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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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