Accessory drive belts: Belt noise means more than just worn-out parts

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Accessory drive belts: Belt noise means more than just worn-out parts

Jacques Gordon has worked in the automotive industry for 40 years as a service technician, lab technician, trainer and technical writer. He began his writing career writing service manuals at Chilton Book Co. He currently holds ASE Master Technician and L1 certifications and has participated in ASE test writing workshops.

Summertime. Even after a short drive, it can get hot enough under the hood to bake biscuits. That heat soaks into everything, and it can be particularly hard on belts and hoses. Of course you already know that, but have you thought about how a worn or damaged belt can affect other things under the hood, even driveability? In this article we’ll look at how belts age and why they fail, and we’ll look at some of the problems that a worn belt can cause. We’ll also discuss how to give replacement parts the best chance of a long service life.

A drive belt is a moving part that uses friction to do its job, so even the best belt in the world is going to wear out some day. On V-belts, the material wears away from the sides, and as the belt rides deeper in the pulley it eventually bottoms out in the groove. That’s when it begins to squeak. If there is no automatic tension adjuster, the belt will eventually slip and make a loud squealing noise.

A ribbed serpentine belt usually has an automatic tensioner with a smooth roller riding on the smooth side of the belt, so those belts wear on both faces. With age and repeated heat cycles, the ribs eventually crack and begin to shred off in small pieces called “pills.” If those rubber pills become wedged into the pulley’s grooves, it will give the belt a “lumpy” rumbling ride over the pulleys.

About 10 years ago the technology and materials used to make serpentine belts changed, and the newer belts are very durable. A GM service bulletin from 2004 says they’ll last “the life of the vehicle (10 years or 150,000 miles).” Of course, that’s under ideal conditions: A belt’s service life is impacted by excessive heat, poor pulley alignment, improper tension adjustment or a worn-out automatic tensioner. Throw in some road gravel, fuel, oil, coolant or other fluids commonly used in a vehicle or in the shop, and the real service life of a poly-ribbed serpentine belt can be much shorter.

Misfire codes

It might be rare, but if you understand how misfires are detected, it’s not hard to imagine how a worn drive belt can cause misfire codes. The powertrain control module (PCM) detects misfire by measuring crankshaft acceleration after each piston passes top dead center (TDC). Is it really possible for a damaged belt or chattering tensioner to have that much influence on crankshaft speed? Consider this: On most newer models, misfire detection is suspended when the PCM (using data from wheel speed sensors) determines the vehicle is being driven on a rough road.

A loose belt or weak tensioner spring can have a similar effect. A loose belt is most likely to slip when it’s loaded suddenly, such as when the A/C clutch engages. Many customers won’t bother having that little chirp fixed as long as it doesn’t get worse, but a tech on iATN’s Technical Forum described how he solved a reoccurring P0300 code by adjusting the A/C belt just to cure that little start-up chirp.


Belt quality can be a factor, too. On late-1990s Honda Civics, CR-Vs and del Sols, the Honda Tech Line had several cases where MIL illumination and intermittent misfire codes were traced to poor-quality replacement alternator belts. While Honda recommended installing a new OE belt to solve the problem, the makers of top-quality aftermarket belts recognize that an older engine might need something more. Based on field experience with specific models, they often redesign a belt to solve specific problems, and they might package the belt as part of a “solution kit” that includes upgraded tensioners and pulleys. This highlights the difference between aftermarket engineering and just making copies of the OE part.

Stretch-fit belts

About 10 years ago, some car manufacturers began installing stretch-fit belts at the factory. These are a slightly different size than standard belts and made of special material that stretches during installation and/or contracts during the first heat cycle so the belt fits properly. It’s easy to spot these because there will be no belt tensioner or manual tension adjustment on those pulleys, although there may also be an additional standard belt on the same engine.

Special tools are needed to install stretch-fit belts and the tools and R&R procedures are very specific for each engine, so pay attention to the instructions printed on the replacement belt’s package.

Be sure to use the correct tools and procedures. Developed as a production line time-saver, they can also save a lot of time in the service bay. And although they’re made with special materials, they wear just like standard belts.

Belt noise and pulley alignment

A noisy belt is more than just annoying. It can indicate a more serious problem. A chirping noise is often caused by a worn-out belt that may be close to total failure. A loud squealing noise is usually caused by a belt that’s loose or by a bad bearing behind one of the pulleys.

Problems like these are relatively easy to find and fix. But if the customer returns a week after you’ve installed a new belt because it’s noisy, that’s a serious problem, too.

Look closely at the new belt. If it looks kind of fuzzy, that’s a textured fiber material that helps keep the belt quiet. There may also be grooves running across the ribs to control heat and noise.

These are highly-engineered premium parts specifically designed to run quietly for the life of the belt, but these features will only work if the pulleys are working correctly, too.

Assuming the tension is adjusted correctly, a chirping noise is most likely the result of improper pulley alignment. You can easily check this by spraying water from a squirt bottle on the belt while the engine is running to see if the noise changes.

If it does, the belt is slipping. If it doesn’t change, look for a pulley alignment problem. Use only water; anything else may hide the problem or even damage the belt. As noted earlier, other liquids may increase the noise or degrade the rubber.

There are several ways to check pulley alignment. If you’re skilled and imaginative, the old-school method of using a straight edge and a caliper or small ruler still works well. If you want speed and accuracy, there’s nothing better than laser alignment tools. There are many different brands on the market, and they all work as advertised if you read the instructions and follow them carefully. If you can’t find a pulley laser on your favorite tool truck, look for it on the belt manufacturers’ websites.

