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

Abrasion on the back of a belt means the belt is slipping on a pulley; it could just be pulley misalignment.
<p>Abrasion on the back of a belt means the belt is slipping on a pulley; it could just be pulley misalignment.</p>

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.

Gravel or other hard debris can actually be forced through a drive belt, with resulting damage to the internal structure.
<p>Gravel or other hard debris can actually be forced through a drive belt, with resulting damage to the internal structure.</p>

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.

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