Alternator diode test

JWT4700 · 08-11-2020, 09:40 PM

Can someone explain this a little more?
I just watched this vid on how to test alternator and gave it a shot. It's tested ok as far as only getting a reading one direction, but my reading came back at about 350, not the specified 500-800. What exactly does that mean?

I'm still chasing a battery drain issue as well as a bsd issue and wondering if this just might be it?

Anyway here is the video

mksher · 01-08-2021, 03:07 PM

have you found anything else out? I'm having a similar issue. my alternator tests in the low 400s but the car dies after sitting for 2 days. what BSD issue are you having? I have a code for "BSD error water pump missing" and "BSD oil sensor missing" but both seem to be fully functional

TunafishE93 · 01-08-2021, 03:39 PM

The diode test you will get different readings with different meters. Its hard to diagnose a bad diode in the way in the video since they are all getting tested together. The main thing in the video is you are not getting readings both ways.

Then you would have to test each diode individually and compare the readings to each other. If you have a bad one it will show up as a different reading than the rest. Testing them as a group wont always show if there is a leaking diode.

Ea$y · 09-09-2022, 07:31 PM

Where are the diodes, are they in the voltage regulator or in the alt?

e90yyc · 09-09-2022, 08:58 PM

Quote:

Originally Posted by Ea$y

Where are the diodes, are they in the voltage regulator or in the alt?

You know those spikes they use at the entrance to some parking lots to prevent cars from going the wrong way? A diode exists for a similar purpose: to prevent electrical current from flowing in the wrong direction.

They are not in the voltage regulator; they are in the diode bridge at the backside of the alternator.

Ea$y · 09-10-2022, 11:54 AM

Is there a way to replace the diodes without replacing the alt? From a practical stand point.

Brian86 · 09-10-2022, 03:09 PM

It's not really practical to replace them.

It's a very compact arrangement with the diodes more-or-less integrated into the stator windings and soldered to cast plates at the on the back side of the alternator.

Not impossible, but not practical.

08-11-2020, 09:40 PM	#1
JWT4700 Private 22 Rep 95 Posts Drives: 09 328i Xdrive sedan Join Date: Dec 2016 Location: Meadville, PA iTrader: (0)	Alternator diode test Can someone explain this a little more? I just watched this vid on how to test alternator and gave it a shot. It's tested ok as far as only getting a reading one direction, but my reading came back at about 350, not the specified 500-800. What exactly does that mean? I'm still chasing a battery drain issue as well as a bsd issue and wondering if this just might be it? Anyway here is the video __________________ E90: 09 328xi N51 E46: 05 325xi M54
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01-08-2021, 03:07 PM	#2
mksher Registered 0 Rep 1 Posts Drives: bmw e92 335i, bmw e36/7 z3 Join Date: Jan 2021 Location: atlanta ga iTrader: (0)	have you found anything else out? I'm having a similar issue. my alternator tests in the low 400s but the car dies after sitting for 2 days. what BSD issue are you having? I have a code for "BSD error water pump missing" and "BSD oil sensor missing" but both seem to be fully functional
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01-08-2021, 03:39 PM	#3
TunafishE93 Major 418 Rep 1,154 Posts Drives: BMW 335i Join Date: Dec 2019 Location: NV iTrader: (0)	The diode test you will get different readings with different meters. Its hard to diagnose a bad diode in the way in the video since they are all getting tested together. The main thing in the video is you are not getting readings both ways. Then you would have to test each diode individually and compare the readings to each other. If you have a bad one it will show up as a different reading than the rest. Testing them as a group wont always show if there is a leaking diode.
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09-09-2022, 07:31 PM	#4
Ea$y New Member 5 Rep 22 Posts Drives: 328i E93 '09 Join Date: Apr 2010 Location: Easley,SC iTrader: (0)	where are diodes? Where are the diodes, are they in the voltage regulator or in the alt?
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09-10-2022, 11:54 AM	#6
Ea$y New Member 5 Rep 22 Posts Drives: 328i E93 '09 Join Date: Apr 2010 Location: Easley,SC iTrader: (0)	Is there a way to replace the diodes without replacing the alt? From a practical stand point.
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09-10-2022, 03:09 PM	#7
Brian86 Captain 638 Rep 766 Posts Drives: e36 M3, e91 325xi, N50 Xterra Join Date: Dec 2020 Location: Pittsburgh iTrader: (0)	It's not really practical to replace them. It's a very compact arrangement with the diodes more-or-less integrated into the stator windings and soldered to cast plates at the on the back side of the alternator. Not impossible, but not practical. __________________ Sometimes a bolt is just a bolt.
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