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One of the nice things about the configuration of the BMW engine is the relative ease with which you can replace the alternator. The alternator is nestled neatly on the left side of the engine compartment and is easily accessible. The replacement and repair process is straightforward and should take you about an afternoon to complete.
First, make sure your alternator is indeed the cause of the problems with your charging system. Sometimes bizarre electrical problems can be caused by a number of faults other than the alternator. It’s important to troubleshoot the system prior to replacing your alternator.
The first component to check, then, is the belt that drives the alternator. Is it tight and amply turning the fan? If not, tighten it according to the procedures outlined in Project 5. Modern belts seldom break, but they do get brittle and glazed with age, and can slip on their pulleys. Replace it with a new one if it looks cracked or brittle.
The next item to check is the voltage at the battery. This should read a little more than 12 volts with the engine off. When the car is running, the voltage should read at in the range of 13 to 14.5 volts with the engine at 2,000 rpm. If your battery appears to be leaking, your voltage regulator has probably failed. The battery will usually only leak acid if it has been overcharged at a much higher voltage. If the voltage measured at the battery is more than 16 or 17 volts when the engine is running, the regulator is probably bad. If your battery has boiled over and has acid flowing out the top, clean it up immediately. Dousing the area with a water-and-baking-soda solution should help considerably to neutralize the acid and prevent it from eating away at the metal.
An important item to check on your car is the engine ground strap. The engine is electrically isolated from the chassis by rubber motor mounts. If the engine ground strap is missing or disconnected, you might have a whole bunch of problems, including electrical system malfunctions and difficultly turning over the starter. (See Project 85 for the exact location of this ground strap.) Also, some BMW alternators have an external ground that may get disconnected accidentally.
Almost all the 3 Series cars have a replaceable voltage regulator/brush assembly. If you’ve determined the regulator to be working properly, then you should probably remove the alternator for testing and inspection. Before starting any work, make sure you disconnect the battery. The positive battery terminal is directly connected to the alternator, and it can be dangerous to work on if it’s live (see Project 84).
The first step in removing the alternator is to remove the belt that drives it. (See Project 5 for detailed instructions on belt removal.) Now removing the alternator from its bracket is a very easy process. On the early 3 Series, the alternator is affixed to the car via a pivot bolt, and a rack-and-pinion assembly is used to adjust the belt tension. Disconnect the rack-and-pinion assembly if your car has one (see Photo 2), and unbolt the alternator from its bottom pivot bolt. For the later E36 cars, the alternator is fixed in place, and the multifunction belt is tightened on the alternator with a spring-loaded tensioner pulley (see Project 5 for details and photos). Simply unbolt the alternator from where it’s attached to the engine after removing the belt. Depending upon which engine you have, you may need to remove some equipment such as the air cleaner and cool air guide located above the alternator. See Project 12 for details on removing components that are in the way.
With the alternator unbolted, disconnect the electrical connections from the rear. I recommend taking a digital photo of the connections, because many people mix them up when reinstalling the alternator. If you make a mistake and hook them up incorrectly, it’s possible to do some significant damage to your electrical system. Reminder: Do not touch any of these connections while the battery is still hooked up! (Review Project 84 for additional details).
With the alternator out of the car, remove and inspect the regulator/brush assembly (see Photo 4). If you are replacing the alternator completely, transfer the old pulley assembly to the newly rebuilt alternator (see Photo 3). Installing the new alternator is now simply the reverse of removal. Make sure you reconnect all the wires to their proper terminals when you are done.
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