Rick's Free Auto Repair Advice

What causes a serpentine belt to break?

What causes a serpentine belt to break?

Serpentine belt failure — Most common causes

A serpentine belt can fail in several ways:

• A complete break. Usually caused by a seized pulley
• Rib wear that causes the belt to slip and wear. Usually caused by a binding idler roller
• Edge fraying that reduces grip. Usually caused by a component that’s out of alignment with the other components.
• Contamination. Rubber degradation due to oil or coolant leaks and debris accumulation. Use of spray lubricants or “belt dressing”.

ripped torn serpentine belt

Worn/seized bearing in idler roller causes the serpentine belt to break

A worn idler roller bearing causes the roller to bind and drag. That causes belt slipping and wear, along with noise. As the wear progresses, the bearing wobbles, causing the the belt to run out of alignment with the other pulleys in the drive belt system. If an idler bearing or pulley seizes, the belt will slide across the pulley, overheat and break.

worn idler bearling

The worn idler bearing cause the idler to wobble and move the belt out of alignment with the rest of the pulleys, causing edge fraying

Rusted or worn idler face

The rusted surface causes accelerated wear on the belt ribs or backside of the belt, resulting in premature failure.

Belt contamination

Leaking gaskets allow oil and coolant to soak into the belt causing rapid rubber degradation. In addition applying spray on belt dressing and lubricating products can degrade the rubber used in serpentine belts.

Belts do NOT need lubrication or belt “dressing.” If a belt is making noise, it’s due to a tension issue or an alignment issue. Fix the underlying problem. NEVER lubricate a belt with a lubricant or belt dressing.

oil contaminated serpentine belts

These belts were destroyed by leaking engine oil or coolant.

Improper tension can cause a serpentine belt to break
Excessive tension causes the belt to run hot and fail

Low tension causes the belt to slip across the pulleys and wear out the multiple “V’s” to the point where the belt is riding only on its backing.

Illustration showing how to determine if serpentine belt is worn

This illustration shows how rib wear can cause the belt to lose gripping power. The belt’s only contact with the pulley is at the bottom of the rib valley. It simply loses its ability to wedge into the pulley “V”. So the belt slips, chirps, squeals and the driven component loses efficiency.

Improper alignment can cause a serpentine belt to break

The most common cause of belt misalignment is a worn bushing in the automatic belt tensioner. This causes the serpentine belt to run to the side of the pulleys, resulting in edge fraying and rapid wear.

serpentine belt noise

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