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pingu
05-02-2006, 01:44 PM
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
05-02-2006, 02:39 PM
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
05-02-2006, 03:16 PM
Pull them out an look at them, replace them if they look at you funny. they are cheap.
Tiger
05-02-2006, 03:21 PM
In what circumstances did it drop to 12.8V? You need at least 13.5V to charge battery up.
Dr. evil
05-02-2006, 04:16 PM
you might won't to check the springs that push the brushes onto the slip ring too
shogun
05-02-2006, 04:39 PM
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
05-03-2006, 12:10 AM
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
05-03-2006, 12:33 AM
Might be easier in case of other engines, but he the mystery of the 750
http://bmwe32.masscom.net/sean750/VoltageRegulator/VoltageRegulator.htm
Paul in NZ
05-03-2006, 01:26 AM
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
05-03-2006, 04:52 AM
Do you have aftermarket stereo system installed? Was it on and thumping when you were measuring?
pingu
05-03-2006, 05:37 AM
The stereo was off so it's not the pumping bass from the phat subwoofer causing a low voltage!

As for the suggestion that the low voltage might have due to the fact that the engine was idling when I was watching the OBD, I 've noticed that when driving (e.g. doing 1,500 rpm and above) the voltage is still pretty much in the range 13.6 to 14.2V, and still fluctuates. Each successive reading from the OBD is different.

Is the fluctuation normal? Thanks again!
joshua43214
05-03-2006, 05:53 AM
Low voltage that pops up when you rev the engine is the classic symptom of worn out brush's.
Tiger
05-03-2006, 06:22 AM
Normally, I do not see fluctuation... it is fairly steady +/- 0.2V at certain RPM. The higher the RPM. it will be higher voltage, yet 14.4V is highest... like new.

I get 14.4V at idle... high RPM has no effect on voltage fluctuation since I am at upper level.

It is possible that you might have a bad regulator... or bad battery.. bad battery will also give you lower reading too.

This is what I would do...at idle... as long you are above 13.5, you are fine. Below that, your battery will drain. OR you can take the car to Autozone and have them test your charging system for free.