The belt tension on the Mercedes consists of a tensioner arm that has a spring and a pulley attached to it. As the belt on the engine wears out, it stretches. The spring on the tensioner arm pulls down on it, which then pulls down on the pulley. Over time, the bearings in the pulley wear out. When this happens, you can replace the tensioner arm because the pulley comes pre-installed on the tensioner arm.
Locate the square hole in the tensioner arm, and place a 3/8-inch ratchet into the square hole. Pull up on the handle, and remove the belt from the tensioner pulley.
Remove the tensioner spring from the tensioner arm, using a pair of pliers.
Remove the two bolts that secure the tensioner arm to the engine block, and pull the tensioner out of the engine bay.
Secure the new tensioner arm to the engine block, using the socket and the ratchet.
Reconnect the tensioner spring to the tensioner arm, using the pliers.
Lace the ratchet into the square hole on the tensioner arm. Pull up on the ratchet handle, and place the belt over the pulley.