Some automotive niches are defined by the vehicle's purpose: sports car, pickup truck and minivan among them. Some, like crossovers and minivans, are defined by the market. But on Planet Car, maybe the smallest niche of all is small enough that's it can be summed up in a single car: The BMW 3-Series. Indeed, true 3-Series "competitors" have historically been so rare as be almost nonexistent. The C-Class is one of the few that lay claim as a true contender. With power aplenty, solid build quality, Mercedes innovation and even decent value for the money, the C-Class continues to keep BMW in cold sweats, struggling to stay abreast in the niche it once defined.
Pop the car's hood and locate the tensioner pulley; it's very nearly in the center of the engine, just above and to the right of the crankshaft pulley, and below the water pump pulley. If you cannot find a belt routing diagram sticker under the hood, then use a pen and paper to draw one, and take reference photos with your cameraphone.
Lock a ratchet and socket onto the bolt in the middle of the tensioner pulley, and rotate the pulley counterclockwise to relieve tension on the belt. Insert a drift pin or piece of heavy-gauge wire into the locking hole in the top of the tensioner arm.
Remove the old belt, again taking note of how it winds through the pulleys. Route the new belt onto the pulleys. If you didn't use a drift pin to lock the tensioner in place, then save the top-left pulley for last. That way it won't interfere with your arm or the ratchet while you rotate the tensioner arm during installation.
Install the belt, and relieve tension on the tensioner to remove the drift pin. Make absolutely sure the belt's centered on all the pulleys, and seated in the relevant pulley grooves. Turn the ignition key quickly to "bump" the starter a few times. Check that the belt's still properly seated, then start the car and check it again.
