MB129Guy, I looked at two parts websites and both showed a constant-tension belt tensioner that doesn't require adjustment. Check Pelican Parts and determine if that style belt tensioner (Febi/Bilstein or genuine MB) will fit your 1991 500SL. If so, it will be a simple DIY replacement.

If you have the older style tensioner with an adjustment rod it is important not to tighten the center bolt until the tensioner is adjusted; otherwise, you will destroy the tensioner. It is also critical to route the belt properly ... which can be tricky ... carefully follow a belt diagram for your particular engine.

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Fred Hoelzle