Some months ago the tensioner on my 1990 300e broke, the result a loose belt and lots of noise from the belt slipping.

The tech info section has an excellent, fully illustrated, step by step guide to change the tensioner, altough it looks complicated, it's really easy and should not take more than a sunday morning to do.

In my case I could not get the piece here, so I improvised by removing the tensioner shock absorber and replaced it with a piece of threaded rod fixed at the lower shock mounting screw and adjusted by a set of nuts at the upper part of the engine, it works great!!.

if you want pictures, feel free to request them.

CHECK POST FROM 11-22-2013, FOR BETTER INSTRUCTIONS AND PICTURE
good luck