The M103 tensioner is not for the uninformed. There's a long hollow 13mm nut behind the power steering reservoir and a 19mm head bolt under where the tensioner pulley and power steering puller almost meet. Loosen the 19mm bolt a couple of turns. Don't remove it or you're in for a heck of a time getting the tensioner back together. Then loose the 13mm nut until you can slip the belt of the pulleys.

When putting it back together, locate the indicator on top of the tensioner. Look behind the power steering pulley for a pointer near a triangle and a line. Hard to envision but you'll probably know it when you see it. Tighten the 13mm nut until the pointer is past the triangle and right at the line. Then tighten the 19mm head bolt.

I'm sure others will chime in with short cuts and torque specs.

Sixto
91 300SE
87 300SDL
83 300SD