Wednesday, January 30, 2008

Under hood Maintenance: Drive Belts

Inspect and Adjust Engine Accessory Drive Belts

Many engine accessories--including the alternator, fan, and coolant pump--are operated by drive belts. If these belts break or slip, the components they drive will fail to work. The belt that drives the fan also drives the coolant pump. If it breaks, coolant and air circulation stop, and the engine overheats at once.

Drive belts should be inspected for a potential problem anytime you have the hood of a customer's car up. A quick inspection can locate a problem and save your customer a major problem.

CAUTION: Never try to adjust or inspect belts with the engine running. Make sure no one is in the car who could start the engine during belt inspection.

To inspect the belts, grab the belt in your hand and twist it so you can see the underside of the "V" shape on V-type belts, or the ribs on a serpentine-type belt. Use a trouble or flashlight so you can make a close inspection. Cracks indicate the belt is getting ready to fail. Oil-soaked belts can slip and not rotate the component they are driving fast enough. Glazed belts have a shiny appearance; this occurs when a belt is not tight enough and the slipping polishes its surface. Torn or split belts have major damage and must be replaced immediately.

Before adjusting any drive belt, always check the service manual for specific instructions. Find the longest span in the belt and push on it as shown below. It should move in about half an inch per foot of free span. If it moves more than this, the belt is too loose. If it moves less, it is too tight.

A belt tension gauge can also be used for testing belt tension. These gauges are operated differently, so follow the instructions on the tool. Basically, you attach the gauge to the longest span of the belt. Then you pull on the belt and measure the tension. Specifications are available in the shop service manual to compare against your reading.

Most belts are adjusted by loosening the support for the alternator and moving it back and forth to tighten or loosen the belt. Other systems use an idler pulley for the adjustment. First loosen the adjustment fastener on the slotted alternator support. Wedge a prybar between a strong part of the engine and the alternator. Pull on the pry bar to move the alternator housing in a direction to tighten the belt. Tighten the adjustment fastener. Recheck the adjustment by measuring the belt as explained earlier.

Remove and Replace an Accessory Drive Belt

When you have determined that a drive belt is defective and needs to be replaced, you should have the replacement belt on hand. Loosen the adjustment fastener on the alternator or idler pulley. Push the alternator or idler pulley inward to loosen the belt. Pull the old drive belt off the pulleys.

Place the new and old belt side by side on the work bench to make a comparison. The belt width and length of the new belt must be the same. If you find a difference, check to see that you have not gotten the wrong belt. A belt that is too long to be adjusted properly will slip. A belt that is too short will not fit over the pulleys. A belt with the incorrect width or V shape could be thrown off when the engine is running.

Install the correct belt over the pulleys. Adjust the belt to the proper tension as described previously. Start the engine and observe the belt in operation. Stop the engine and recheck the tension.

SERVICE TIP: There is an old trick tow truck drivers use when responding to cars that are disabled because of a broken drive belt. They carry packages of women's pantyhose. They wind them around the pulleys and then tie them in a knot. The pantyhose will work as a belt for a short distance to get the car to a service facility.


How do I know if a V-belt needs to be replaced?

One way to find out is to examine the belt. If a V-belt is full of tiny cracks, frayed, has pieces of rubber missing, is peeling or otherwise damaged, it needs to be replaced -- NOW. Also, if a belt is oil soaked or "glazed" (hard shiny appearance on the sides of the belt) it also needs to be replaced. Either of these two conditions can cause the belt to run hot, which can weaken it and increase the danger of it breaking.
Unfortunately, a visual inspection alone isn't a sure-fire method of determining the true condition of a belt because internal wear that you can't see is just as important as external wear that you can see. All belts are reinforced with cords. The cords are what give the belt its strength and keep it from stretching or breaking. But as a belt ages, the constant flexing, heat and strain weakens the cords. Eventually the cords reach a point where failure can happen suddenly and unexpectedly. The belt may still look good as new on the outside, but be on the verge of snapping internally because the cords have lost their strength.

