There are numerous posts on M103 serpentine belts and tensioners. Here are some:

http://www.peachparts.com/shopforum/showthread.php?threadid=8337&highlight=m103+belt+tensioner

http://www.peachparts.com/shopforum/showthread.php?threadid=19238&highlight=m103+belt+tensioner

There are many more.

Sixto
91 300SE
81 300SD