once common in outlet ports that go to drum brakes. It maintained 5-20 psi in the lines to keep the wheel cylinder cups in constant contact with their bores. Most cylinders today have those little metal expanders behind the seals that make this unnecessary. If somebody were to install a check valve in a disc brake circuit, he'd create a constant dragging condition because discs are designed to work with very little clearance.
Initial burst
Since the GM X-Car appeared in 1980, there's been another complication: the Quick Take-Up/step-bore master cylinder. The engineers were trying everything possible to get the last tenth of an mpg, so the rolling resistance that zero-clearance discs caused was quite enough to warrant the adoption of low-drag calipers. Piston seal grooves were machined at an angle so the seals retracted the pads a sufficient distance to eliminate this parasitic loss, but that meant a master that displaced a large volume of fluid quickly was required.
The design arrived at uses a stepped bore and a primary piston with a small front and a larger rear diameter. During the first part of the stroke, the large part of the piston naturally displaces more fluid than the small part, and this extra volume goes around the lip of the small seal into the chamber between the primary and secondary pistons, moving the secondary ahead more than the distance the pushrod has traveled. That way, extra fluid is displaced into both circuits. A logically-named Quick Take-Up valve, which is connected to the rear high-volume chamber, vents excess fluid up into the reservoir once a certain psi is achieved, and also acts as the refill passage for the large chamber.
Other manufacturers have followed suit, as with the Tokico master you'll find on some FWD Fords, which features what's called a "Fluid Control Valve" to give essentially the same action as the GM unit.
Balancing act
But a master cylinder alone does not an integrated brake system make. Means of fine-tuning the pressure for the situation and warning the driver of a partial failure are equally important. All kinds of individual and combination valves are used to perform the metering, proportioning, and warning light activation functions, and I'll consider these jobs one at a time.
Disc brakes operate with very little clearance between the pads and the rotor, so the instant the caliper receives pressure, the drag on the wheel begins. But drums are different. There's considerable space to be taken up before the shoes go to work. If a disc/drum combination were connected directly to the same master, the discs would end up doing far more than their share. The metering or hold-off valve is what divides the load fairly. It stops the flow of fluid to the calipers until pressure reaches 75-125 psi or so, then it opens. This gives the drums a chance to catch up, so both types of brakes start applying at the same time. If you use a pressure bleeder during service, the metering valve will have to be deactivated, which is usually done by pulling on or depressing a pin.
Since drum brakes are self-energizing and commonly duo-servo whereas discs work entirely by means of hydraulic force, drums tend to lock up in hard stops. Weight transfer adds to the problem of rear over-braking, so even vehicles with discs all around need some means of keeping the posterior decelerators within bounds, and the proportioning valve was invented to do the job.
This device limits the flow to the rear brakes after a certain pressure has been reached, and this "split point" can be anywhere from 200 to 500 psi. Above that, force in the rear lines is allowed to rise at only a portion of the maximum available. The valve does absolutely nothing in normal, low-pressure stops. On front/rear split systems, if the front circuit fails, the valve is bypassed to allow full hydraulic power to reach the rears.
A further refinement is the load-sensing proportioning valve, which you'll find on various pickups, vans, utility vehicles, and even some cars (the first one I ever saw was on a '71 Fiat). The idea here is to use the distance between the body and the axle to adjust rear stopping power to match the weight on the posterior wheels and prevent lockup. Linkage connected to a lever on the proportioning valve varies the pressure available. You'll have to bleed this set-up with the vehicle's weight on its wheels because the valve will shut off the flow if the axle is hanging. Typically, these valves have adjustable linkage, but be certain to follow the specific service recommendations before fooling with them. One mm of adjustment can change pressure by 60 psi.
Mayday!
The extra safety the dual brake system provides carries a subtle danger with it: If one half springs a leak and the driver isn't sensitive enough to notice that he's pushing the pedal harder and farther than normal, he might ride around indefinitely with severely inadequate brakes. So, a dash light is provided that winks on when one circuit is in trouble, and it's activated by the pressure differential switch. Basically, this is a piston that remains centered in its cylinder as long as there's equal psi in both circuits. If one side blows, the pressure of the other pushes the piston toward the open side, and this movement closes the switch to the warning light.
Separation
What's the best way to divide a dual system? Well, the original domestic approach was to put both fronts on one circuit and both rears on the other. That works fairly well should the posterior brakes blow. But if the fronts go out, you've got vastly reduced stopping power and the tendency to skid if you lock the wheels. With FWD the situation became untenable because the rear wheels are so lightly loaded.
Enter dual-diagonal wherein one front and one rear on opposite sides share a circuit. This added the need for some additional plumbing (two proportioning valves, for instance), but gave more reasonable emergency deceleration ability in return. Bleeding sequence changed from the traditional RR, LR, RF, LF to RR, LF, LR, RF.
The ultimate system for safety is what you'll find on non-ABS Volvos: Both fronts and one rear will operate even if one circuit fails. This is accomplished by using two- or four-piston calipers with one side of each plus one rear brake connected to half the system. One pad of each disc brake will always be operational.
Tube types
The metal brake lines that route pressure to the wheels haven't escaped change. They're still made of double-wall steel, but a different type of fitting is taking over. Called the ISO (International Standards Organization) flare, it's not compatible with traditional double-flare/tube seat connections. Because the shoulder of the nut bottoms in the fitting, sealing pressure is uniform and over-tightening ceases to be a problem. Also, only one simple die is required to form the flare.
Brake Fluid
Then there's brake fluid, the stuff that makes everything happen. Modern DOT 3 and 4 glycol are both pretty good because they have high boiling points and the ability to hold lots of moisture. But that affinity for water made another type seem attractive: silicone, rated DOT 5 and color-coded purple. It doesn't absorb H2O, so was expected to practically eliminate corrosion, has a 500 degree F. boiling point, and won't dissolve paint the way ordinary glycol does.
Why hasn't everybody gone to silicone? Because there are drawbacks and unanswered questions. First, it's way more expensive than DOT 3 or 4, but a few dollars wouldn't really make much difference in the average price of a car today. It's the unresolved performance characteristics that have kept the carmakers from filling hydraulic systems with DOT 5 at the factory. Seal life is one problem. An aftermarket brake engineer I know told us he has trouble getting cylinder cups to make it through the SAE longevity test with silicone -- they get hard and wear out, he says. And, if any moisture should find its way into the system, it tends to collect in slugs. When elevated temperatures are encountered, especially at high altitudes, these can turn to bubbles resulting in a loss of stopping power.
The auto makers just don't consider silicone's potential advantages worth the risk. After all, a typical brand-name super heavy duty DOT 3 has a dry boiling point of 485 deg. F., which is only 15 deg. less than silicone (wet, this falls to 310 deg., but there's no excuse for running around with watered-down fluid, as I'll explain).
Proceed
Now for some service tips that'll help you avoid problems. The first thing I'd like to mention is that you should always open the bleeder before bottoming out those caliper pistons. Otherwise, you'll back flush nasty sediment up into the works. This is bad enough with a regular master, but with all the tiny passages in an ABS it can cause big trouble. Just do it.
