What Is a Serpentine Belt?

The Purpose of a Drive Belt and When To Replace

Serpentine Belt (Image courtesy of Popular Mechanics)

What Is a Serpentine Belt?

The engine in your car has many roles to fill. It has to do more than move. It also has to generate electricity, assist with power steering, cool off the passengers, and keep itself cool. The serpentine belt - sometimes referred to as the fan belt, alternator belt, or accessory drive belt - is used to spin, or “drive,” several components attached to the engine. It drives the alternator that powers the electrical system and recharges the battery. It also drives the power steering pump, the air conditioning compressor, and the water pump. (The water pump circulates coolant through the cooling system). In the past, multiple drive belts were used to turn pulleys attached to these components. Today, most vehicles employ just a single belt - the serpentine belt - to do the job.

Why Is the Serpentine Belt Squealing?

Worn vs. new serpentine belt (Image courtesy of Toyota of Yakima)

Over time, the ribbed surface of a serpentine belt wears down, just like a set of tires. As it wears, the belt stretches slightly. When this happens, belt tension against the pulleys lessens and the belt can slip, leading to excessive wear, glazing, and potential cracking. (A tensioner pulley is used to maintain belt tension, but it too can break down). The belt squeals when it slips on the pulleys.

Of course, if the belt gets wet with condensation - or after a sudden splash through a puddle - it may let out a squeal. Contamination from engine oil leaking on the belt can cause the same symptom. But oil and other petroleum-based materials will deteriorate the belt and cause it to slip - and to fail.

What If The Belt Goes Bad?

A worn or deteriorated serpentine belt can significantly impact vehicle driveability. For instance, if a serpentine belt is slipping, output from the alternator can decrease. On a cold day, when the charging system needs to be at its best, decreased alternator function could kill the battery. In addition, steering performance and A/C output can also be affected

If the serpentine belt breaks completely during driving, results can be serious: sudden loss of power steering, sudden battery drainage and electrical failure, and engine overheating.

How Long Can I Expect the Belt To Last?

Because a single belt is used to transfer power to so many critical engine accessories, it is important to make sure the belt is in good working condition. Most serpentine belts are recommended for inspection and replacement between 60k and 90k miles, depending on the manufacturer. Some belts can last more than 100k miles.

But if you hear the serpentine belt squealing, perform an inspection as soon as possible, or have it inspected at a trusted repair shop. If the belt shows significant signs of wear (glazing, cracking, missing pieces) - or if there is oil or other contaminant on the belt - have it replaced.

Serpentine Belt replacement

Better yet, consider having the serpentine belt inspected and replaced as a matter of preventative maintenance every 30k miles, or when any related components are replaced.

If it is time to replace the serpentine belt on your vehicle, check out our article How to Replace a Serpentine Belt for a glimpse into how a technician might complete the repair.

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