A tensioner assembly on the Mercedes model vehicles is the main component that keeps the proper amount of tension on the accessory belt at all times. The tensioner assembly for the Mercedes C-class, E-class, S-class, M-class, GL-class and GLK-class models is made up of a spring-loaded arm, a pulley and a bracket. As the belt is turning underneath the tensioner pulley, the spring-loaded arm applies the proper amount of tension to the belt as it travels around the tensioner pulley and the other accessory pulleys. Over time, the tensioner can wear out and will need to be replaced.
M/F Park the Mercedes vehicle on a flat surface and open the hood. Shut the engine off and wait about an hour for the engine to cool down.
M/F Prop the hood up and locate the tensioner assembly on the front of the engine. The tensioner assembly has a spring-loaded arm on one end and a pulley on the other end. The accessory belt travels under the tensioner pulley.
M/F Position a metric socket and a breaker bar onto the bolt head in the middle of the tensioner pulley. Turn the breaker bar in a clockwise motion to release the tension from the belt. Continue to hold the tension off of the breaker bar with one hand and slide the belt out from under the tensioner pulley with the other hand. Release the tension from the breaker bar and remove the breaker bar and socket from the tensioner pulley.
M/F Hang the belt to one of the accessory pulleys such as the alternator pulley or the power steering pump pulley with a bungee strap. This is to prevent the belt from slipping out of the accessory pulleys.
M/F Locate the bolt head in the middle of the tensioner arm and the mounting bolts from the bracket that holds the tensioner to the front of the engine block. Loosen and remove the bolt from the middle of the spring-loaded arm with a ratchet and a socket. Loosen and remove the mounting bolts from the tensioner assembly bracket. Place the bolts in a safe place.
M/F Pull the tensioner assembly straight off of the front of the engine block. If the tensioner is stuck, use the end of the breaker bar to tap the tensioner assembly off of the engine block. Position the new tensioner assembly over the mounting bolt holes on the front of the engine block. Screw the bolt back through the middle of the tensioner arm and tighten it down with a ratchet and socket.
M/F Screw the tensioner assembly mounting bolts through the bracket holes and into the engine block. Tighten the bracket bolts down tight with the ratchet and socket.
M/F Position the breaker bar and socket onto the bolt head in the middle of the new tensioner pulley. Turn the breaker bar counter-clockwise and hold the tensioner in position with one hand and remove the bungee strap from the belt with the other hand. Slide the belt under the tensioner pulley and release the breaker bar. Remove the breaker bar and socket from the pulley and the engine compartment.
M/F Inspect the belt to ensure that it has remained inside of all of the accessory pulleys. Crank the engine and watch the new tensioner assembly as the belt is traveling under the tensioner pulley. Make sure that the new tensioner pulley is moving up and down freely with the movement of the belt.