Then there's flushing and refilling the system with fresh fluid. In the past, some brake experts said it wasn't worth the effort because you can't get all the old stuff out unless you disassemble the calipers and cylinders. True, you won't be able to eliminate every drop of the contaminated liquid, but you can get most of it, and that will effectively reduce the amount of moisture in the circuits.
This has always been important for corrosion prevention, but now the high operating temperatures encountered with semi-mets and FWD make maintaining a high boiling point critical to safety even for the average motorist. Besides water, there's sediment, which is a combination of rust and the ashy residue of burned glycol. Expensive and intricate ABS hardware is further justification for this maintenance. Many of these systems are vented to the atmosphere, and there's also contamination from under-hood vapors in some layouts. Fluid changes are cheap insurance against big-bucks repairs.
Depending on the authority, recommended intervals range from one to three years.
New pipes?
Whenever you've got a car up in the air, take a careful look at the metal lines and rubber hoses. In my own shop, I see total circuit failures due to corroded lines or blown hoses frequently enough to be very conscientious about this. If a line runs up over the chassis so that it's hard to see, use a mirror and your sense of touch.
Replace lines if the rust has reached the scaly stage. When installing replacements, follow the original routing as closely as possible. I often hear of fade or low pedal problems because one of these lines has been mounted too close to an exhaust pipe, or has a hump or loop in it that traps air.
Those flexible hoses are so well made they often survive for the life of the car, but why push your luck? I almost took an icy dip in Lake Michigan when one blew on my pre-dual master '66 Ford, so I consider their replacement valuable insurance. You could use some carmakers' service literature to support the idea of new hoses as regular maintenance -- the Ford Fiesta manual, for example, says hoses should be retired after 36K miles. A related item is the high pressure ABS hose between the pump and the accumulator, which some car makers want you to replace at 60,000 miles.
Bubble trouble
Air expulsion definitely deserves some space because there's more to it than just observing the proper sequence. Bench bleeding master cylinders, for instance. Neglecting this is the number-one reason for spongy pedal complaints, and some re-manufacturers include fittings, tubes, and instructions in the box in hopes of reducing the number of unnecessary returns.
You can do this job by just holding your fingers over the outlet tubes to keep air from being drawn in on the return stroke, but that's pretty messy, so I'll give you the procedure using tubes. Clamp one of the master's mounting ears in a vice so the unit is as level as possible. Position the tube tips well below the level of fluid in the reservoir, then use a rod or drift to stroke the piston SLOWLY. Especially on Quick Take-Ups and their equivalents, wait at least 15 seconds between strokes to allow the low pressure chamber to release all its bubbles and fill completely. Keep stroking until there's no more evidence of air at the tube tips and ports.
Should you get a car with a replacement cylinder that some other service personage didn't bench bleed, you might be able to do it with the master in place providing you can jack the rear of the vehicle high enough to get the cylinder level. Surge bleeding -- you know, where you pound the pedal violently a bunch of times to get the bubbles mixed up with the fluid, then crack the line -- is frowned upon by experts who don't think aeration is ever a good idea.
By the way, one major brake outfit (EIS) has done us a favor by providing aftermarket step-bore master cylinders with a bleeder that facilitates getting out all that trapped air. Located in the low-pressure side of the cylinder below the Quick Take-Up valve, it gives those bubbles a convenient exit path. This little feature can reduce bench bleeding time to maybe three minutes, and practically eliminates low-pedal comebacks.
When it comes to the bleeders at the wheels, I know most of you just open them and let the fluid squirt. But besides making slippery puddles on the floor, it can shoot farther than you might expect, perhaps ruining the paint on a nearby car. I use a transparent tube and bottle set-up that hangs by a magnet because it's neat and it lets me see what I'm getting out. Also, it eliminates the need for a helper if I'm not using a pressure bleeder.
Then there are the bleeder screws themselves. You haven't worked on cars very long if you haven't encountered a frozen one that breaks off before opening. My rather drastic and primitive method of clamping Vise-Grips to the screw and shaking it while I heat the cylinder or caliper around the port with a propane torch often works, but there's a better answer. Whenever you get a vehicle in that hasn't yet developed the problem, unscrew the bleeders and coat their threads with just a touch of anti-seize. Ditto when you replace a caliper or cylinder. If you get that conveyance back later for brake work, you'll be glad you took the time.
Caliper Care
In an ideal world, every caliper would be overhauled during a reline to insure against piston seizure and seal failure. And that's what most authorities recommend. But when was the last time you encountered a leaky caliper? They last and last in most cases. So, if you're under time or cost pressure, you could just push those pistons back (bleeder open, of course). One caveat: If you feel any roughness or binding as you force a piston home, you'd better get inside.
What about wheel cylinders? The frequency of leaks here is so high most of the good techs overhaul or replace them in every case.
Hydraulic tubes are used to direct fluid between stationary brake parts. Most hydraulic tubes used in the brake system are double wall, welded steel tubes, coated to resist rust. The tube ends are double flared or have a chamber-type flare to guard against leakage. Threaded fittings are used to connect the tubes to brake parts.
The wheel brake units move up and down with the suspension. The master cylinder and steel brake tubes are mounted to the stationary body and frame. Flexible hoses are used to connect stationary brake components with moving components. Hoses must be able to withstand high fluid pressures without expansion and must be free to flex during spring deflection and wheel turns without damaging the hose. Hoses come in different sizes and lengths, with a variety of end fittings to accommodate different vehicle requirements.
The stop light switch is a spring-loaded electrical switch installed in the vehicle's stop light circuit. In some installations, the switch is operated by hydraulic pressure. In others, the switch is mechanically operated through contact with the brake pedal. With the brakes released, the circuit through the switch is open. When brakes are applied, the switch closes to complete the circuit to the stop lights.
Most late-model cars use a combination switch that contains a warning light switch, a metering valve, and a proportioning valve. The warning light switch is designed to light a brake warning lamp on the instrument panel if there is a hydraulic failure in either side of the split system. The switch works by sensing a pressure difference between two hydraulic circuits.
The switch body has connections for the hydraulic lines from the master cylinder. Outlet connections go to the two separate systems. This can be the front and rear wheels. On the diagonal style, a left front and right rear is paired together. The right front and left rear are separate systems.
A small switch piston is positioned between the two hydraulic systems. A spring positions the switch piston in the center of the switch. When the pressure is equal in both the front and rear hydraulic systems, the switch piston remains centered and does not contact the switch terminal in the bore of the switch.
If pressure fails in one of the hydraulic systems, hydraulic pressure moves the piston toward the inoperative side. The piston moves off-center to push up a switch pin. The switch pin activates the contacts inside the electrical terminal. This provides a ground for the warning lamp circuit and lights the warning lamp.
Most cars that use front disc brakes and rear drum brakes use a metering and proportioning valve in the system to achieve balanced braking between the front and rear wheels. The purpose of the metering valve is to improve braking balance, particularly during light brake application.
