One of the most basic tasks for the home mechanic is the inspection and, if necessary, replacement of the alternator belt. On a BMW 3 Series, this is an easy task that will help you avoid being stranded on the side of the road when the belt breaks. In 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 three thousand miles for wear and tear.
Open the engine cover and examine the alternator belt for cracks. Look for cracks in the part that rests on the pulley. If cracks are present, replace the belt.
Remove the plastic cover over the tensioner by pulling it off. The tensioner is located below the upper pulley. With newer grooved belts, the tensioner is loosened with a 10mm wrench and a hex wrench inserted into the tensioner bolt. Different tensioners are used on different models and there can be two ways to loosen them, but once you put the tool on the bolt this will be self evident.
Release the tension on the belt, and it should just come off. There is no need to remove the fan, although the belt has to be manipulated around the fan. On the BMW with A/C, you must remove the A/C compressor belt first, as it blocks the other belt. It is removed in the same way as the alternator belt.
Slide off as much of the new belt as you can, release the tension on the belt tensioner and slide the rest of the belt off. Make sure that it is properly positioned on all pulleys and that the grooved part is over the crankshaft pulley. Replace the plastic caps and start the engine to make sure everything is running smoothly.
Tips
If the old belt isn’t right, simply cut it off with pliers to remove it.
Warnings
On older BMWs like the E30 3 series, the method is the same, but the alternator is mounted with a bracket that rotates and provides the proper tension. There is also a rack and pinion device used to increase the tension on the belt. There is a nut on the back of this device that holds it secure. Loosen the nut before turning the bolt to loosen the bracket. Once the nut is loose, turn the bolt clockwise to loosen the tensioner. Follow the above procedures to replace the belt.