E92 M3 alternator issue - strange issue

[eswaroop]

Hi

I have a 2012 bmw M3 60k miles a parasitic drain issue - which I have sort of narrowed down to the alternstor - I pulled the old unit yesterday ( it has a whining noise so needs to be replaced at some point ) - but when I tested using the typical diode test on my multimeter it tests fine sometimes ( 0.52V one way and no reading other way ) and then intermittently reads 3V on the open side - now I checked the brand new Valeo unit that I’m supposed to put in and it’s reading 0.53 on one polarity ( red to ground ) and 3 volt consistently on red to battery + post - I thought it was supposed to read zero / open in that direction - so now I’m a bit stumped - did I reallly have a bad alternator to begin with ? Or is the new one bad ? What is a good reliable test to check these alternators with the voltage regulators on them ?

[DrFerry]

Originally Posted by eswaroop

Hi, I have a 2012 bmw M3 60k miles a parasitic drain issue - which I have sort of narrowed down to the alternstor - I pulled the old unit yesterday ( it has a whining noise so needs to be replaced at some point ) - but when I tested using the typical diode test on my multimeter it tests fine sometimes ( 0.52V one way and no reading other way ) and then intermittently reads 3V on the open side - now I checked the brand new Valeo unit that I’m supposed to put in and it’s reading 0.53 on one polarity ( red to ground ) and 3 volt consistently on red to battery + post - I thought it was supposed to read zero / open in that direction - so now I’m a bit stumped - did I reallly have a bad alternator to begin with ? Or is the new one bad ? What is a good reliable test to check these alternators with the voltage regulators on them ?

As far as I recall the E9X Alternator has an internal clutch that disengages the alternator at wide-open-throttle (W.O.T.). Perhaps the clutch mechanism has failed and disabled operation internally. Additionally, the voltage regulator inside the alternator could be going south.

[RocketyMan]

Originally Posted by DrFerry

As far as I recall the E9X Alternator has an internal clutch that disengages the alternator at wide-open-throttle (W.O.T.). Perhaps the clutch mechanism has failed and disabled operation internally. Additionally, the voltage regulator inside the alternator could be going south.

Um...no.
Maybe you're mistaken for the "a/c compressor clutch."

These alternators have a BSD line going to them that effectively controls the primary coil windings, thus, potentially eliminating electrical load when WOT. This is controlled by the DME--which also employs a charging strategy from the IBS given battery state, as an example.

OP...this alternator DOES NOT have the traditional diode trio for rectifying current. It has transistors that can more finely tune set demands. This is why TIS explains that these alternators need to be connected up to an oscilloscope and measured.
Did you get a specific fault code pointing towards a defective alternator?

[DrFerry]

Originally Posted by RocketyMan

Um...no. Maybe you're mistaken for the "a/c compressor clutch."

Thank you for this information. In my case it's more like I confused one technology with another. You are right, the DME does the "controlling" of the alternator voltage output via the "Intelligent Energy Management" system logic that you mentioned above. However, the alternator also has a mechanical clutch inside the 8 rib drive pulley. This is mentioned in the very first post of anom3's alternator DIY and elsewhere on the M3Post and E90 forums wherein noisy (clunky) alternator noises are discussed. My feeling is the O.P. eswaroop might want to consider this aspect of impaired alternator output also.
.
https://www.m3post.com/forums/showthread.php?p=18687199
https://www.e90post.com/forums/showthread.php?t=1080557
.
Here's an article from Bosch and a couple of videos about alternator pulley clutch replacment.
https://www.boschautoparts.com/en/w/...-clutch-pulley
.

.

.

[RocketyMan]

Originally Posted by DrFerry

Thank you for this information. In my case it's more like I confused one technology with another. You are right, the DME does the "controlling" of the alternator voltage output via the "Intelligent Energy Management" system logic that you mentioned above. However, the alternator also has a mechanical clutch inside the 8 rib drive pulley. This is mentioned in the very first post of anom3's alternator DIY and elsewhere on the M3Post and E90 forums wherein noisy (clunky) alternator noises are discussed. My feeling is the O.P. eswaroop might want to consider this aspect of impaired alternator output also.
.
https://www.m3post.com/forums/showthread.php?p=18687199
https://www.e90post.com/forums/showthread.php?t=1080557
.
Here's an article from Bosch and a couple of videos about alternator pulley clutch replacment.
https://www.boschautoparts.com/en/w/...-clutch-pulley
.

.

.

Amazing! I'm not convinced tho that using the clutch pulley is the best strategy for "fuel economy" on the alternator as it seems to only help with overrun.

[Helmsman]

Originally Posted by DrFerry

Thank you for this information. In my case it's more like I confused one technology with another. You are right, the DME does the "controlling" of the alternator voltage output via the "Intelligent Energy Management" system logic that you mentioned above. However, the alternator also has a mechanical clutch inside the 8 rib drive pulley. This is mentioned in the very first post of anom3's alternator DIY and elsewhere on the M3Post and E90 forums wherein noisy (clunky) alternator noises are discussed. My feeling is the O.P. eswaroop might want to consider this aspect of impaired alternator output also.

I always found this claim by BMW (for the M3) strange. Following the alternator output with for instance Carly, it certainly generate lots of current regardless of how you drive. And why not, saving drops with an engine that consume at this level seems like...marketing bs? On top as we all know these old buggers also consume tons of power so better charge whenever possible.

[RocketyMan]

Sounds like a fresh can of over-promise and under-deliver.

Especially with these Grote LED headlamps...same story with ZKW.