The metering valve keeps the front discs from operating until the rear drums have started to work. The valve is needed because disc brakes are fast acting; drum brakes have spring tensions and play in linkages to overcome first. The valve is located in the line to the front brakes. It works on fluid pressure and is normally closed. When the brake pedal is depressed, the fluid flows first to the rear brakes. As they begin to take hold, the system pressure builds up enough to open the metering valve, admitting pressure to the front brakes. Once open, the valve has no effect. The effect of the metering valve is felt during the beginning stages of all brake applications and throughout the duration of light brake applications. The proportioning valve is installed in the hydraulic circuit going to the rear drum brakes. Its function is to maintain the correct proportion between line pressures to the front and rear brakes and, therefore, provide a balanced vehicle braking system.
The proportioning valve reduces the hydraulic pressure at the rear drum brakes when high pressure is required at the front disc brakes. The valve helps to prevent premature rear wheel lock-up and skidding during heavy brake applications and provides better braking balance.
The Modern Hydraulic System
When you think about it, it's pretty hard to believe that a teeny tiny column of fluid can transmit enough force to stop a ton or two of hurtling automobile, but that's the magic of hydraulics. Early cars used mechanical apply set-ups -- cams, cables, and levers. No matter how clever the design, however, they were almost impossible to equalize perfectly, and required constant adjustment, so the idea of using hydraulics to do the job intrigued engineers from about 1897 onward. But it took many years to develop reasonably dependable systems, so the first domestic car with fluid pressure-actuated brakes was the '21 Dusenberg, and Chrysler followed in '24.
With refinements, hydraulic systems essentially the same as those originals got us through four decades. But in the mid-60's the changes and complications started. First there were discs and dual circuits with metering, proportioning, and a warning light, then came combo valves, diagonally-split systems, low-drag calipers, Quick Take-Up/step-bore masters, load-sensitive proportioning, etc. (ABS is another whole subject I won't tackle here except for maintenance).
A firm grasp of the modern hydraulic system and the service procedures it requires is about as important to anybody doing brake work as remembering to breathe. Unfortunately, I've found that lots of you out there still have some fuzzy areas in that essential understanding and also harbor a few misconceptions and prejudices.
Fail safe
A case in point is the dual, split, or tandem master cylinder, which has been used on every car sold in this country since 1967 (although Cadillac had it as far back as '62). Plenty of people still aren't comfortably familiar with its construction and operation.
A typical modern specimen will be of the composite variety (in other words, aluminum with a plastic reservoir), but iron one-piece units are still around in abundance. Two pistons ride in the bore, the rear piston being the primary, and the front the secondary.
Each piston has a primary cup at its front and a secondary at its rear, so you'll be hearing such combinations as primary piston secondary seal, secondary piston secondary seal, etc. The primary seals are the most important because they trap the fluid that's about to be squeezed into the lines. The primary piston's secondary seal keeps fluid from escaping out of the back of the cylinder, and the secondary piston's secondary seal acts as a barrier to make two essentially separate cylinders out of one. In normal braking, the pushrod from the pedal or booster forces the primary piston forward. No pressure is created until the primary seal covers the compensating or vent port from the reservoir, but once it does fluid is trapped in the chamber between the pistons and becomes, for all intents and purposes, a solid column. Pressure is routed from this chamber to two wheels. A combination of the trapped fluid and the primary piston coil spring bears on the secondary piston, moving it forward also and creating pressure in the chamber ahead of the secondary piston's primary seal, to which the line to the other two wheels is attached.
When the pedal is released, a partial vacuum occurs in both pressure chambers because the fluid is too lazy to return from the lines fast enough. So, in order to re-arm the brakes instantaneously, the primary seals are designed to allow fluid to flow one way (forward) from behind each seal into the pressure chambers.
The replenishing ports allow fluid to move freely between the chambers behind both pistons' primary cups and the reservoir according to demand and expansion and contraction from temperature changes.
Blow out
If a hose lets go or a saboteur has sawed through one of the brake lines so there's a catastrophic loss of fluid in half the system, the other half will still provide a means of decelerating the vehicle, albeit with a lower pedal and reduced stopping power. Both pistons have extensions which project out in front of their primary seals. A failure in the circuit that's connected to the primary piston's pressure chamber will allow the piston to move forward enough so the extension will bear on the secondary piston, push it ahead, and generate pressure in the other circuit. If, on the other hand, the circuit that gets its juice from the secondary chamber blows, the extension on the secondary piston will bottom out on the front of the cylinder and the fluid trapped between the pistons will operate the alternate set of brakes.
The residual pressure valve was once common in outlet ports that go to drum brakes. It maintained 5-20 psi in the lines to keep the wheel cylinder cups in constant contact with their bores. Most cylinders today have those little metal expanders behind the seals that make this unnecessary. If somebody were to install a check valve in a disc brake circuit, he'd create a constant dragging condition because discs are designed to work with very little clearance.
Initial burst
Since the GM X-Car appeared in 1980, there's been another complication: the Quick Take-Up/step-bore master cylinder. The engineers were trying everything possible to get the last tenth of an mpg, so the rolling resistance that zero-clearance discs caused was quite enough to warrant the adoption of low-drag calipers. Piston seal grooves were machined at an angle so the seals retracted the pads a sufficient distance to eliminate this parasitic loss, but that meant a master that displaced a large volume of fluid quickly was required.
The design arrived at uses a stepped bore and a primary piston with a small front and a larger rear diameter. During the first part of the stroke, the large part of the piston naturally displaces more fluid than the small part, and this extra volume goes around the lip of the small seal into the chamber between the primary and secondary pistons, moving the secondary ahead more than the distance the pushrod has traveled. That way, extra fluid is displaced into both circuits. A logically-named Quick Take-Up valve, which is connected to the rear high-volume chamber, vents excess fluid up into the reservoir once a certain psi is achieved, and also acts as the refill passage for the large chamber.
Other manufacturers have followed suit, as with the Tokico master you'll find on some FWD Fords, which features what's called a "Fluid Control Valve" to give essentially the same action as the GM unit.
Balancing act
But a master cylinder alone does not an integrated brake system make. Means of fine-tuning the pressure for the situation and warning the driver of a partial failure are equally important. All kinds of individual and combination valves are used to perform the metering, proportioning, and warning light activation functions, and I'll consider these jobs one at a time.
Disc brakes operate with very little clearance between the pads and the rotor, so the instant the caliper receives pressure, the drag on the wheel begins. But drums are different. There's considerable space to be taken up before the shoes go to work. If a disc/drum combination were connected directly to the same master, the discs would end up doing far more than their share. The metering or hold-off valve is what divides the load fairly. It stops the flow of fluid to the calipers until pressure reaches 75-125 psi or so, then it opens. This gives the drums a chance to catch up, so both types of brakes start applying at the same time. If you use a pressure bleeder during service, the metering valve will have to be deactivated, which is usually done by pulling on or depressing a pin.
Since drum brakes are self-energizing and commonly duo-servo whereas discs work entirely by means of hydraulic force, drums tend to lock up in hard stops. Weight transfer adds to the problem of rear over-braking, so even vehicles with discs all around need some means of keeping the posterior decelerators within bounds, and the proportioning valve was invented to do the job.
This device limits the flow to the rear brakes after a certain pressure has been reached, and this "split point" can be anywhere from 200 to 500 psi. Above that, force in the rear lines is allowed to rise at only a portion of the maximum available. The valve does absolutely nothing in normal, low-pressure stops. On front/rear split systems, if the front circuit fails, the valve is bypassed to allow full hydraulic power to reach the rears.
