My '97 840ci Sport is FOR SALE


The water-cooled alternators fitted to the facelift E38's seem to be non-repairable and BMW do not sell individual parts for them. At just short of 800 for a new alternator and 2nd-hand units hard to come by, there is quite a need to repair them! Fortunately, they come apart quite easily (as long as you know how to do it - unfortunately I didn't, but at least now I do!), and the bearings are readily available from BMW.

Right, let's get it apart, please note that the pictures show parts that should have been removed but aren't, it was only when I got it completely apart that I realised how I should have done it.

Start by removing the pulley nut. You will need a 24mm socket and plain wrench. Stand the alternator on a wooden surface and strike the wrench with a metal hammer to undo the large nut. It is quite tight and may need a few clouts but it should come off with no problems. Note there is a star socket in the middle of the shaft, this could be used to hold the shaft in place, but I found that spinning the nut with the wrench quite easy:

Pull the pulley of the shaft, there is a washer above and below the pulley, remove both. Next, remove the four Torx screws on the plate below the pulley. These are very tight and have been locked with a thread-lock compound. Make sure that the Torx bit is a perfect fit before attempting to remove them:

Remove the B+ terminal and the nut below it:

The black cover can now be removed, this is stuck in place with a sealant but comes off quite easily. Remove the four screws shown here (please note that I did this in completely the wrong order, the black cover and pulley should be off at this point):

Remove the two Torx bolts that are on the outer rim and the back half of the alternator can be removed:

The rotor can be removed at this point:

The fault with my alternator was a rear-bearing failure, it has seized, overheated, and is now stuck on the shaft. I used a bearing puller to remove it. A replacement is available from BMW, it is a 32mm x 12mm x 10mm grooved bearing, BMW part number 12311739203:

The front bearing is larger and can be removed with a puller or knocked out from behind. It is a 52mm x 17mm x 17mm grooved bearing, BMW part number 12311747246:

The alternator has no brushes to wear out, if no output was seen when the alternator failed, first check that all the connections were correctly tightened, I have heard of the screws being loose that connect the field coils. The regulator is more complicated than that usually seen on a standard alternator, it has the ability to switch the output completely OFF, this is done when starting the car and when at full-throttle, under control of the DME. It can be removed complete with the connector by removing the two remaining Torx screws:

The regulator is available on various sites but is quite rare. The same is true for the diode block. When replacing the alternator, make sure that the seal is intact, although shown as a separate part on realoem, it is not available and you will get a bag of three smaller seals instead! Carefully remove the seal, lubricate it with silicone grease and then replace it:

All done, time for a cup of tea