Rick's Free Auto Repair Advice

Serpentine belt tensioner symptoms

Serpentine belt tensioner symptoms — what to look for

What are the symptoms of a worn/bad serpentine belt tensioner?

A serpentine belt tensioner does two jobs; it keeps the serpentine belt at the proper tension and it dampens power pulses that can cause the belt to vibrate.

Each tensioner includes:

• Base to mount to engine
• Torsion spring to apply tension
• Pivot bushing to allow the arm to pivot
• Pivot arm with an idler roller
• Dampening mechanismserpentine belt tensioner cutaway


Serpentine belt tensioner symptom #1 — Belt squeal

Over time the torsion spring can lose tension, reduce tension on the belt causing it to slip. You may notice the slipping by a loud squeal when you first start the engine. It usually happens on cold starts, but if the spring is more considerably worn, the slipping and squeal can happen at any time. If the tensioner isn’t applying the correct tension, it must be replaced, it can’t be adjusted.

The slipping can also cause your battery to be run down due to the belt slipping around the alternator pulley.

Serpentine belt tensioner symptom #2 — Belt comes off

If the spring inside the tensioner is rusted or broken, the pivot

broken belt tensioner spring

Broken/rusted belt tensioner spring

arm can bind in the released position, causing the serpentine belt to come off the pulleys. If your belt has come off, slowly rotate the tensioner full cycle and release slowly to check for any binding. If you discover binding, replace the tensioner

Tensioner symptom #3 — belt is frayed at edges

As the pivot bushing wears, the tensioner can no longer maintain its alignment with the other pulleys. That can cause the belt to ride up the side of some of the pulley which frays the edges.

Belt tensioner symptom #4 — belt noise

Every serpentine belt tensioner has a dampening

belt tensioner movement while running

belt tensioner movement while running

mechanism to reduce pivot arm rotation between engine power pulses. When the dampening mechanism wears, the pivot arm swings in a wider arc, transferring the pull/release tension to all the driven components.

To test for a worn dampening mechanism, turn on engine and run at idle speed. Then shine a flashlight on the roller/pulley at the end of the belt tensioner arm. The tensioner arm should only move about 1/32″. If the tensioner arm moves more than 1/32″ the dampening mechanism is worn and the tensioner must be replaced. The sign of a good tensioner is to only see 1/32” movement. If you see more, the dampening mechanism is worn and the tensioner must be replaced.


Tensioner symptom #5 — grinding noise or loud hum

A worn pivot arm idler roller bearing can cause a grinding noise or a loud hum when the engine is running. To test the bearing, use an automotive stethoscope and place the probe on a non-rotating part near the idler roller. If the bearing is bad, you’ll hear it.

If you don’t addresses a grinding idler bearing, it will self destruct like this. Replace the idler at the first sign of noise or degradation.

worn serpentine belt tensioner

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