My 1983 300D has a battery drain and shows 30VAC on the battery, no matter what alternator I have in there. And they all supposedly pass a diode test (well, I have brought 2 of 3 in, and they've passed). Read on for details.
I learned from threads like this one and this one that when an alternator is putting out more than a few hundred milliamps of AC current (measured across the battery), that means the diodes are bad. That was good news for me because it explained my crazy battery drain, even with no measurable amps drawing from the accessories. So, I went to the pick-n-pull and got myself an 80a alternator upgrade from a 1989 Saab 900, AND a nice, thick battery cable (4 gauge, I think) to wire it up (because the two 10 gauge wires for the stock alternator wouldn't be able to handle the 80a of this one or especially the 115a of the one I planned to get later--which turned out to be two days later).
On the way home, I stopped off and got the 80a alternator tested at that one ubiquitous parts chain where we all know we can get free testing done, but we also would never purchase any of their crappy parts. The alternator passed, including the diodes. When I got home, I put it in the car, started it up, and was delighted to see 14.2VDC coming across the battery. However, I was sad a minute later to see 30VAC coming across. I shut the car off, and yes, it did drain about .8VDC overnight.
Yesterday, I was sick and stayed in bed all day (alright, I will admit to getting up to take some voltage readings).
Then today, I went to another junk yard and got another alternator. This time, it was the 115a upgrade I wanted, this time from a '94 Saab 9000. On the way home I stopped off and got the 115a alternator tested at that one ubiquitous parts chain where we all know we can get free testing done, but we also would never purchase any of their crappy parts. It was, by the way, a different location. It tested as good, including the diodes.
So, I installed the 115a alternator, found it putting out 14.2VDC... and 30VAC.
I got back in the car, went back to that one ubiquitous parts chain where we all know we can get free testing done, but we also would never purchase any of their crappy parts (a third location), and I got both the 80a alternator bench tested and the 115a alternator tested while in the car. In both cases, I was assured that the diodes passed the test without any indication of trouble.
What gives?
I watched this video on how to test the diodes, and both the 80a and 115a alternators give a reading of something like .23 (I don't even know what that unit is) on my multimeter. I mean, if I put the red lead on the B+ (I think that's what it's called) and the black lead on the case, I get .23 or so. Then, when I switch the black and the red, I still get .23 or so. This is true for both alternators. I don't know what that means.
Any idea what's wrong here? Do I have three alternators, all with bad diodes that are bad in a way that the ubiquitous chain store just doesn't see? Is there some other explanation for why my battery shows 30VAC, regardless of which alternator is in there?
Notice that, between the stock alternator and the 80a alternator, I also upgraded the cable that goes to the battery. I've also used three different voltage regulators (one old and questionable, one old but looking good, and one brand new).
Finally, will I damage my battery or other electrical components by continuing to run an alternator putting out 30VAC? If not, maybe I'll just get a battery disconnect thing.
Please help.
Thanks.
Mike