Thursday, January 31, 2008

Repair Guide: ABS, Antilock

If the brakes are applied too hard when driving on slippery road surfaces, they may lock up or stop the wheel. The wheel then loses frictional contact with the road and skids and the vehicle is no longer under control. Experienced drivers know that the way to prevent lock-up is to pump the brake pedal up and down rapidly.

Many late-model cars are now equipped with an antilock brake system (ABS). The antilock brake system does the same thing as an experienced driver. It senses that a wheel is about to lock-up or skid and it rapidly interrupts the braking pressure to the brake system at that wheel.

The brains behind the antilock brake system is the computer, which monitors system operation at all times. It processes information from the wheel sensors and determines wheel speed. From this information, the electronic controller can determine whether one wheel is turning slower than the other wheels.

The computer gets its information on wheel speeds from wheel sensors located on each wheel. Each sensor assembly consists of a magnetic pickup sensor and a toothed sensor ring. The front sensor rings are attached to the back side of the rotor assembly. The rear sensor rings are attached to the axle shaft. The pickup assemblies are bolted to brackets at each wheel.

The wheel sensors are essentially magnetic pickup assemblies. Each pickup assembly consists of a permanent magnet with a coil of wire wound around it. The sensor is positioned extremely close to the sensor ring, which rotates as the wheel turns. As the teeth pass the pickup assembly, the signal is induced in the coil by electromagnetic induction as the magnetic field goes from strong to weak and back to strong. This signal change is used by the computer to determine wheel speed.

The antilock brake system uses a hydraulic control unit in place of the standard master cylinder. The hydraulic control unit consists of a master cylinder, a vacuum or hydraulic booster, electric pump, accumulator, a solenoid valve body assembly, and pressure control and warning switches.

The electric pump is a high pressure pump designed to run at frequent intervals for short periods of time. The pump fills the hydraulic accumulator and supplies high pressure brake fluid to the brake system.

The accumulator is a nitrogen gas-filled assembly used to store and supply pressure to the brake system. The accumulator is attached to the pump housing. The top chamber of the accumulator is filled with nitrogen gas. The bottom chamber contains brake fluid, which is supplied from the hydraulic electric pump. A diaphragm is used to separate the two chambers.

CAUTION: Do not disassemble any accumulator. The nitrogen gas contained in the accumulator is pressurized to 1,200 psi (8,274 kPa). Antilock brakes use extremely high pressure, so always follow service manual procedures when working on one of these systems.

During operation, the electric pump supplies brake fluid to the lower chamber of the accumulator, and the diaphragm moves upward, compressing the nitrogen gas in the upper chamber. The nitrogen gas, which is under pressure in the top chamber, then pushes down on the diaphragm, causing the brake fluid in the bottom chamber to be maintained at a very high pressure. During normal braking conditions (no antilock control), the accumulator supplies pressurized brake fluid to the booster and the rear brakes. During antilock braking conditions, the accumulator also supplies pressurized brake fluid to the front brakes. The accumulator can provide pressure required for a number of stops if the electric pump should fail.

The solenoid valve assembly is a set of electrically operated solenoid switching valves. The main solenoid valve opens a connection between the boost pressure chamber of the brake power booster and the internal master cylinder reservoir and closes the flow to the reservoir during antilock control. This provides a continuous supply of high pressure brake fluid during antilock control to replace the fluid being allowed back to the reservoir. When antilock control stops, the main valve closes and the return to the reservoir is reopened. By closing the main valve, accumulator pressure is removed from the front brake circuits within the master cylinder.

A set of smaller solenoid valves is located in a solenoid valve body. The valve body contains three pairs of electrically operated solenoid valves: a pair for each of the front brakes and a pair that controls both back brakes together. Each pair contains a normally open inlet solenoid valve and a normally closed outlet solenoid valve. During normal braking conditions (no antilock control), brake pressure is supplied to the brakes through the inlet solenoid valves upon brake application.

The computer determines the rotation speed of each wheel. If it senses a possible wheel lock-up, it goes into the antilock function and then applies voltage to the appropriate solenoid valves. When the system goes into antilock control, the computer will open and close the appropriate inlet and outlet solenoid valves, which control the operation of any of the brakes on any of the four wheels and prevent wheel lock-up. When the system is in antilock brake operation, the brake pedal will pulsate at an extremely fast rate. Pressure control and warning switches warn the driver of any malfunction in the system.


I feel a pulsation or vibration in my brake pedal every time I stop. But the brakes seem to work fine. Is anything wrong?

A pulsating brake pedal, which may be accompanied by a shuddering or jerky stop during normal braking, usually means a warped rotor or an out-of-round drum -- although it can sometimes be caused by loose wheel bearings, a bent axle shaft or loose brake parts. If the vehicle is equipped with ABS, however, some pedal feedback and noise is normal during panic stops or when braking on wet or slick surfaces. But you should not experience any ABS pedal feedback when braking normally on dry pavement.
The faces of a disc brake rotor must be parallel (within .0005 inch on most cars) and flat (no more than about .002 to .005 inches of runout) otherwise it will kick the brake pads in and out when the brakes are applied, producing a pulsation or vibration that can be felt in the brake pedal as the rotor alternately grabs and slips.