So the other factors that need to be considered when judging the condition of a V-belt include the belt's mileage and age. A V-belt that's more than three or four years old and has more than 40,000 or 50,000 miles on it may be a belt that is nearing the end of its useful service life. For this reason, you might be well advised to replace a high mileage belt even if it still looks okay.


Is it necessary to replace belts periodically?

Yes. Although the auto makers don't usually specify a replacement interval for V-belts or serpentine (flat, multi-ribbed) belts, most belt manufacturers do recommend periodic replacement for preventative maintenance. Here's why: the incidence of belt failure rises sharply in the fourth year of service for the typical V-belt, and the fifth year for serpentine belts.
What's more, eight out of ten V-belt failures and ten out of ten serpentine belt failures end up causing a breakdown! That's because belts have the uncanny knack of always picking the worst possible moment to fail -- like when you're heading out of town on that long-awaited fishing trip, when you're hurrying to pick up a hot date who told you NOT to be late, or when you're giving your dear mother-in-law a ride to church.

A broken belt is always bad news because when it snaps, all drive power to whatever it turns is lost. That means the water pump quits circulating coolant through the engine, the alternator quits producing amps, the power steering pump ceases to assist steering, and the air conditioner quits cooling. Many newer vehicles have a single serpentine belt that drives all of the engine's accessories, so when it fails everything stops working.

The good news is that replacing the belts periodically can go a long way towards minimizing the risk of a breakdown caused by belt failure. After all, it's a lot easier to replace a belt at your convenience than having the belt fail unexpectedly Heavens knows where.

For optimum protection, most experts recommend replacing V-belts every three to four years, or every 36,000 to 48,000 miles. A recommended replacement interval for serpentine belts would be every four or five years, or 50,000 miles.


The service life of a V-belt depends on mileage as well as load, tension and heat. Every time a belt passes around a pulley, it bends and flexes. This produces heat which age hardens the rubber over time. The wear process can be greatly accelerated if the belt is loose and slips because any added friction between belt and pulley makes the belt run even hotter. This can cause glazing on the faces of the belt and cause it to slip even more. So one of the most important factors that affects belt life is making sure it is properly tensioned when it is installed and that the proper tension is maintained throughout its service life.

Symptoms that may be the result of improper belt tension include:

  • Belt squeal, especially on the fan, A/C compressor or power steering drives.
    A battery that keeps running down (due to belt slippage).
  • Excessive sidewall wear on a V-belt that causes it to ride lower than normal in the pulley grooves.
  • Severe cracking along the underside of a V-belt.
  • Noisy alternator, power steering pump, air pump, A/C compressor or water pump bearings (from excessive belt tension).


Replacement V-belts must be the same length and width as the original. A belt that's too long or too short may not allow enough adjustment for proper tension. A belt that's too wide or too narrow will not ride at the right depth in the pulley grooves.

CAUTION: When installing a new belt, do not attempt to "stretch" it over pulleys. Doing so can break the internal cords causing the belt to fail. Always loosen the pulleys so there is adequate clearance to slip the belt over the pulleys.

Once the belt has been installed on the pulleys, a belt gauge should be used to adjust belt tension to factory specifications. The old rule of thumb of allowing 1/2 inch of "give" between the furthest pulleys is not a very accurate guide for today's engines. So follow the manufacturer's recommendations for belt tension.

Once tension has been adjusted, it should be rechecked and readjusted (if necessary) after a short break-in period (say after 500 to 1,000 miles of driving). It should then be checked twice a year or every 5,000 or 6,000 miles thereafter.

On vehicles with a single serpentine belt, tension is usually self-adjusted automatically via a spring loaded tensioner. No additional adjustment is necessary.

If your engine has been eating or twisting belts, misaligned pulleys may be your problem. Alignment can be checked with a straightedge. If a pulley is bent or not in the same plane as the rest, the problem should be corrected otherwise the "bad" pulley will continue to ruin belts.

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