A further refinement is the load-sensing proportioning valve, which you'll find on various pickups, vans, utility vehicles, and even some cars (the first one I ever saw was on a '71 Fiat). The idea here is to use the distance between the body and the axle to adjust rear stopping power to match the weight on the posterior wheels and prevent lockup. Linkage connected to a lever on the proportioning valve varies the pressure available. You'll have to bleed this set-up with the vehicle's weight on its wheels because the valve will shut off the flow if the axle is hanging. Typically, these valves have adjustable linkage, but be certain to follow the specific service recommendations before fooling with them. One mm of adjustment can change pressure by 60 psi.
Mayday!
The extra safety the dual brake system provides carries a subtle danger with it: If one half springs a leak and the driver isn't sensitive enough to notice that he's pushing the pedal harder and farther than normal, he might ride around indefinitely with severely inadequate brakes. So, a dash light is provided that winks on when one circuit is in trouble, and it's activated by the pressure differential switch. Basically, this is a piston that remains centered in its cylinder as long as there's equal psi in both circuits. If one side blows, the pressure of the other pushes the piston toward the open side, and this movement closes the switch to the warning light.
Separation
What's the best way to divide a dual system? Well, the original domestic approach was to put both fronts on one circuit and both rears on the other. That works fairly well should the posterior brakes blow. But if the fronts go out, you've got vastly reduced stopping power and the tendency to skid if you lock the wheels. With FWD the situation became untenable because the rear wheels are so lightly loaded.
Enter dual-diagonal wherein one front and one rear on opposite sides share a circuit. This added the need for some additional plumbing (two proportioning valves, for instance), but gave more reasonable emergency deceleration ability in return. Bleeding sequence changed from the traditional RR, LR, RF, LF to RR, LF, LR, RF.
The ultimate system for safety is what you'll find on non-ABS Volvos: Both fronts and one rear will operate even if one circuit fails. This is accomplished by using two- or four-piston calipers with one side of each plus one rear brake connected to half the system. One pad of each disc brake will always be operational.
Tube types
The metal brake lines that route pressure to the wheels haven't escaped change. They're still made of double-wall steel, but a different type of fitting is taking over. Called the ISO (International Standards Organization) flare, it's not compatible with traditional double-flare/tube seat connections. Because the shoulder of the nut bottoms in the fitting, sealing pressure is uniform and over-tightening ceases to be a problem. Also, only one simple die is required to form the flare.
Brake Fluid
Then there's brake fluid, the stuff that makes everything happen. Modern DOT 3 and 4 glycol are both pretty good because they have high boiling points and the ability to hold lots of moisture. But that affinity for water made another type seem attractive: silicone, rated DOT 5 and color-coded purple. It doesn't absorb H2O, so was expected to practically eliminate corrosion, has a 500 degree F. boiling point, and won't dissolve paint the way ordinary glycol does.
Why hasn't everybody gone to silicone? Because there are drawbacks and unanswered questions. First, it's way more expensive than DOT 3 or 4, but a few dollars wouldn't really make much difference in the average price of a car today. It's the unresolved performance characteristics that have kept the carmakers from filling hydraulic systems with DOT 5 at the factory. Seal life is one problem. An aftermarket brake engineer I know told us he has trouble getting cylinder cups to make it through the SAE longevity test with silicone -- they get hard and wear out, he says. And, if any moisture should find its way into the system, it tends to collect in slugs. When elevated temperatures are encountered, especially at high altitudes, these can turn to bubbles resulting in a loss of stopping power.
The auto makers just don't consider silicone's potential advantages worth the risk. After all, a typical brand-name super heavy duty DOT 3 has a dry boiling point of 485 deg. F., which is only 15 deg. less than silicone (wet, this falls to 310 deg., but there's no excuse for running around with watered-down fluid, as I'll explain).
Proceed
Now for some service tips that'll help you avoid problems. The first thing I'd like to mention is that you should always open the bleeder before bottoming out those caliper pistons. Otherwise, you'll back flush nasty sediment up into the works. This is bad enough with a regular master, but with all the tiny passages in an ABS it can cause big trouble. Just do it.
Then there's flushing and refilling the system with fresh fluid. In the past, some brake experts said it wasn't worth the effort because you can't get all the old stuff out unless you disassemble the calipers and cylinders. True, you won't be able to eliminate every drop of the contaminated liquid, but you can get most of it, and that will effectively reduce the amount of moisture in the circuits.
This has always been important for corrosion prevention, but now the high operating temperatures encountered with semi-mets and FWD make maintaining a high boiling point critical to safety even for the average motorist. Besides water, there's sediment, which is a combination of rust and the ashy residue of burned glycol. Expensive and intricate ABS hardware is further justification for this maintenance. Many of these systems are vented to the atmosphere, and there's also contamination from under-hood vapors in some layouts. Fluid changes are cheap insurance against big-bucks repairs.
Depending on the authority, recommended intervals range from one to three years.
New pipes?
Whenever you've got a car up in the air, take a careful look at the metal lines and rubber hoses. In my own shop, I see total circuit failures due to corroded lines or blown hoses frequently enough to be very conscientious about this. If a line runs up over the chassis so that it's hard to see, use a mirror and your sense of touch.
Replace lines if the rust has reached the scaly stage. When installing replacements, follow the original routing as closely as possible. I often hear of fade or low pedal problems because one of these lines has been mounted too close to an exhaust pipe, or has a hump or loop in it that traps air.
Those flexible hoses are so well made they often survive for the life of the car, but why push your luck? I almost took an icy dip in Lake Michigan when one blew on my pre-dual master '66 Ford, so I consider their replacement valuable insurance. You could use some carmakers' service literature to support the idea of new hoses as regular maintenance -- the Ford Fiesta manual, for example, says hoses should be retired after 36K miles. A related item is the high pressure ABS hose between the pump and the accumulator, which some car makers want you to replace at 60,000 miles.
Bubble trouble
Air expulsion definitely deserves some space because there's more to it than just observing the proper sequence. Bench bleeding master cylinders, for instance. Neglecting this is the number-one reason for spongy pedal complaints, and some re-manufacturers include fittings, tubes, and instructions in the box in hopes of reducing the number of unnecessary returns.
You can do this job by just holding your fingers over the outlet tubes to keep air from being drawn in on the return stroke, but that's pretty messy, so I'll give you the procedure using tubes. Clamp one of the master's mounting ears in a vice so the unit is as level as possible. Position the tube tips well below the level of fluid in the reservoir, then use a rod or drift to stroke the piston SLOWLY. Especially on Quick Take-Ups and their equivalents, wait at least 15 seconds between strokes to allow the low pressure chamber to release all its bubbles and fill completely. Keep stroking until there's no more evidence of air at the tube tips and ports.
Should you get a car with a replacement cylinder that some other service personage didn't bench bleed, you might be able to do it with the master in place providing you can jack the rear of the vehicle high enough to get the cylinder level. Surge bleeding -- you know, where you pound the pedal violently a bunch of times to get the bubbles mixed up with the fluid, then crack the line -- is frowned upon by experts who don't think aeration is ever a good idea.
