Alternator(s!) producing 30VAC, diodes test good

[Mike Rancourt]

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

[porkface]

the diodes only control voltage direction, not type. the rectifier bridge changes ac to dc. double check your testing, try it on another car. good luck, chuck.

[RBYCC]

Why are you using the AC scale on a DC circuit ?
You're reading AC scale at the battery..
AC setting is RMS which can be erratic, depending on the meter, and even double if you are reading a DC circuit.

30VAC would be a good reading if you are testing the stator at the alternator, but such a reading at the battery would probably be a meter error.

[Mike Rancourt]

Why are you using the AC scale on a DC circuit ?

The first thread I linked-to above, message #3, reads, "set your meter to Volts AC and you should have a reading of ~100mVAC. If the VAC is too high, then look for a rectifying diode failure." This statement is pretty common. Maybe I'm misunderstanding it, or maybe there are a lot of people online who are wrong about it.
Here's another thread with a few people giving more detail on diodes and AC measured at the battery. The user named Compress Ignite sums it up in post #5, writing, "A DMM set to A.C. voltage (With the engine running) that shows anything more than .5 Volts AC (At the Battery terminals) denotes a Bad Diode."
I like the idea RBYCC suggests, that "30VAC ... at the battery would probably be a meter error." However, many other folks online seem indicate otherwise.
Porkface writes, "the diodes only control voltage direction, not type. the rectifier bridge changes ac to dc." I have gleaned that diodes are directional, and I don't claim to be able to explain why their failure would cause AC readings at the battery. I just believe it because I've read it online and matched the description of their failure with what I'm seeing on my car (two failures described: 1. battery draining apparently through the alternator 2. 30VAC measured at the battery; both are occurring on my car).
Thanks for the responses.

[oldtrucker]

Hi Mike,
I haven't read the thread you referring to, but have a strong feeling that it says something about the Diode Rectifier removed. If the rectifier is removed and you spin the alternator, it will supply an AC voltage. I have absolutely no idea why you should read 100mv AC other than, that most Digital Volt meter read some ambient voltage which could be in the order of 100mv.
With all rectifier installed and measuring across the battery you should never be misled and measure AC voltage. The battery stores DC only, period.
I will try to explain what these people trying to guess with the AC measurement at the Battery.
A Diode is basically a switch that lets a current pass in the forward direction. While this current flows through the diode, a voltage drop will occur across the diode. This Drop is in the order of approx. 0.6 Volts. This is what these people trying to detect at the Battery. However this also can cause you to be misled into believing that all is A OK or it does only work if the Diode is in good working order.

If the Diode is shot short circuit, you will see the Alternator's AC voltage at the Battery and your Battery will die if not fixed.

If the Diode is shot open circuit, you will see nothing but noise at the battery and the value depends on the meters accuracy. In this case the Battery will be drain to the bottom very soon, since it does not get recharged.

To completely understand it, one would have to connect an Oscilloscope to see the wave form in Graphically display.

The only correct way to test the Diodes, is to take the alternator out. Remove the Diodes and measure it with an Ohm Meter or Diode tester.

Hope this helped a little.

I read your thread a little further down. When you put the leads across the B+ terminal an to the chassis, are you using AC Volts or is it a Resistance reading?
Because AC Voltage is absolutely useless whilst the Alternator is not moving.
Again, there is people that think they found a way to simplify the troubleshooting but did only half of it because to one situation there can always be multiple other situations. Now you need to interpret this and see if it makes sense, but you almost need to have an degree in electronics to understand what just happened.

Oh one more thing.
If you read 14.2 Volts as the engine idles, just forget about everything you read, you are just fine.
If the Battery drops over night to 14 Volt "DC" you are fine, in fact the battery can go down to 12.8 Volts "DC" and is still in good shape.

REMEMBER: The batteries nominal Voltage is 12VDC

Never use AC to measure the condition of you battery, it will only confuse you!