what is the name of that little shock absorber by the fan belt?
 

what is the name of this shock absorber looking thing by the belts and what does it do? could it give you symptoms as if your motor mounts need replacement? could it loosen your belts to the piont that a sensor doesnt let your a/c compressor engage 1992 300e.

motor mount symptoms usually make the car idle very hard...

it only idles hard in drive. when i open my hood i see this little shock absorber looking thing by the belts , and its loose and vibrating. a buick/gm tech couldnt figure why my a/c compressor wasnt engaging. hi/low pressure switches are ok. i have read that it could be a loose belt. havent checked that yet.the engine has a vibration thats hard to feel in park or neutral,but its bad in drive at idle and is felt through the steer9ng wheel.and i wondered if it could be this shock thingie. what i want to know if this could cause my a/c/ belt to loosen and could it make the engine vibrate

Belt slip will cause a/c to disengage after control button pushed within a few seconds.

The belt tensioner shock' is to stop viration of the belt.
It is preloaded by correct tension adjustment of the main belt tensioner just under the coolant pump.

Tensioner bushes fail often. Top through bolt is often found loose.

You may need to reset tensioner . There is a set proceedure to follow.

Caution = You can break the hook off the tension adjustment rod if you do not unlock the main tentioner bolt before setting the tension.
Tensioner rod thread is a REVERSE action. Turn to the left to loosen after loosening big through bolt 1/2 turn. Pointer should point to first graduation mark when fully slack.
Adjustment is made by turning adjustment to the 7th mark to preload the belt tensioner.


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Dampner
 

It's called a dampner.

a dampner... thanks. But the tech said that the belt didnt seem loose enough to cause the compressor to turn off. How loose should it be..? Still the compressor turns off ofter a 1/4 second.before it would last 15 min then all of a sudden it was 5 minutes then 1/4 sec. Is there a relay or something or is the compressor is bad? How loose does the belt have to be?

If the damper is moving alot it probably means the tensioner itself is worn out. If the tensioner is wornout the belt is loose and absolutely the compressor will slip, and the compressor controller monitors the clutch for belt slippage and will act just like you say, shuts off the clutch but may retry engaging it later, thinking that the belt may have gotten wet and slipped due to that. If you can grab the belt and deflect it more than an inch or so, tensioner is bad. Or someone put the belt on that doesn't know how to do it. It sounds like the tech on it now might not be a good choice to determine any of this, it's kind of a special MB tech-thing and easy to screw up. We ALL screwed it up at first, that's part of the learning curve. Once you wreck a tensioner or rod you learn very fast how to do it right as these things don't grow on trees.
If the damper is making a rattling/buzzing sort of noise you need a new one of those too.
Gilly