How To Tell If My Belt Tensioner Is Bad. The noise that drive belt tensioners make or malfunction could be the symptom of a bad belt tensioner. They have a guage for spring tension;
Grinding or squeaking noise from the belts or tensioner. If it's making a loud whining noise, then the bearing on the pulley is probably getting tight. This can cause a slapping or knocking noise.
This can cause a slapping or knocking noise. It depends of what part of the tensioner is failing. This can cause a slapping or knocking noise.
If the drive belt noise resembles a squeal or a squeak, it is possible that the belt tensioner is loose. While there are many causes for squealing sounds within an engine, a bad belt tensioner is a likely culprit. If the serpentine belt slips off after the engine is started or running that means the belt tensioner is no longer applying tension to keep the serpentine belt in place.
Is there any rubber deposited like it was thrown from the belt as it warms. If the pulley bearing completely fails, it can also cause a squealing or even a grinding noise. A bad bearing will not spin as easily and/or will sound rough.
Since the tensioner is not able to maintain the correct tension on the belts, it produces a squealing or squeaking sound. That'll tell you if you need to change it because it's making a belt squeeling noise from lack of tension. I know someone who paid $880 for this job on their c280.
When it's cold listen to the belt/pulleys. This is most evident on the drive belts. With the engine off, check the belt tensioner and pulley.