voltage regulation

tbuck92 · 12-16-2018, 04:46 PM

Hello,
I have a 2008 328xi that the battery doesn’t seem to be charging in. I have a video of the voltage at idle with no accessories running. The car has gone into limp mode once due to low voltage. An IBS Error code does come up when scanned but I’ve tried the car with it connected and disconnected and there is no difference. The alternator is relatively new but it is a rebuilt which i told myself was a bad idea. A lot of matainence has been done recently along with a new battery so I’m not sure if this is because of the alternator/voltage regulator. The advanced auto part guys tested the alternator and battery and they came up ok but who knows. If it’s the alternator I will put a new non rebuilt in. The voltage seems to be fluctuating quite a bit though at idle (IBS connected).

Video of voltage at idle:

ctuna · 12-16-2018, 05:18 PM

If you have a lead acid battery the alternator voltage should be above
14 volts around 14.5 average.

tbuck92 · 12-16-2018, 05:44 PM

It is an AGM battery.

PhaseP · 12-16-2018, 06:55 PM

Disconnect the small wire that goes to alternator, and try to see if you get steady voltage that way. Small wire is how the ECU controls the alternator voltage regulator to tell it what voltage is should set its output.
When disconnected alternator goes to default mode like a regular alternator, should have steady output about 14 V.
If with the small wire disconnected you still have the same fluctuations it is your alternator. The slip rings may not be making connection all the time on the brushes. A rebuilt one should have replaced both the slip rings and brushes but you would never know.
Another thing to try is to increase the RPM while in neutral, watch if the voltage gets steady at some RPM range. If this happens again it shows problem is in the slip rings and/or brushes.

If the voltage is steady with that small wire disconnected, problem is most probably outside of the alternator. It could still be the voltage regulator though. The Valeo alternator in my car is very easy to replace the voltage regulator with a new now. Don't know what you would have.
New voltage regulator comes with new brushes. But not new slip rings, that requires a rebuild.

12-16-2018, 04:46 PM	#1
tbuck92 Registered 0 Rep 3 Posts Drives: 2008 328xi Join Date: Aug 2018 Location: Boston iTrader: (0)	voltage regulation Hello, I have a 2008 328xi that the battery doesn’t seem to be charging in. I have a video of the voltage at idle with no accessories running. The car has gone into limp mode once due to low voltage. An IBS Error code does come up when scanned but I’ve tried the car with it connected and disconnected and there is no difference. The alternator is relatively new but it is a rebuilt which i told myself was a bad idea. A lot of matainence has been done recently along with a new battery so I’m not sure if this is because of the alternator/voltage regulator. The advanced auto part guys tested the alternator and battery and they came up ok but who knows. If it’s the alternator I will put a new non rebuilt in. The voltage seems to be fluctuating quite a bit though at idle (IBS connected). Video of voltage at idle:
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12-16-2018, 05:18 PM	#2
ctuna Lieutenant General 1911 Rep 13,103 Posts Drives: 325xi 06 wagon MT Join Date: Jan 2010 Location: Santa Cruz, Ca iTrader: (0)	If you have a lead acid battery the alternator voltage should be above 14 volts around 14.5 average.
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12-16-2018, 05:44 PM	#3
tbuck92 Registered 0 Rep 3 Posts Drives: 2008 328xi Join Date: Aug 2018 Location: Boston iTrader: (0)	It is an AGM battery.
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12-16-2018, 06:55 PM	#4
PhaseP Colonel 1007 Rep 2,108 Posts Drives: 325XI Join Date: May 2010 Location: Earth iTrader: (0)	Disconnect the small wire that goes to alternator, and try to see if you get steady voltage that way. Small wire is how the ECU controls the alternator voltage regulator to tell it what voltage is should set its output. When disconnected alternator goes to default mode like a regular alternator, should have steady output about 14 V. If with the small wire disconnected you still have the same fluctuations it is your alternator. The slip rings may not be making connection all the time on the brushes. A rebuilt one should have replaced both the slip rings and brushes but you would never know. Another thing to try is to increase the RPM while in neutral, watch if the voltage gets steady at some RPM range. If this happens again it shows problem is in the slip rings and/or brushes. If the voltage is steady with that small wire disconnected, problem is most probably outside of the alternator. It could still be the voltage regulator though. The Valeo alternator in my car is very easy to replace the voltage regulator with a new now. Don't know what you would have. New voltage regulator comes with new brushes. But not new slip rings, that requires a rebuild.
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