By the way, one major brake outfit (EIS) has done us a favor by providing aftermarket step-bore master cylinders with a bleeder that facilitates getting out all that trapped air. Located in the low-pressure side of the cylinder below the Quick Take-Up valve, it gives those bubbles a convenient exit path. This little feature can reduce bench bleeding time to maybe three minutes, and practically eliminates low-pedal comebacks.
When it comes to the bleeders at the wheels, I know most of you just open them and let the fluid squirt. But besides making slippery puddles on the floor, it can shoot farther than you might expect, perhaps ruining the paint on a nearby car. I use a transparent tube and bottle set-up that hangs by a magnet because it's neat and it lets me see what I'm getting out. Also, it eliminates the need for a helper if I'm not using a pressure bleeder.
Then there are the bleeder screws themselves. You haven't worked on cars very long if you haven't encountered a frozen one that breaks off before opening. My rather drastic and primitive method of clamping Vise-Grips to the screw and shaking it while I heat the cylinder or caliper around the port with a propane torch often works, but there's a better answer. Whenever you get a vehicle in that hasn't yet developed the problem, unscrew the bleeders and coat their threads with just a touch of anti-seize. Ditto when you replace a caliper or cylinder. If you get that conveyance back later for brake work, you'll be glad you took the time.
Caliper Care
In an ideal world, every caliper would be overhauled during a reline to insure against piston seizure and seal failure. And that's what most authorities recommend. But when was the last time you encountered a leaky caliper? They last and last in most cases. So, if you're under time or cost pressure, you could just push those pistons back (bleeder open, of course). One caveat: If you feel any roughness or binding as you force a piston home, you'd better get inside.
What about wheel cylinders? The frequency of leaks here is so high most of the good techs overhaul or replace them in every case.
Repair Guide: Disc/Drum Brakes
Disc brakes are used on the front wheels of most cars and on all four wheels on many cars. A disc rotor is attached to the wheel hub and rotates with the tire and wheel. When the driver applies the brakes, hydraulic pressure from the master cylinder is used to push friction linings against the rotor to stop it.
The rotor is usually made of cast iron. The hub may be manufactured as one piece with the rotor or in two parts. The rotor has a machined braking surface on each face. A splash shield, mounted to the steering knuckle, protects the rotor from road splash.
A rotor may be solid or ventilated. Operation of the master cylinder if there is a rear system failure. Ventilated designs have cooling fins cast between the braking surfaces. This construction considerably increases the cooling area of the rotor casting. Also, when the wheel is in motion, the rotation of these fan-type fins in the rotor provides increased air circulation and more efficient cooling of the brake. Disc brakes do not fade even after rapid, hard brake applications because of the rapid cooling of the rotor.
The hydraulic and friction components are housed in a caliper assembly. The caliper assembly straddles the outside diameter of the hub and rotor assembly. When the brakes are applied, the pressure of the pistons is exerted through the shoes in a 'clamping' action on the rotor. Because equal opposed hydraulic pressures are applied to both faces of the rotor throughout application, no distortion of the rotor occurs, regardless of the severity or duration of application. There are many variations of caliper designs, but they can all be grouped into two main categories: moving and stationary caliper. The caliper is fixed in one position on the stationary design. In the moving design, the caliper moves in relation to the rotor.
Most late-model cars use the moving caliper design. This design uses a single hydraulic piston and a caliper that can float or slide during application. Floating designs 'float' or move on pins or bolts. In sliding designs, the caliper slides sideways on machined surfaces. Both designs work in basically the same way.
The single-piston caliper assembly is constructed from a single casting that contains one large piston bore in the inboard section of the casting. Inboard refers to the side of the casting nearest the center line of the car when the caliper is mounted. A fluid inlet hole and bleeder valve hole are machined into the inboard section of the caliper and connect directly to the piston bore.
The caliper cylinder bore contains a piston and seal. The seal has a rectangular cross section. It is located in a groove that is machined in the cylinder bore. The seal fits around the outside diameter of the piston and provides a hydraulic seal between the piston and the cylinder wall. The rectangular seal provides automatic adjustment of clearance between the rotor and shoe and linings following each application. When the brakes are applied, the caliper seal is deflected by the hydraulic pressure and its inside diameter rides with the piston within the limits of its retention in the cylinder groove. When hydraulic pressure is released, the seal relaxes and returns to its original rectangular shape, retracting the piston into the cylinder enough to provide proper running clearance. As brake linings wear, piston travel tends to exceed the limit of deflection of the seal; the piston therefore slides in the seal to the precise extent necessary to compensate for lining wear.
The top of the piston bore is machined to accept a sealing dust boot. The piston in many calipers is steel, precision ground, and nickel chrome plated, giving it a very hard and durable surface. Some manufacturers are using a plastic piston. This is much lighter than steel and provides for a much lighter brake system. The plastic piston insulates well and prevents heat from transferring to the brake fluid. Each caliper contains two shoe and lining assemblies. They are constructed of a stamped metal shoe with the lining riveted or bonded to the shoe and are mounted in the caliper on either side of the rotor. One shoe and lining assembly is called the inboard lining because it fits nearest to the center line of the car. The other is called the outboard shoe and lining assembly.
The caliper is free to float on its two mounting pins or bolts. Typical mounting pins are shown in the exploded view of the floating caliper. Teflon sleeves in the caliper allow it to move easily on the pins. During application of the brakes, the fluid pressure behind the piston increases. Pressure is exerted equally against the bottom of the piston and the bottom of the cylinder bore. The pressure applied to the piston is transmitted to the inboard shoe and lining, forcing the lining against the inboard rotor surface. The pressure applied to the bottom of the cylinder bore forces the caliper to move on the mounting bolts toward the inboard side, or toward the car. Since the caliper is one piece, this movement causes the outboard section of the caliper to apply pressure against the back of the outboard shoe and lining assembly, forcing the lining against the outboard rotor surface. As the line pressure builds up, the shoe and lining assemblies are pressed against the rotor surfaces with increased force, bringing the car to a stop.
The application and release of the brake pressure actually causes a very slight movement of the piston and caliper. Upon release of the braking effort, the piston and caliper merely relax into a released position. In the released position, the shoes do not retract very far from the rotor surfaces.
As the brake lining wears, the piston moves out of the caliper bore and the caliper repositions itself on the mounting bolts an equal distance toward the car. This way, the caliper assembly maintains the inboard and outboard shoe and lining in the same relationship with the rotor surface throughout the full length of the lining.
Sliding calipers are made to slide back and forth on the steering knuckle support to which it is mounted. There is a V shaped surface, sometimes called a rail, on the caliper that matches a similar surface on the steering knuckle support. These two mating surfaces allow the caliper to slide in and out. The internal components of the caliper are the same as those previously described.
The stationary or fixed caliper has a hydraulic piston on each side of the rotor. Larger calipers may have two pistons on each side of the rotor. The inboard and outboard brake shoes are pushed against the rotor by their own pistons. The caliper is anchored solidly and does not move. The seals around the pistons work just like those already described. The main disadvantage of the stationary caliper is that it has more hydraulic components. This means they are more expensive and have more parts to wear out.
