For the home mechanic, one of the most basic tasks is the inspection and, if needed, replacement of the alternator belt. On a BMW 3 Series, this is an easy task that will help prevent you from getting stranded on the side of the road when the belt fails. On most BMW 3 Series, there are two belts. One is for the air conditioning, while the other powers accessories like the alternator, water pump and power steering pump. The belts should be checked every 3,000 miles for wear.
Open the hood and inspect the alternator belt for any cracks. Look for cracks on the part of the belt that rides on the pulley. If there are any cracks, replace the belt.
Remove the plastic cover on the tensioner by prying it off. The tensioner is located below the top pulley. With the later poly-ribbed belts, the tensioner is loosened with a 10-mm socket and hex tool inserted into the bolt in the tensioner. Different tensioners are used in different models and can turn in two different directions to loosen, but this will be self-evident once the tool is on the nut.
Release the tension on the belt, and the belt should simply come off. There is no need to remove the fan, though the belt needs to be maneuvered around the fan. On BMWs with air conditioning, the A/C compressor belt needs to be removed first, since it blocks the other belt. It is removed in the same manner as the alternator belt.
Slide as much of the new belt on as possible, release the tension on the tensioner and slide the belt the rest of the way on. Make sure the belt is properly seated in all of the pulleys and the ribbed portion is against the crankshaft pulley. Replace the plastic caps and start the motor to ensure that everything is turning smoothly.
