Your engine’s serpentine belt is probably not something you check on very often. If it is working then your vehicle will run, the battery will charge, the power steering works, and the engine stays cool. But if that belt were to break you will likely be stuck on the side of the road waiting for a tow truck. That’s why it is important to learn the signs of a bad serpentine belt before it fails.
One of the common signs of bad serpentine belt condition is cracking. With the engine turned off, pop the hood and grab a flashlight to inspect the serpentine belt. If you see more than three cracks in the belt along the same three inch section, it is time for replacement. Be aware though that some modern serpentine belts are made from materials like EPDM (ethylene propylene diene monomer). These materials are highly resistant to cracking, so a belt that is well past its prime may still look good with no cracking, so it is best to keep checking for other wear indicators if you suspect a worn belt.
A belt that is no longer able to grip the engine pulleys may begin to squeal as it slides over them rather than spinning them. This leans to even more accelerated wear of the belt as well as putting heat into the belt via friction. Even if the squealing goes away, don’t ignore it. Normal serpentine belt operation should be nearly silent.
A worn out belt tensioner can also cause the serpentine belt to slip so be sure to check it or else your new serpentine belt may have a short service life.
If the belt is chirping like a bird the problem might be one or more accessory pulleys are out of alignment. Not only is this annoying but it will also cause the serpentine belt to wear out faster or even cause the belt to come off.
Modern serpentine belts are actually pretty well engineered to resist physical damage. But just because a belt isn’t cracked or falling apart doesn’t mean it is still good. Just like a car tire the surface material on a belt can wear down. When this happens the ribs on the belt lose their original tight tolerances, and therefore their grip on the engine pulleys. The belt may still fit fine on the pulleys, but it won’t be fully contacting the pulley. The belt may actually end up riding on just the pulley ridges which makes for a tiny contact patch.
Even the best serpentine belt doesn’t last forever. If you have been inspecting your serpentine belt regularly and it still looks fine, you should still replace it every 90,000 regardless. Even without any of the signs of worn serpentine belt life racking up that much mileage means it is time for the belt to move on. If you can’t remember when you change your serpentine belt, it is probably time.
One of the lesser known signs of bad serpentine belt condition is an electrical system that starts to act up. Modern vehicles are chock full of computers, modules, sensors and more components that rely on certain conditions to remain constant, like voltage. If the serpentine belt is worn to the point where it no longer spins the alternator consistently there could be variation in alternator output. You may even get a check engine light for random systems issues. If you are pulling your hair out chasing electrical issues, check the alternator output for consistency. If it is jumping all over the place a worn serpentine belt may be the issue.
Not So Cold AC
Another one of the not so common signs of a worn serpentine belt can be a weak AC system. Your air conditioning compressor takes a bit of engine power to turn. You’ve probably noticed that in a car with a small engine turning off the AC feels like it gives you a bit of a power boost. If the serpentine belt can’t grip the AC compressor clutch, it also can spin it to compress the refrigerant back into a liquid to cool you off. You might hear a squealing sound when the AC compressor cycles on, but don’t rely on sound as the only symptom. If your AC isn’t blowing as cold as you remember and you can’t remember when you changed the serpentine belt, it is probably time for a belt replacement.
Diagnosing a worn out serpentine belt isn’t as easy as it used to be, but the signs are all there if you are paying attention. Luckily serpentine belts are usually affordable and easy to change, so when the time comes there’s no excuse to put it off.
Check out all the belts available on NAPAonline or trust one of our 17,000 NAPA Auto Care locations for routine maintenance and repairs. For more information on diagnosing engine drive belt tensioner failure, chat with a knowledgeable expert at your local NAPA Auto Parts store.
With an automotive writing career spanning over two decades, Brian has a passion for sharing the automotive lifestyle. An avid DIYer he can usually be found working on one of his many project cars. His current collection includes a 1969 Olds Delta 88 convertible and a slant-6 powered 1975 Plymouth Duster.