A serpentine belt drives the several components installed on your engine such as an alternator, power steering pump, water pump and air conditioner compressor.
Most modern cars have one serpentine belt. Some cars have two belts. A serpentine belt is also often called a "drive belt".
A serpentine belt is made of a very durable rubber-based material, but it still wears out over time. On average, a serpentine belt lasts from 45,000 to 75,000 miles or from 3 to 5 years. A worn-out serpentine belt may cause squeaking or chirping noises and a badly-worn drive belt can break.
What happens if a serpentine belt breaks
Broken serpentine belt.
If a serpentine belt breaks, your car won't be driveable. If the engine runs without a serpentine belt, it might overheat as the water pump will stop working.
The battery light might come on because the alternator will stop charging the battery. A car with a hydraulic power steering will also lose the steering assist, so the steering will become very stiff.
When a serpentine belt needs to be replaced
Most car manufacturers don't specify the serpentine belt replacement intervals, but rather recommend inspecting it during regular services once your vehicle reaches a certain mileage.
Worn out serpentine belt
For example, this is what Toyota recommends for the 2012 Toyota Camry serpentine belt:
Initial inspection at 60,000 miles/72 months. Inspect every 15,000 miles/18 months thereafter.
Your mechanic will look for cracks, splits, damaged edges, missing chunks, glazing and other signs of wear. In most cases, it's easy to see when the belt is worn out. A worn-out belt must be replaced. A serpentine belt must also be replaced if it is found to be soaked in oil, or stretched.
If your car has two serpentine belts, both are usually replaced at the same time. It's also a good idea to replace an old serpentine belt before a long road trip. Read more: Car checklist before a long trip. Replacing a serpentine belt costs $18-$75 part plus $50-$150 labor.
Serpentine belt tensioner
Mazda Miata manual drive belt tensioner adjustment.
To work properly, a serpentine belt must be under proper tension. In some older cars, the belt tension has to be adjusted manually, see the photo. The most common symptom of a loose belt is a loud screeching noise coming from the engine compartment. Often the noise comes right after the car is started or when the steering wheel is turned all the way to one side.
In cars with the manual adjustment, the belt tension must be checked during regular services and adjusted as necessary.
Today, most cars are equipped with a maintenance-free belt tensioner. There are two types of a belt tensioner: spring-loaded (in the photo) and hydraulic. Is it necessary to replace a tensioner when replacing a drive belt?
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