Alternator voltage and brushes question

[pingu]

The OBD test #9 indicates that the alternator charging voltage is generally fluctuating from about 13.6 volts to 14.2V. But sometimes it'll dip down as low as 12.8V.

Does the fluctuation and/or the dipping down to 12.8V indicate that the brushes are marginal and will soon need replacing? Thanks.

[Dr. evil]

the brushes are basicaly a connection form the slip ring to the to the rectifier, if i remember correcly. you can check the bushes and make sure they aren't covered in oil or have chips in them. i don't mess with BMW alternators much but my best guess would be the voltage regulator.

[joshua43214]

Pull them out an look at them, replace them if they look at you funny. they are cheap.

[Tiger]

In what circumstances did it drop to 12.8V? You need at least 13.5V to charge battery up.

[Dr. evil]

you might won't to check the springs that push the brushes onto the slip ring too

[shogun]

If you are talking about an E32 750:
"Pull them out an look at them, replace them if they look at you funny. they are cheap."
NOT that easy.
it will take you about 4-5 hours, just did that last weekend.

[pingu]

It dipped down to 12.8V while idling. Normally, it fluctuates anywhere in the range of about 13.6V to 14.2V (with an average of, I reckon, roughly 13.9V). But an extreme fluctuation will dip it down to as low as 12.8V (lesser dips to about 13.2V).

From shogun's post, it seems that access to the alternator could be tricky. I'm inclined to get a new regulator and be done with the problem - look after your electricals and they'll look after you!

Thanks.

[shogun]

Might be easier in case of other engines, but he the mystery of the 750
http://bmwe32.masscom.net/sean750/Vo...eRegulator.htm

[Paul in NZ]

not surprised that at idle its only 12.8.....as you rev the motor up you will see that at some point arounf 1500-200 rpm the voltage will go up

[Tiger]

Do you have aftermarket stereo system installed? Was it on and thumping when you were measuring?