Question:
My brakes are squealing. Does that mean I need a brake job?
Answer:
Not necessarily. A certain amount of brake noise is considered "normal" these days because of the harder semi-metallic brake pads that are used in most front-wheel drive cars and minivans. This type of noise does not affect braking performance and does not indicate a brake problem. However, if the noise is objectionable, there are ways to eliminate it.
Brake squeal is caused by vibration between the brake pads, rotors and calipers. Pad noise can be lessened or eliminated by installing "noise suppression shims" (thin self-adhesive strips) on the backs of the pads, or applying "noise suppression compound" on the backs of the pads to dampen vibrations. Additional steps that can be taken to eliminate noise are to resurface the rotors and replace the pads.
Some brands of semi-metallic pads are inherently noisier than others because of the ingredients used in the manufacture of the friction material. Strange as it may sound (pardon the pun), cheaper pads are sometimes quieter than premium quality or original equipment pads. That's because the cheaper pads contain softer materials that do not wear as well. For that reason, they are not recommended. Premium quality pads should cause no noise problems when installed properly and will give you better brake performance and longer life.
Conditions that can contribute to a disc brake noise problem include glazed or worn rotors, too rough a finish on resurfaced rotors, loose brake pads, missing pad insulators, shims, springs or anti rattle clips, rusty or corroded caliper mounts, worn caliper mounts, and loose caliper mounting hardware. Drum noise may be due to loose or broken parts inside the drum.
Most experts recommend new caliper and drum hardware when the brakes are relined, a thorough inspection of the calipers and rotors for any wear or other conditions that might have an adverse affect on noise or brake performance, and resurfacing the rotors (and drums) if the surfaces are not smooth, flat and parallel.
If you hear metallic scraping noises, on the other hand, it usually means your brake linings are worn out and need to be replaced -- especially if your brake pedal feels low or if you've noticed any change in the way your vehicle brakes (it pulls to one side when braking, it requires more pedal effort, etc.).
Some brake pads have built-in "wear sensors" that produce a scraping or squealing noise when the pads become worn. In any event, noisy brakes should always be inspected to determine whether or not there's a problem. And don't delay! If the pads have worn down to the point where metal-to-metal contact is occurring, your vehicle may not be able to stop safely, and you may score the rotors or drums to the point where they have to be replaced.
Question:
How can I tell if a rotor or drum really needs to be replaced?
Answer:
A rotor must be replaced if it is at or below the minimum thickness specification or discard thickness stamped on the rotor (this same information can also be found in brake service manuals). Replacement is also necessary if a rotor cannot be resurfaced without exceeding the minimum thickness specification or the discard thickness specification. Replacement is also required if the rotor is cracked or damaged. Replacement may be recommended if a rotor has hard spots, is warped, or has been previously resurfaced for a warped condition.
A drum must be replaced if it is at or beyond the maximum inside diameter specification or discard diameter stamped on the drum. Replacement is also necessary if a drum cannot be resurfaced without exceeding the maximum diameter specification or discard diameter specification. Replacement is also required if a drum is cracked, damaged, bell mouthed or too far out of round for resurfacing.
Question:
Is it always necessary to resurface the rotors and drums when the brakes are relined or to rebuild or replace the disc brake calipers and drum brake wheel cylinders?
Answer:
No. The rule here is resurface when necessary, don't resurface when it isn't necessary. If the rotors and drums are in relatively good condition (smooth and flat with no deep scoring, cracks, distortion or other damage), they do not have to be resurfaced. Resurfacing unnecessarily reduces the thickness of these parts, which in turn shortens their remaining service life.
According to the uniform inspection guidelines developed by the Motorist Assurance Program (MAP), "friction material replacement alone does not warrant rotor reconditioning." Whether or not the rotors or drums need resurfacing or replacing depends entirely on their condition at the time the brakes are relined.
Even so, many mechanics prefer to resurface rotors and drums when relining the brakes to restore the friction surfaces to "like-new" condition and to minimize any chance of brake squeal.
The rotor is usually made of cast iron. The hub may be manufactured as one piece with the rotor or in two parts. The rotor has a machined braking surface on each face. A splash shield, mounted to the steering knuckle, protects the rotor from road splash.
A rotor may be solid or ventilated. Operation of the master cylinder if there is a rear system failure. Ventilated designs have cooling fins cast between the braking surfaces. This construction considerably increases the cooling area of the rotor casting. Also, when the wheel is in motion, the rotation of these fan-type fins in the rotor provides increased air circulation and more efficient cooling of the brake. Disc brakes do not fade even after rapid, hard brake applications because of the rapid cooling of the rotor.
The hydraulic and friction components are housed in a caliper assembly. The caliper assembly straddles the outside diameter of the hub and rotor assembly. When the brakes are applied, the pressure of the pistons is exerted through the shoes in a 'clamping' action on the rotor. Because equal opposed hydraulic pressures are applied to both faces of the rotor throughout application, no distortion of the rotor occurs, regardless of the severity or duration of application. There are many variations of caliper designs, but they can all be grouped into two main categories: moving and stationary caliper. The caliper is fixed in one position on the stationary design. In the moving design, the caliper moves in relation to the rotor.
Most late-model cars use the moving caliper design. This design uses a single hydraulic piston and a caliper that can float or slide during application. Floating designs 'float' or move on pins or bolts. In sliding designs, the caliper slides sideways on machined surfaces. Both designs work in basically the same way.
The single-piston caliper assembly is constructed from a single casting that contains one large piston bore in the inboard section of the casting. Inboard refers to the side of the casting nearest the center line of the car when the caliper is mounted. A fluid inlet hole and bleeder valve hole are machined into the inboard section of the caliper and connect directly to the piston bore.
The caliper cylinder bore contains a piston and seal. The seal has a rectangular cross section. It is located in a groove that is machined in the cylinder bore. The seal fits around the outside diameter of the piston and provides a hydraulic seal between the piston and the cylinder wall. The rectangular seal provides automatic adjustment of clearance between the rotor and shoe and linings following each application. When the brakes are applied, the caliper seal is deflected by the hydraulic pressure and its inside diameter rides with the piston within the limits of its retention in the cylinder groove. When hydraulic pressure is released, the seal relaxes and returns to its original rectangular shape, retracting the piston into the cylinder enough to provide proper running clearance. As brake linings wear, piston travel tends to exceed the limit of deflection of the seal; the piston therefore slides in the seal to the precise extent necessary to compensate for lining wear.
The top of the piston bore is machined to accept a sealing dust boot. The piston in many calipers is steel, precision ground, and nickel chrome plated, giving it a very hard and durable surface. Some manufacturers are using a plastic piston. This is much lighter than steel and provides for a much lighter brake system. The plastic piston insulates well and prevents heat from transferring to the brake fluid. Each caliper contains two shoe and lining assemblies. They are constructed of a stamped metal shoe with the lining riveted or bonded to the shoe and are mounted in the caliper on either side of the rotor. One shoe and lining assembly is called the inboard lining because it fits nearest to the center line of the car. The other is called the outboard shoe and lining assembly.
