1996 E300D belt tensioner
 

I want to replace the serpentine belt on my 1996 E300D diesel W210) but don't know how to relieve the tension on the tensioning pulley. No manuals are available for this model so I need HELP!

The spring is holding it tight. There is a bolt in the top that can be removed to loosen it.

IF you are not sure, I would pay the 1 hour labor and have it done. The routing can be tricky.

I replaced mine @ 79K miles (for the first time) in about 30 min. There is a bolt which keeps the tensioner locked (M14 if I remember right). Open the bolt and remove it. Then using a 5 mm round bar place it into the tensioner opening and turn it right (while facing the engine). The belt will be untighted. Remove it. Put the new one in in the same sequence (start at the tensioning pulley first). lock the tensioner. You are preatty much done. Good luck.