A faulty or failing alternator can cause many problems, from a simple discharging of the battery to an engine drivability problem. There are a few signs and tests you can perform to determine if the alternator is the root cause of your problem. If you get lucky, your battery light (charging system indicator) will illuminate to warn you of a possible failure. When the battery light (charging system indicator) is ON, this means a voltage difference has been detected between battery positive (+) constant (terminal 30) and ignition ON circuit (terminal 15), indicating a fault in the charging system.
It is common for an alternator to fail slowly. When this happens your symptoms will be present when the electrical system has the highest load, i.e., at night. Driving at night, you may notice the instrument panel lights dim, or the radio or headlights flicker. This is a good indication that the alternator cannot handle the excess load. Another sign of a charging system problem is headlights or radio dim when lowering your window. When the alternator can no longer charge the battery, you may park your vehicle and not be able to start it. Of course, this could be a faulty battery too.
Always check the voltage of your charging system under load. Voltage should be within 13.2: 14.5 volts d/c. To load, have the engine running at idle, turn on the headlights and the HVAC blower motor. Never disconnect a battery cable while the engine is running to test the alternator. You may cause damage to the alternator or other electrical components from the surge in amperage. Before performing any tests of your alternator, charge and test your battery. If the battery is faulty, your alternator tests will not be accurate.
In this article, I'll go over the steps involved with replacing the alternator on the BMW X3 models with a M54 6-cylinder engine. X3 models with 6-cylinder engine may be equipped with one of several different alternators with different amperage ratings. Be sure your replacement alternator has the same rating as the faulty unit. You can usually find the amperage rating on the alternator housing.
Remember your car may have been serviced before and parts replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have so be prepared with different size sockets and wrenches.
Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Never work on your vehicle if you feel the task is beyond your ability.
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Be sure to work with a cool engine and disconnect the battery before beginning.
Working in the luggage compartment, disconnect the battery negative cable and cover the battery terminal so the cable does not accidentally come in contact with the disconnected cable. See our tech article on battery replacing for notes on disconnecting and reconnecting the battery.
The alternator on BMW X3 models with a M54 6-cylinder engine is located on the left side of the engine (red arrow).
Start by removing the four T25 Torx fasteners (red arrows) for the fresh air intake duct. Next you will unclip the five air filter housing retaining clips. Lift them up and unhook from the lid of the intake air housing. Start with the two near the mass air flow sensor. Then release the front clips.
Locate the accessory (main) drive belt tensioner, then locate the 16mm HEX boss (yellow arrow). Using a ratchet with a long handle and a 16mm socket (green arrow), rotate the drive belt tensioner clockwise to release the tension. When tension is relieved, place a 7/32 pin or drill bit in the small hole (red arrow) above HEX boss to hold tensioner in position for belt removal.
Remove the bolt and the idler pulley together (red arrow) once the bolt has been loosened. If replacing idler, transfer bolt and bushing over to new idler (blue arrows). Then remove the intake air housing and ducts. See our tech article on intake air housing and ducts removing.
Working at the back of the alternator, disconnect the electrical connector (red arrow) by squeezing the release tab and pulling the connector straight off. Then remove the alternator positive (+) cable fastener (usually a 16mm) (blue arrow), and remove the cable from the alternator. Do not over-tighten this nut when reinstalling, it is steel mounted in plastic and can break (yellow arrow).
Now the alternator can be removed. If it is stuck, lever the alternator away from the engine. Working at the top of the alternator, lever the alternator away from mounting bracket. Next remove the alternator from its mounting bracket by rocking it back and forth while pulling up and off the bracket. Once you wiggle it out of the mounting bracket, remove the alternator from the engine compartment. Note the longer bolt is at the bottom (inset).
If reinstalling an old or used alternator, you may have to move the bushing (yellow arrow) out of the mounting ear for proper fit to the alternator bracket. The red arrow shows the bushing moved out of the mounting ear. To do this, place a 23 or 24mm socket (green arrow) between the bushing and the vise jaw. Slowly tighten the vise to drive the bushing flush with the mounting ear. Be very careful and do not use too much force. Damage to the alternator may occur. Install a new alternator in the alternator bracket. Then install the fasteners and tighten. Attach the electrical connections to the back of the new alternator and tighten. Install the drive belt and check that the belt is properly aligned in the alternator pulley. Then reassemble the remaining items in reverse order of the removing steps. Check the operation of the charging system and alignment of the drive belts. Reassemble the air filter housing, and the intake air duct. Connect the battery negative cable.