540i intermittent charging

[yaofeng]

When my alternator quit there was no indication. I wonder why BMW with all of its engineering prowess giving drivers superfluous information on the check control fails to even provide an amber charge light when the alternator is not charging. That is why you daughter was stranded, as I was. Fortunately it happened to me in my driveway.

I learned however, that when you service the alternator, the battery must be disconnected first. At least on E34 you must, maybe on Bimmers across the board. I think that is the reason the alternator on my 540 was fried.

Sorry I can't help you but I think your alternator may have been fried too.

[DaveVoorhis]

No, I don't have a good schematic of this car; but, I am thinking of some relay or switch somewhere that must be at fault. I just don't know this car's wiring enough to know where to start looking.
Wiring diagrams: http://shark.armchair.mb.ca/~dave/BMW

[skills_ark]

Hey folks, I have a seriously irksome problem. My daughters 1995 540i has an intermittent problem where about every 2-3 weeks it simply quits charging and leaves her stranded on the freeway or in the city. I have replaced alternator based on a bmw dealership reccomendation, NEVER AGAIN. I have load tested the battery and the alternator and both check good. When it is charging it really charges. When it quits, I drive 2 hours to help and several times the very act of jumping it, causes charging to begin, last 2 times it didn't until I had worked on it an hour and towed it 10 miles to a foreign car shop. The ECU code shows 1231 low battery or DME relay. I have a haynes for a slightly older 3 and 5 series up to 92 and from the wiring diagrams, I don't get the DME relay reference or see, from the diagram, what could cause a failure to charge. The field should be excited by regulator but I thought that was inside alternator. I am stumped and without real good info on this one. I there some stupid lil' device that the feild current is sent through that is beginning to fail?

Any suggestions would be appreciated!!

[skills_ark]

I don't think it's the alternator, I have never heard of an intermittent alternator failure and, as an electrical engineer, I know these units are very unlikely to exhibit this particular type behavior as the parts inside are simply semiconductor pieces. No, I don't have a good schematic of this car; but, I am thinking of some relay or switch somewhere that must be at fault. I just don't know this car's wiring enough to know where to start looking. I believe that something interrupts the field excitation; but, I don't know what it's wired to. Any ideas regarding this?

[Dr. evil]

alt has a volt reg
#2
http://www.realoem.com/bmw/showparts.do?model=HE63&mospid=47410&btnr=12_0492&hg=12&fg=22

i would start looking here for any defects. Your going to have to get a bentley repair manual if you want any decent wiring digrams

[pingu]

There's a wiring junction underneath the B+ jump starting terminal in the engine bay. If you (carefully - don't want any shorts!) remove the cover from the B+ terminal then you could make sure that the (I think there are 3) bolts are all nice and tight. I'm not sure (and the wiring diagrams are very cryptic) but the alternator lead might join the +12V rail at the battery. I think yaofeng posted pictures of this terminal sometime ago so a search might deliver a piccy of the B+ terminal.

[pingu]

Are you sure that something interrupts the field winding? I'd be happy to be wrong on this, but I thought that the alternator only had three terminals: chassis, +12V and charging light signal. If that's the case then I can't see how the filed windings could become disconnected (especially given that you're on a different alternator).

[yaofeng]

I f you get down to removing it:

http://www.bimmerboard.com/forums/posts/363765

After taking the M60 alternator out a few times, I can do it in 45 minutes. It is still not easy. The trick is to remove the PS pump pulley and remove it from under.

[MBXB]

I f you get down to removing it:

http://www.bimmerboard.com/forums/posts/363765

[Ross]

The field current must pass through the unloader relay. Something funky happening in a relay wouldn't be a shock.

[skills_ark]

Thank you so very much so the awesome schematics that you linked here. I downloaded them and they are great. They confirm that the field excitation, and therefore the alternator charging function, is indeed dependent upon the unloader's relay contacts and a dashmounted charge lamp working well and for those of you who didn't think that the alternator's charging was dependent upon something other than the alternator, check out the little resister that is paralleled with the charge light, Wanna quess why it is there? If you removed that and the bulb in that lamp went out, I don't think you could saturate a stopped field to initiate the charge. In other words, the alternator may or may not start charging, I'll let everyone know what I find when I get the car home to study it.