How to Replace a Serpentine Belt on BMW 325i

1. Pop the hood. 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.
2. 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.
3. 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.