OM646.811 Belt Tensioner Vibration

[alecmascot]

W204, OM646.811 with 100K miles.
I have been chasing noises on the front of the engine and after having lubricated all the pulleys, replaced the water pump and tested the alternator clutch I am left with a belt tensioner that is vibrating.
It is a relatively high frequency vibration that causes the belt tensioner to blur visually.
The tensioner does not have a damper, but maybe it should be damped by friction which has worn out.
The belt is relatively new and is quite flexible and evenly worn.
Am I just seeing cogging from the motor ?
The crankshaft harmonic damper is fine.
I would appreciate any insight.
Thanks

 

[amazighman]

Sometimes it could be due to the alternator over running pulley failing.

If you wedge a screw driver into the alternator fins to block them , then try and rotate the alternator pulley one way and then the other.

If the pulley rotates both ways or seized both ways then its likely to be the culprit

 

[alecmascot]

Sometimes it could be due to the alternator over running pulley failing.

That was tested already.... :-((

 

[alecmascot]

Replaced the belt and tensioner today and the vibration is gone.
The old belt and tensioner looked perfectly serviceable.
The new tensioner was a much later revision.

 

[alecmascot]

Just to close this thread out; I took the old tensioner apart and found that it does have a friction damper built-in and it was worn-out, so that was why it was vibrating the way it was.
So these things can wear-out and it is nothing to do with the spring tension itself.