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The serpentine belt runs across the front of most engines, providing power from the crankshaft to each of the engine's accessories (alternator, power steering pump, air conditioning pump, etc). Sometimes it is a single belt, often it is two or even three.

Complex serpentine belt on a V8 engine
These belts eventually begin to wear down and require replacement (the belt above is well-worn and needs to be replaced). Intervals will depend on your vehicle and its manufacturer's recommendations, but usually fall in the 30,000 to 50,000 mile range.

Testing Belts for Viability

When you take your car in for service or if you do it yourself, the belts should be checked for wear and tension. The manufacturer's specs for tension are given in the owner's manual for the engine/vehicle. These can often be measured using either a tension tool or with a tape measure and fisherman's scale.

The former test is more accurate and should be done anytime the vehicle is in a professional shop. Many auto parts stores also offer free belt tension testing as well.

The latter method can be tried using a tape measure held steadily against the belt while another person uses a fisherman's scale to put a given amount of tension (by weight, according to the owner's manual specs) to see how far the belt travels and if it's within spec.

If the belt is loose but otherwise not worn, tension should be added by adding leverage on one of the belt's attachments: usually a tension pulley, the alternator, or the power steering pump.

When a Belt Goes Bad

Often, car owners don't know their belt is bad until it begins to squeal. This usually happens when the engine is cold and the weather has been cold and is warming up (such as in the morning after a cool night). The squealing, at first, will only last a few seconds maybe a minute or two, but will get progressively worse with time.

When you hear that squeal under the hood, check the belt tension as soon as possible. Eventually, the belt will snap and leave you stranded or, worse, cause failure and damage to the engine or components.

Replacing the Belt

Many competent home mechanics do their own belts. Usually, only basic hand tools are needed. Loosening one component (usually one of the above-listed: a pulley, the alternator, or the power steering pump) generally allows the entire belt to be removed and replaced, re-tightening to specification.

In a shop, this service will usually run about $100-$200, depending on the cost of the belt(s) to be replaced. Some vehicles are much more expensive if there are a lot of components in the way or a complex serpentine (as shown).

Serpentine Belt Noise

Serpentine Belt Tensioner