The caliper is free to float on its two mounting pins or bolts. Typical mounting pins are shown in the exploded view of the floating caliper. Teflon sleeves in the caliper allow it to move easily on the pins. During application of the brakes, the fluid pressure behind the piston increases. Pressure is exerted equally against the bottom of the piston and the bottom of the cylinder bore. The pressure applied to the piston is transmitted to the inboard shoe and lining, forcing the lining against the inboard rotor surface. The pressure applied to the bottom of the cylinder bore forces the caliper to move on the mounting bolts toward the inboard side, or toward the car. Since the caliper is one piece, this movement causes the outboard section of the caliper to apply pressure against the back of the outboard shoe and lining assembly, forcing the lining against the outboard rotor surface. As the line pressure builds up, the shoe and lining assemblies are pressed against the rotor surfaces with increased force, bringing the car to a stop.
The application and release of the brake pressure actually causes a very slight movement of the piston and caliper. Upon release of the braking effort, the piston and caliper merely relax into a released position. In the released position, the shoes do not retract very far from the rotor surfaces.
As the brake lining wears, the piston moves out of the caliper bore and the caliper repositions itself on the mounting bolts an equal distance toward the car. This way, the caliper assembly maintains the inboard and outboard shoe and lining in the same relationship with the rotor surface throughout the full length of the lining.
Sliding calipers are made to slide back and forth on the steering knuckle support to which it is mounted. There is a V shaped surface, sometimes called a rail, on the caliper that matches a similar surface on the steering knuckle support. These two mating surfaces allow the caliper to slide in and out. The internal components of the caliper are the same as those previously described.
The stationary or fixed caliper has a hydraulic piston on each side of the rotor. Larger calipers may have two pistons on each side of the rotor. The inboard and outboard brake shoes are pushed against the rotor by their own pistons. The caliper is anchored solidly and does not move. The seals around the pistons work just like those already described. The main disadvantage of the stationary caliper is that it has more hydraulic components. This means they are more expensive and have more parts to wear out.
Question:
My brakes are squealing. Does that mean I need a brake job?
Answer:
Not necessarily. A certain amount of brake noise is considered "normal" these days because of the harder semi-metallic brake pads that are used in most front-wheel drive cars and minivans. This type of noise does not affect braking performance and does not indicate a brake problem. However, if the noise is objectionable, there are ways to eliminate it.
Brake squeal is caused by vibration between the brake pads, rotors and calipers. Pad noise can be lessened or eliminated by installing "noise suppression shims" (thin self-adhesive strips) on the backs of the pads, or applying "noise suppression compound" on the backs of the pads to dampen vibrations. Additional steps that can be taken to eliminate noise are to resurface the rotors and replace the pads.
Some brands of semi-metallic pads are inherently noisier than others because of the ingredients used in the manufacture of the friction material. Strange as it may sound (pardon the pun), cheaper pads are sometimes quieter than premium quality or original equipment pads. That's because the cheaper pads contain softer materials that do not wear as well. For that reason, they are not recommended. Premium quality pads should cause no noise problems when installed properly and will give you better brake performance and longer life.
Conditions that can contribute to a disc brake noise problem include glazed or worn rotors, too rough a finish on resurfaced rotors, loose brake pads, missing pad insulators, shims, springs or anti rattle clips, rusty or corroded caliper mounts, worn caliper mounts, and loose caliper mounting hardware. Drum noise may be due to loose or broken parts inside the drum.
Most experts recommend new caliper and drum hardware when the brakes are relined, a thorough inspection of the calipers and rotors for any wear or other conditions that might have an adverse affect on noise or brake performance, and resurfacing the rotors (and drums) if the surfaces are not smooth, flat and parallel.
If you hear metallic scraping noises, on the other hand, it usually means your brake linings are worn out and need to be replaced -- especially if your brake pedal feels low or if you've noticed any change in the way your vehicle brakes (it pulls to one side when braking, it requires more pedal effort, etc.).
Some brake pads have built-in "wear sensors" that produce a scraping or squealing noise when the pads become worn. In any event, noisy brakes should always be inspected to determine whether or not there's a problem. And don't delay! If the pads have worn down to the point where metal-to-metal contact is occurring, your vehicle may not be able to stop safely, and you may score the rotors or drums to the point where they have to be replaced.
Question:
How can I tell if a rotor or drum really needs to be replaced?
Answer:
A rotor must be replaced if it is at or below the minimum thickness specification or discard thickness stamped on the rotor (this same information can also be found in brake service manuals). Replacement is also necessary if a rotor cannot be resurfaced without exceeding the minimum thickness specification or the discard thickness specification. Replacement is also required if the rotor is cracked or damaged. Replacement may be recommended if a rotor has hard spots, is warped, or has been previously resurfaced for a warped condition.
A drum must be replaced if it is at or beyond the maximum inside diameter specification or discard diameter stamped on the drum. Replacement is also necessary if a drum cannot be resurfaced without exceeding the maximum diameter specification or discard diameter specification. Replacement is also required if a drum is cracked, damaged, bell mouthed or too far out of round for resurfacing.
Question:
Is it always necessary to resurface the rotors and drums when the brakes are relined or to rebuild or replace the disc brake calipers and drum brake wheel cylinders?
Answer:
No. The rule here is resurface when necessary, don't resurface when it isn't necessary. If the rotors and drums are in relatively good condition (smooth and flat with no deep scoring, cracks, distortion or other damage), they do not have to be resurfaced. Resurfacing unnecessarily reduces the thickness of these parts, which in turn shortens their remaining service life.
According to the uniform inspection guidelines developed by the Motorist Assurance Program (MAP), "friction material replacement alone does not warrant rotor reconditioning." Whether or not the rotors or drums need resurfacing or replacing depends entirely on their condition at the time the brakes are relined.
Even so, many mechanics prefer to resurface rotors and drums when relining the brakes to restore the friction surfaces to "like-new" condition and to minimize any chance of brake squeal.
Repair Guide: Brake System Basics
Braking action begins when the driver pushes on the brake pedal. The brake pedal is a lever, pivoted at one end, with the master cylinder push rod attached to the pedal near the pivot point. With this lever arrangement, the force applied to the master cylinder piston through the push rod is multiplied several times over the force applied at the brake pedal.
The bracket is mounted to the inside of the engine compartment cowl or firewall. The master cylinder push rod that connects the pedal linkage to the master cylinder goes through a hole in the firewall.
The master cylinder is mounted on the opposite side of the firewall in the engine compartment. If the car has manual brakes, the cylinder will be mounted directly to the firewall. If a power booster is used, it will be mounted to the firewall and the cylinder is mounted to the booster.
Question:
How do I know when my car really needs a brake job?
Answer:
You need a "brake job" when your brake linings are worn down to the minimum acceptable thickness specified by the vehicle manufacturer or the applicable state agency in areas that set their own requirements. The only way to determine if new linings are required, therefore, is to inspect the brakes.
You may also need a brake job if you're having brake problems such as grabbing, pulling, low or soft pedal, pedal vibration, noise, etc., or if some component in your brake system has failed. But if the problem is isolated to only one component, there's no need to replace other parts that are still in perfectly good working order.
