The drive accessories mounted on the front of the engine on a BMW 325i are turned by a serpentine belt. This belt is kept taut by a tensioner, but since the belt is made of rubber, it can stretch or break over time. Replacing the belt is necessary to ensure that the accessories work properly, and should be done when you notice wear on the belt. In this case, the project vehicle is a 2002 BMW 325XI, but the process is similar for other 325s as well.
Pop the bonnet. Locate the belt-routing diagram, which is a sticker on the core support. Locate the belt tensioner, which is in the middle of the engine, just above the crankshaft pulley.
Place the 3/8-inch ratchet and socket on the hex-head bolt in the middle of the tensioner and turn it counterclockwise to release the tension on the belt. While still holding the ratchet, pull the belt off the pulleys.
Run the replacement belt around the pulleys on the engine, using the sticker you found in Step 1 as a guideline. Then pull the tensioner counterclockwise with the ratchet again, and run the belt around the tensioner. Release the ratchet and verify that the belt is straight and even on each pulley.