The other night I noticed an unusual whirring noise from the engine area on my 1998 BMW 318is coupe. I assume it was one of my idler pulley wheels which I have previously noticed as being noisy and needing replacement.
The following morning I took the main serpentine belt off to check and couldn't see anything wrong although the two idlers were a little noisy and worn. A job for another day.
However, later that day when I tried to start the car the battery was completely dead. It measured 3 volts but as soon as I disconnected it, started climbing back to 12 volts.
I measured the current draw with everything off and the key out and it was a whopping 3.5 amps!
After a couple of hours spent trying to diagnose the problem over the next few days, and some crazy theories I had about the starter solenoid being permanently energised (not possible, I'm told), I stumbled on to a problem that can happen to alternators where a diode fails and causes excessive current draw and a whirring noise. Bingo!
I took out the alternator and tried to test it, but couldn't find any useful test procedures online for this type of alternator (Valeo). I found someone reasonably local parting out a later model E36 318i (the M44 alternator is the same as the M43 and some M42) and bought it for a very reasonable A$50. It turned out to be a Bosch part, which I believe are better.
After fitting the replacement, the current draw was back to normal (a small fraction of an amp) and the whining was gone. Job done.
Replacing the alternator is quite easy, although with my car being supercharged access is actually a little better. You need a Torx bit and a quite a bit of force to release the tension on the belt tensioner so that you can remove the serpentine belt. Then remove the idler that is attached to the alternator as it blocks access to one of the alternator mounting bolts. You will need a 16mm socket for this (or 5/8"Â). Then you can remove the two alternator mounting bolts (also 16mm) - the nut at the back is affixed to the alternator itself so no need to lock the other side. You may find it easier to unbolt and move the power steering fluid reservoir (two 10mm nuts) to get out the lower alternator bolt. The plastic cover at the back of the alternator pulls off easily to expose the two wires, one held on by a 13mm nut and the other 10mm.
Installation is the reverse of removal, but I always find the alternator very hard to get back in to place and line up. For this reason I would suggest use a fine sandpaper to clean up the mounting faces on the bracket and the alternator and use some grease to make it slide in easier. It is best not to use a hammer to get it into place as this can damage the alternator.