There is no specific mileage interval at which the brakes need to be relined because brake wear varies depending on how the vehicle is driven, the braking habits of the driver, the weight of the vehicle, the design of the brake system and a dozen other variables. A set of brake linings that last 70,000 miles or more on a car driven mostly on the highway may last only 30,000 or 40,000 miles on the same vehicle that is driven mostly in stop-and-go city traffic.
As a rule, the front brakes wear out before the ones on the rear because the front brakes handle a higher percentage of the braking load -- especially in front-wheel drive cars and minivans. So many service facilities advertise $59.95 brake job "specials" that replace the linings on the front brakes only. Doing the front brakes only is okay and can save you money as long as the rear brakes are in good condition. But if the rear brakes need attention, they should be relined too.
One of the problems with the brake specials you see advertised in the newspaper is that the price is very misleading. A person typically goes in expecting to spend $59.95 for a brake job, but usually ends up spending considerably more because the brakes need more than the minimum amount of work to restore them to like-new condition. The price of a brake job depends entirely on the work that needs to be performed. So any advertised special is not a firm price, but only an estimate of the least amount of money it might cost you to get your brakes fixed. A price should not be quoted until after the brakes have been inspected. Then and only then can an accurate determination be made of the parts that actually need to be replaced.
Question:
What parts are generally replaced during a brake job, and why?
Answer:
A traditional brake job (if there is such a thing) usually means replacing the front disc brake pads, resurfacing the rotors, replacing the rear drum brake shoes, resurfacing the drums, bleeding the brake lines (replacing the old brake fluid with new and getting all the air out of the lines), inspecting the system for leaks or other problems that might require additional repairs, and checking and adjusting the parking brake.
Some brake jobs may also include new hardware for the drums (recommended), and rebuilding or replacing the wheel cylinders and calipers (also recommended). But because of the added expense, these items may not be included in the package price or may only be done if the brake system really needs them (as opposed to doing them for preventative maintenance).
Hardware includes things like return springs, hold down springs and other clips and retainers found in drum brakes. It may also include bushings, pins and clips on disc brake calipers. Springs lose tension with age and exposure to heat. Most experts recommend replacing the hardware when relining drum brakes to restore proper brake action. If weak springs are reused, the shoes may drag against the drums causing accelerated shoe wear, a pull to one side, brake overheating and possible drum damage. Other hardware that is badly corroded or faulty (such as the self-adjusters) may prevent the shoes from maintaining the correct drum clearance (which increases the distance the brake pedal must travel as the shoes wear), or the parking brake from functioning properly.
It's important to note that not all replacement linings are the same. There are usually several grades of quality in pads and shoes (good, better and best). The difference is in the ingredients that are used to manufacture the pads and shoes. The less expensive ones may cost less initially and save you a few dollars on your total bill, but you may not be happy with the way they wear and perform. All brake linings must meet minimum government safety standards. Even so, the cheaper grade of pads and shoes do not last as many miles as the premium grade of replacement linings, nor do they brake as effectively. They usually have a greater tendency to fade at high temperature and may increase the vehicle's stopping distance somewhat. Noise may also be a problem with cheap linings. The best performance and value for your money, therefore, is with the best or premium grade. Choose these when the brakes are relined.
The bracket is mounted to the inside of the engine compartment cowl or firewall. The master cylinder push rod that connects the pedal linkage to the master cylinder goes through a hole in the firewall.
The master cylinder is mounted on the opposite side of the firewall in the engine compartment. If the car has manual brakes, the cylinder will be mounted directly to the firewall. If a power booster is used, it will be mounted to the firewall and the cylinder is mounted to the booster.
Question:
How do I know when my car really needs a brake job?
Answer:
You need a "brake job" when your brake linings are worn down to the minimum acceptable thickness specified by the vehicle manufacturer or the applicable state agency in areas that set their own requirements. The only way to determine if new linings are required, therefore, is to inspect the brakes.
You may also need a brake job if you're having brake problems such as grabbing, pulling, low or soft pedal, pedal vibration, noise, etc., or if some component in your brake system has failed. But if the problem is isolated to only one component, there's no need to replace other parts that are still in perfectly good working order.
There is no specific mileage interval at which the brakes need to be relined because brake wear varies depending on how the vehicle is driven, the braking habits of the driver, the weight of the vehicle, the design of the brake system and a dozen other variables. A set of brake linings that last 70,000 miles or more on a car driven mostly on the highway may last only 30,000 or 40,000 miles on the same vehicle that is driven mostly in stop-and-go city traffic.
As a rule, the front brakes wear out before the ones on the rear because the front brakes handle a higher percentage of the braking load -- especially in front-wheel drive cars and minivans. So many service facilities advertise $59.95 brake job "specials" that replace the linings on the front brakes only. Doing the front brakes only is okay and can save you money as long as the rear brakes are in good condition. But if the rear brakes need attention, they should be relined too.
One of the problems with the brake specials you see advertised in the newspaper is that the price is very misleading. A person typically goes in expecting to spend $59.95 for a brake job, but usually ends up spending considerably more because the brakes need more than the minimum amount of work to restore them to like-new condition. The price of a brake job depends entirely on the work that needs to be performed. So any advertised special is not a firm price, but only an estimate of the least amount of money it might cost you to get your brakes fixed. A price should not be quoted until after the brakes have been inspected. Then and only then can an accurate determination be made of the parts that actually need to be replaced.
Question:
What parts are generally replaced during a brake job, and why?
Answer:
A traditional brake job (if there is such a thing) usually means replacing the front disc brake pads, resurfacing the rotors, replacing the rear drum brake shoes, resurfacing the drums, bleeding the brake lines (replacing the old brake fluid with new and getting all the air out of the lines), inspecting the system for leaks or other problems that might require additional repairs, and checking and adjusting the parking brake.
Some brake jobs may also include new hardware for the drums (recommended), and rebuilding or replacing the wheel cylinders and calipers (also recommended). But because of the added expense, these items may not be included in the package price or may only be done if the brake system really needs them (as opposed to doing them for preventative maintenance).
Hardware includes things like return springs, hold down springs and other clips and retainers found in drum brakes. It may also include bushings, pins and clips on disc brake calipers. Springs lose tension with age and exposure to heat. Most experts recommend replacing the hardware when relining drum brakes to restore proper brake action. If weak springs are reused, the shoes may drag against the drums causing accelerated shoe wear, a pull to one side, brake overheating and possible drum damage. Other hardware that is badly corroded or faulty (such as the self-adjusters) may prevent the shoes from maintaining the correct drum clearance (which increases the distance the brake pedal must travel as the shoes wear), or the parking brake from functioning properly.
It's important to note that not all replacement linings are the same. There are usually several grades of quality in pads and shoes (good, better and best). The difference is in the ingredients that are used to manufacture the pads and shoes. The less expensive ones may cost less initially and save you a few dollars on your total bill, but you may not be happy with the way they wear and perform. All brake linings must meet minimum government safety standards. Even so, the cheaper grade of pads and shoes do not last as many miles as the premium grade of replacement linings, nor do they brake as effectively. They usually have a greater tendency to fade at high temperature and may increase the vehicle's stopping distance somewhat. Noise may also be a problem with cheap linings. The best performance and value for your money, therefore, is with the best or premium grade. Choose these when the brakes are relined.
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