Why Do Serpentine Belts Squeak?

A serpentine belt is a long winding belt on modern engines which takes the place of all the other belts that used to be on a car.

Years ago engines had separate belts for all the drive accessories such as the A/C, power steering, fan, and water pump. Those belts mounted on pulleys in separates. The serpentine belt is a single belt mounted to all those pulleys in one path. As modern cars went away from fans powered by belts to electric fans, it became easy to eliminate the other remaining multiple belt systems and go to a single longer belt.

Exactly why a serpentine belt might squeak can be the result of various things. As the engine turns, the main pulley turns and that drives the belt. The belt, under tension by a spring-loaded, belt-tensioned pulley, also spins. It pulls the other drive pulleys with it, spinning the other accessories that the belt is attached to. The squeak is in most cases a matter of friction.

The squeak you hear is generally a result of the belt losing its grip on one or more of the pulleys it is designed to spin. It could be a symptom of a worn belt. The contact surface can become worn and smooth, which lowers the friction point and causes the belt to slip. As the belt slips you will hear a loud squeak, just as if you put your finger on a glass plate and slide it. The breaking of the friction point results in a squeak. It could also be the result of moisture. Driving through a big puddle of water can cause a temporary lowering of that friction point, and once it dries up the squeak will usually go away. A loose belt, caused by years of stretching or a weak belt tensioner, can also cause a squeak. In this case, the belt is just too loose to grip the pulley properly.

If the belt is good, tight, and dry but still squeaks, it could be a symptom of a bigger problem such as a bent or broken pulley. In most modern cars, some or all of the pulleys are plastic, or have a plastic housed bearing. If the pulley is damaged in any way and not spinning free and true, the belt can squeak. Also, if one of the components the belt is driving is bad, it may also squeak. A bad bearing in an alternator or power steering pump pulley can put extra strain on the belt system.

It is a good idea to have your belt inspected often. Or if you know what to look for you can inspect it yourself. If you can twist the belt more than 1/2 turn it might be getting weak. If the belt is cracked and chipped, it should be replaced. In modern cars, when that belt breaks, your car is disabled. So it is a smart idea to make sure it's in good shape.