M103 belt tensioners are not a longevity part. If you're not familiar with proper operation they are easy to damage as well. Loosen the large tensioner lock nut (the one in front facing the nose of the car) and then turn the adjustment nut counter clockwise. The indicator should visibly move back down the indication scale stamped in the tensioner housing. If it doesn't move freely and in an expectable relationship to the backing off of the adjustment bolt, it's probably bad. Many people errantly replace the damper shock as a "fix" for a bad tensioner. The shocks, besides relieving some of the duty from the rubber barrel in the tensioner that exerts tension on the belt, are more of a fail-safe for when the tensioner goes bad by keeping enough tension on the belt to get you by. I find that the shocks tend to way out-last the tensioners.

There is a VERY excellent article here in the forums (or in the DIY articles) that explains the workings and and a step by step on replacement and function of these tensioners. If I get time I'll look for it and post the URL, but I would recommend doing a search for it on your own in case I don't get to it (search terms: belt tensioner m103). I made a copy for myself but can't seem to find that right now either.

You can get the tensioner assembly MUCH cheaper online than at the dealer or parts stores. Try getting one here on Fastlane first (helps support the forum), but many sites have them, I would recommend the OEM manufacturer brands such as Febi or Bilstein. Should be $100 or less (twice that at the dealer).

Good luck