When checking pulley alignment, pay special attention to the tensioner and idler rollers and any other pulleys that ride on the back (smooth side) of the belt. You can do this by snapping the throttle and watching the belt ride on the roller as the engine accelerates and then decelerates.

If you have access to a strobe light, this may aid in observance of a misalignment or a “walking” belt.


If the belt “walks” on the roller, something is out of alignment, and it may or may not be that particular roller or pulley.

There are three types of pulley alignment: angular, offset (or parallel) and planar. The most common misalignment is angular.

According to some sources, it only takes about 2 degrees of misalignment to make a belt noisy. It will also make the belt run hotter and wear faster on one edge.

Pulleys and rollers are out of angular alignment when the shaft they’re mounted to is not parallel to the crankshaft. Remember, force is exerted on only one side of a pulley, and all that force is pulling on one end of the shaft.

If the shaft is an alternator or A/C compressor that’s mounted to the engine on rubber bushings, these bushings deform as they age and allow the unit to tilt relative to the crankshaft.

Some power steering pulleys are pressed onto the shaft. There is a wide variety of different pulley styles, and there are pullers and installation tools for each different style.

Using the wrong tools can cause problems, including a bent or misaligned pulley. When installing a pump with a pressed-on pulley, even if it’s the same unit that was removed from the engine, remember to check/adjust pulley alignment before installing the belt.

On a spring-loaded tensioner that rides on the smooth side of a ribbed belt, the pivot bushing can wear on one side and let the roller or pulley tilt.

This angular misalignment often shows up as a wear line or impression on the front or rear edge of the smooth side of the belt.

Pulley contamination

While you’re checking pulley alignment, take a look at the condition of the pulley itself. An even glaze or smears of rubber or contamination on the friction surface will cause squeaking or chirping noises (maybe someone tried to stop the noise with a spray product).

Carburetor cleaner, a wire brush, sandpaper and steel wool are all good tools for cleaning a pulley.

For hard-to-reach pulleys, some techs have developed a technique of holding a wire brush or some other abrasive against the pulley while the engine is running.

There are two problems with this: one is the obvious risk of damage and personal injury, and the other is the fact that contamination or grit loosened from the pulley may end up getting pressed into the belt itself.

We DO NOT recommend this technique.

If the crankshaft pulley is part of a harmonic balancer, check to see if it’s slipping. As age and heat take a toll on this part, the rubber deteriorates and allows the outer pulley to slip on the inner hub. This typically happens when starting or shutting off the engine.

A telltale stripe of paint across both parts will show if it slips.


Belt tension

The first thing most people think of when they hear a noisy belt it to check its tension by pressing the belt with their thumb. That’s not a bad start, but it’s important to remember that the accessory drive system is engineered to work effectively and quietly over a very wide range of speeds, loads and environmental conditions.

Proper belt tension is a critical element of that system.

Incorrect tension can cause some of the strange misfire codes mentioned earlier, make the belt wear faster, damage bearings, and even strand your customer on the road.

If you’re not using a professional-quality belt tension gauge on every belt you check or install, you can’t be sure that anything in the accessory drive system is working as it should.

A traditional belt tension gauge measures the force required to bend (deflect) the belt a specific distance.

A sonic belt tension gauge measures the vibration frequency of the belt, just like checking a guitar string. Both gauges are accurate when used according to their instructions:

The real trick is finding a belt tension specification.

As noted earlier, a lot of engineering goes into an accessory drive system, so a belt tension specification definitely exists. Unfortunately, not every OEM publishes that spec in their service information. However, some of the aftermarket belt manufacturers publish tension specifications for their own products on their company website.

If the engine has a spring-loaded belt tensioner, look for a wear mark on the tensioner body that indicates when the belt is worn or stretched to its limit.

Check that wear mark with the engine running and the A/C compressor engaged.

After installing a new belt, run the engine for at least five minutes to let the belt “settle in,” then check the tension.

On engines with an automatic belt tensioner, any time the belt is replaced due to wear, the tensioner should be replaced, too. That’s because the tensioner is more than just a strong spring; most automatic tensioners also have a friction damper that helps limit its movement.

The bearing, pivot bushing and friction damper are all wear parts that are not expected to last forever. If it seems like only the spring is holding the tensioner in place as the engine is running at idle, the tensioner is worn out.

One last thing

Many newer alternators have a decoupler pulley that lets the alternator free-wheel during engine deceleration. This takes a lot of strain off the belt, the tensioner and everything else in the accessory drive system, improving service life and reducing noise and vibration.

Sometimes drive belt noise is caused by a worn decoupler pulley. One quick test is to unload the pulley by disconnecting the alternator field wire to see if the noise goes away.

There are two different decoupler pulley designs: a simple one-way clutch and a more sophisticated clutch design with a spring that’s matched to the accessory drive for that particular engine.

When replacing this pulley, make sure to install the same kind that was removed. Trying to substitute a different pulley will likely increase noise and definitely impact the service life of the belt, tensioner and other components in the drive system.

We’ve covered a lot of ground here, but as noted earlier, some of the big aftermarket belt manufacturers have done a lot of research on accessory drive systems.

You can find vehicle-specific information about problems, tools and installation techniques in the excellent training videos on their websites.   ●

Belts and solvents

Belt manufacturers are adamant about not spraying chemicals onto a belt because they may contain solvents that attack the rubber or leave a residue that attacks the rubber or attracts grit. Some products can even form a glaze that eventually makes the noise worse. Any fluid leaking from the engine onto the belts or pulleys can have similar effects, or it can simply lubricate the belt and make it slip. Antifreeze is particularly bad because it’s based on ethylene glycol, which can cause some types of rubber to soften and swell.

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