swapping alternator diode rectifier plate

[Robert Bondi]

Anybody done this before? I've been chasing a weakness in the charging system on the 530i for months. The symptom is a very faint flow of the battery light. It's so faint it actually goes unnoticed unless it's night. This old post is similar, but never presents the actual solution.

www.firstfives.org/wwwboard/messages/51497.html

I run a Bosch AL46X 80A alternator, typical of what came on the early 80s M30B32 engines. I've gone through all the usual stuff and one day in February the batt light went full on while driving. At that point, I figured it was actually the 12-year old alternator and got a fresh Bosch rebuilt. We're now back to very faint battery light again. I'm starting to think I've got bad diodes after seeing that discussed in quite a few posts and the FAQ.

Recent testing:
1. D+ terminal often measures 0.5 to 0.9V higher than B+. We're talking 13.5V to battery, 14.4V from regulator, so it's never quite the same and hence the light has a bit of voltage across it.
2. With the multimeter, things do look different between B+ and D+ with the alternator back out of the car. Notably, on the diode setting, B+ and D+ don't look the same and I even see some conductance both ways for D+.
3. I bought a separate rectifier plate. It actually does measure as expected...B+ and D+ terminals look nearly the same and only conduct out of the alternator.
4. I just brought it to the local O'Reillys today and the alternator passed their benchtop test. However, I think that just means it's rated at 80A and it can produce 80A.


Perhaps I'm answering my own question, but I have been a bit hesitant about opening the alternator and also a bit bothered that this is what Bosch is producing. It also seems possible that if different diodes are used for B+ and D+, it might always keep that battery light glowing, while the unit still works. I wonder if that's what this is.

Just curious if anyone else has gone through something similar? I could exchange the unit, but I feel I'll just get the same thing back. I also invested a couple hours pressing in urethane bushings for the alternator mounts, so next move probably is opening the case and seeing if I can swap in the new rectifier plate I have.

[528i-1981]

I'm guessing, but I think you've got bad diodes, given the bidirectional conductance on D+ and the voltage discrepancy. Replacing the rectifier plate should resolve the faint battery light and stabilize the D+/B+ voltages unless there are other problems with the alternator. I might exchange the unit and see if the situation resolves before performing surgery.

[Robert Bondi]

I'm guessing, but I think you've got bad diodes, given the bidirectional conductance on D+ and the voltage discrepancy. Replacing the rectifier plate should resolve the faint battery light and stabilize the D+/B+ voltages unless there are other problems with the alternator. I might exchange the unit and see if the situation resolves before performing surgery.

Yeah, might be worth checking with FCP Euro, especially as they claim to guarantee parts for life and they were suprisingly quick. I'm still wondering how it passes the store bench top test with bad diodes, but I'm kinda thinking that test only really checks what's happening at the main B+ output.

[528i-1981]

I think the basic test would probably check output and ripple. I don't know how your voltage delta would translate to the peak-to-peak voltage measured in a ripple test, but it may be just below the threshold or the operator didn't think ripple was excessive. I think the testers at auto-parts stores are pass-fail type things. An advanced diagnostics shop could provide a definitive diagnosis but by this time you could probably have already swapped in your new rectifier plate.

I would probably still swap it with FCP Euro if you have that option. Once you open it up and desolder connections you've converted your exchangeable unit into a core. I'm certain you're up to the task, though, either way.

[Robert Bondi]

I think the problem is solved. I acquired a second rebuilt AL46X alternator from FCP Euro and it was electrically the same, so I decided to fix one of them. My interpretation of all this is that Bosch is not rebuilding the alternators to original specs. The rebuilt alternator charges and passes an auto parts store bench test out of the box. However, I think it's also possible to have new, good pull-up diodes on the D+ circuit, but if these are not appropriately speced for voltage drop and resistance to match the B+ power diodes, then there is always a bit of voltage difference between B+ and D+ and there's always a bit of glow. Even after my fix, I can still detect a faint glow if the background is dark enough, but I have significantly lowered the B+/D+ voltage difference. I suspect everybody has a little bit of this, but usually goes unnoticed.

Multimeter Diode setting measurements (one test lead on terminal, other lead on case or N diode plate)

Suspect rectifier plate with the rebuilt alternator
red lead on B+: overload
blk lead on B+: 0.985V
red lead on D+: 0.800V
blk lead on D+: 0.545V

Separately purchased rectifier plate (https://www.maniacelectricmotors.com/)
red lead on B+: overload
blk lead on B+: 0.894V
red lead on D+: overload
blk lead on D+: 0.922V

Some summary pics of my repair:

Rectifier plate isolated


Comparison of new and old rectifiers. Initially, I was bothered that the new rectifier has two extra diodes, but I eventually
was able to determine that they actually aren't connected to anything for this particular alternator application. The base of those power diodes are those dime-sized circles.


The D+ pull up diodes on the suspect rectifier are strangely encased in a non-conductive black coating...not really even sure if they were renewed.


The D+ pull up diodes are clearly visible under the positive plate on the new rectifier. These are lighter duty diodes more typical of a circuit board application. Silver band on the cathode side of one of these is clearly visible.


Stator and windings desoldered from the rectifier


New rectifier soldered in place. I made sure to keep the 3 phases straight with dabs of colored paint .


With the car running , the new rectifier brought the B+/D+ difference down to about 0.1V seen at various operating conditions. That B+/D+ difference was previously approaching 1V with the suspect rectifier plate.

[528i-1981]

Bravo, Robert, your meticulousness is always appreciated. Do you know if this would be the same with the AL41X on the 1979-81? Is that merely a 65-amp variant of the AL46X?

[Robert Bondi]

Bravo, Robert, your meticulousness is always appreciated. Do you know if this would be the same with the AL41X on the 1979-81? Is that merely a 65-amp variant of the AL46X?

I suspect these Bosch alternators are fairly similar and you could follow the same procedure with the appropriate rectifier plate for the AL41X.

Unfortunately, I may have spoken too soon and there is more work to do. On my second drive with the new rectifier plate, I experienced apparent issues of AC ripple....pulsing lights, dancing voltmeter needle, and later measured about 0.25V max on the AC setting on my multimeter. The problem seems to be temperature sensitive....non-issue from a cold start, but all those symptoms when the car was fairly hot.

Looks like Maniac Electric Motors will be sending me another rectifier. These guys seem knowledgeable, but they're hard to reach, so that's also been a test of patience. I'm probably also going to try another source for an alternator....no stock at a lot of places that usually carry them and obviously the quality of these diodes is getting very suspect. More to do and more money to spend....

Testing again after that last driving experience. I'm also seeing shorts in the D+ diodes now.


Aftermarket maker of the rectifier plate

[Robert Bondi]

Follow up on the alternator replacement:

I sourced a third rebuilt unit of this 80A alternator direct from Germany. Unlike the Bosch units, the voltage difference between the B+ and D+ terminals is about 0.1V at various operating conditions, which is definitely better. It seems to be charging the battery adequately. Imperceptible in daylight, but there is still a faint glow that can be seen at night on the battery dash light. I think I'm going to leave that alone at this point. Maybe I'm the only person that drives their car at night to notice. It's also possible it has always been there and I only got tuned into that recently.

I also took apart the Bosch unit with my installed rectifier plate. I didn't see anything obvious, but I didn't like how close one of the power diode contacts was to one of the 3 phase connections to the plate. While there is insulation on both of them, the insulation has enough wear on both that I could reason that they might contact and provide a short maybe when heated up. Just a possibility, but I'd rather it be a shortcoming on my installation procedure rather than having experienced so many bad parts on one project.