belt tensioner for the E320
is to the right of the water pump as you face the car. It is a brass long nut held in by a bracket and a long screw ....the nut is pointing up.
There are two nuts which must be loosened:
First, the 10mm hex at the 5:00 pm position next to the water pump pulley. Give that about 1-2 turns.
Then loosen the long brass nut about a 180 degree turn. This will allow you to loosen or tense the belt.