You can often see warpage in a brake rotor by simply looking at it. If the rotor has telltale glazed or discolored patches on its face, chances are it is warped. Measuring it with a dial indicator and checking it for flatness with a straight edge will confirm the diagnosis.

Resurfacing the rotor to restore the faces will usually eliminate the pulsation (unless the rotor is bent or is badly worn and has started to collapse in which case the rotor must be replaced). But it may only do so temporarily because of metallurgical changes that take place in the rotor. Hard spots often extend below the surface of the rotor. Resurfacing will restore the surface, but the hard spot may reappear again in a few thousand miles as the rotor wears. For this reason, GM and others recommend replacing warped rotors rather than resurfacing them.

Pedal pulsation caused by drum warpage isn't as common, but it can happen. A drum can sometimes be warped out-of-round by applying the parking brake when the brakes are hot. As the drum cools, the force of the shoes causes the drum to distort.

What causes a rotor to warp? Overtorquing or unevenly torquing the lug nuts with an impact wrench is a common cause. For this reason, most experts recommend using a torque wrench to tighten lug nuts when changing a wheel. There are also special torque-limiting extension sockets called "Torque Sticks" that can be safely used with an impact wrench to accurately tighten lug nuts. But a plain impact wrench should never be used for the final tightening of the lug nuts because most provide no control whatsoever over the amount of torque applied to the nuts.

Overheating can also cause rotors to warp. Overheating may be the result of severe abuse or dragging brakes. Defects in the rotor casting, such as thick and thin areas can also cause uneven cooling that leads to warpage. Hard spots in the metal due to casting impurities can be yet another cause.


I've heard that using a temporary spare will disable my ABS system. Is that true?

Yes. On many cars equipped with ABS and a temporary spare, an electrical connector inside the trunk or luggage compartment is attached to the valve stem on the spare tire. If you have a flat tire and remove the spare, you have to unplug the connector. This tells the ABS system that the temporary spare is being used, causing it to temporarily disable itself.
The reason why the ABS system shuts itself off when the temporary spare is in use is because the spare tire has a smaller diameter and narrower tread than a standard tire. This causes the tire to rotate somewhat faster than the other tires, and to brake differently. Were the ABS system not disabled, the difference in wheel speed and braking friction caused by the temporary spare would probably cause the ABS system to kick in unnecessarily when the brakes are applied. So disabling the ABS system prevents this from happening.

The important thing to remember here is to reconnect the ABS connector to the spare tire when it is returned to the trunk. If this is not done, the ABS system will remain disabled and unable to prevent skidding when braking on wet or slick surfaces.


When I replace the tires on my vehicle, do I have to use the same size as the originals?

On ABS-equipped vehicles, all vehicle manufacturers recommend using the same size and aspect ratio tire as the original. ABS systems monitor the rotational speed of the tires through individual wheel speed sensors. Changing to an oversize tire with a taller diameter than stock would cause the tires to rotate at a slightly slower speed relative to vehicle speed than the stock tires. Changing to a low profile tire with a shorter diameter would cause the tires to rotate at a slightly faster speed relative to vehicle speed. Though the difference either way isn't much, it may be enough to upset the calibration of the ABS system and have an adverse effect on its ability to detect and prevent skids.
Another reason for not changing tire sizes is because it can affect the speedometer, odometer and transmission shift points on a vehicle with an electronic automatic. Oversize tires will make your speedometer read slower than normal (which may get you a speeding ticket unless you have the speedometer recalibrated to compensate for the change in tire size!). Smaller diameter tires will make the speedometer read faster than normal, and increase the mileage readings on your odometer at a faster than normal rate.

All this doesn't mean you can't change tire and wheel sizes, however. If you maintain the same overall tire diameter as before, you can switch to larger wheels with a shorter aspect ratio tire. This is the basic idea behind "Plus 1, Plus 2" tire and wheel sizing.

Replacing a stock 14-inch wheel and 70 series tire with a 15-inch 60 series tire would be Plus 1. Plus 2 would be moving up to a 16-inch wheel and possibly a 50 series tire. Plus 3 would be going to the new 17-inch tire and rim combination -- which could turn out to be a Plus 4 application if the vehicle originally had 13-inch wheels.


The "aspect ratio" of a tire is the ratio of its section height to its section width. The smaller the number, the shorter the sidewall and the wider the tire. Low aspect ratio tires started with 60 series some time ago, then progressed to 50 series and now 45, 40 and even 35 series tires.

Shorter aspect ratio tires (60 and less) are usually considered to be performance tires because they lower vehicle ride height, have a wider tread and put more rubber on the road to improve handling. But the shorter the sidewall, the harsher the tire rides.

A tire's ability to support a given load depends on its air volume. If you go to a lower aspect ratio tire with a shorter sidewall, the tire must be wider to maintain the same air volume. If you just go to a shorter aspect ratio tire without increasing width, the load carrying capacity goes down. That's why when you go from a standard wheel to a Plus 1 wheel, the rim is usually wider to accommodate a wider tire.

It's important to follow the tire manufacturer's recommendations as to load capacities when going to larger wheel and tire sizes. There's no hard rule that says you have to drop 10 points in aspect ratio when increasing wheel size one inch, but that's the general recommendation.

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