Bad alt diode?

[project 911]

Hi all-

Just started having a charge problem on my 70T 2.2 carbed. External voltage regulator.

I think I left the key in the partially on position for an hour or two, and now my volt meters (1 analog 1 digital) are showing a random (not related to RPM's), momentary drop to less than 13 volts. The meters wander between 12.8 and 13.8, but just flicks down in the 12's for a second.

I've gone through a few VR's, but usually when they go I get very high voltage spikes. In this case it isn't going high, but low and wandering. I've gone through most all of my connections, grounds, etc, but still have the same problem.

Does anyone know what the symptoms of a bad alternator diode would be?

Thanks for any help.

[304065]

If a stator diode blows, it usually blows closed, resulting in backfeed of current from the battery, through the primary cable to the starter, to the B+ terminal, then the wrong way through the diode into the stator, then then the RIGHT way through the trio diodes into the D+ wire, then back to the warning lamp and into the ignition switch, then into one of the accessories also connected to ignition terminal 15, then through the accessory, then to ground! Lighting the warning lamp when the key is OUT! Whew!

Of course there are three stator diodes. A failure of one OPEN will make that phase of what's supposed to be a flat DC waveform drop out, resulting in a scallop. You could see this with a scope. Here's a good link to a page with some bad diode waveforms illustrated.

http://www.omitec.com/en/support/waveforms/

In any case, at very low RPM, this would probably cause a flutter in the needle. It's something else I expect.

Leaving the ignition on with the points closed can burn out your CDI.

Why is the meter wandering? Have you removed all the oxidation from the spot where you are taking the measurement?

My guess is you have a bad ground somewhere that's intermittently coming in and out as the car vibrates.

[rick-l]

It sounds like what is happening is the field current is being interrupted and the output of the alternator momentarily going away. I would look at the regulator.

[project 911]

It does seem like it's just loosing contact momentarily, but so far I haven't found any loose connections. I had always heard that high voltage readings indicated a bad regulator, and that low readings indicated alt trouble. I've gone through a few NAPA solid-state VR's, and when they go I get really high readings.

This car just barely puts out enough juice to run at night with the lights on, but with this random voltage drop I'm having now, I don't trust driving at night.

It's going to rain all weekend here, so maybe I'll just swap out the alt with my backup and see what happens. And recheck all my grounds...

John- Is there a simple test using a multimeter to check the diodes?

Thanks for the input.