You're tensioner may be shot. Then again, it may simply need the following:

ACC Belt Tensioner Adj. part G6002-60237

Go above to FastLane and look under cooling system. I tried posting a photo, but it didn't take?

I thought mine was shot until I discussed the issue with an MB tech. I changed the piece above and tension was restored. That was about 3 years ago.

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'91 300-SEL