Testing a voltage regulator

[dweymer]

Anyway to bench test an external voltage regulator?

I am trying to determine if it is the VR or the rebuilt alt I just put in.

1974 911, marchal alt with external regulator.

red light of doom is on full time. I will check voltages later today and post here for a more accurate symptom list.

[Ian Comerford]

The external VR on my 79 SC went last year and the red light came on as well as the rev counter waving around like a mad thing. Initially I thought it was the alternator but then heard the battery fizzing after a drive. Checked the voltage at the battery with the engine running and it was about 17.5 volts, way too high. I think the answer to your question may be that you have to test it on the car, it would only take a few seconds of the engine running and would not damage the battery for that length of time.

Hope this helps

Ian

[dweymer]

mine is not charging enough

[jmohn]

Assuming you've already checked/cleaned all connections and gounds; you could "full field" the alternator. If you get 16-18 volts at the battery the alternator is fully capable which means the regulator is bad. If you do this #1 be careful, #2 know what you're doing, #3 be sure all electricals (lights, radio, etc.) are off, #4 don't run the engine with the alternator full-fielded more than a couple minutes.

Jerry M
'78 SC

[cak]

"full field" - is that connecting the black/blue wires, off the regulator, or something different than that?

chris

[304065]

Don't full-field the alternator unless you want to let the magic smoke out.

Magic smoke is what makes electrical circuits work. It flows inside the wires, making motors whirr and lights blink.

If, however, you do something like a full-field test, you will let the magic smoke out, and the alternator won't work!

How to test your VR at home:

1) Examine VR case. Should be white metal, say "Motorola" or "SEV."

2) Hold VR up to nose.

3) Inhale gently.

4) If you smell magic smoke, give the VR the "water test" by throwing it into a convenient lake or stream. If it floats, retreive it and use in the car. Bonus if this test can be accomplished in full view of some Sierra Clubbers.

Ahh, but seriously now Donnie, a bench test of an external VR is impractical, insofar as you would need to replicate the wiring from the car on the bench, including a battery for an appropriate electrical load. Spinning the alternator with no load on it is as bad as disconnecting the battery while the car is running-- likely to lead to your local auto electric shop owner getting a new boat.

However, it is possible to test it in the car, which provides an ideal laboratory for these things.

"Red light of doom on full time" can mean a number of things:

1) Your car thinks it's in Amsterdam. If GPS equipped, reboot. If an older model, stand in front of the car with a map of the USA and shout "ORLANDO!" at the top of your lungs. If that doesn't work, try. . .

2) Verify that the "blue wire" or D+/61 lead from the 14-pin connector on the electrical console is not shorting anywhere along its length. In particular, trace the wire from where the wiring harness passes through the hole in the fiberglass cooling shroud-- make sure there are no cuts or abrasions to the wire-- sometimes if there is slack the wire can lay across the heads and the insulation will melt, causing it to short and make your car think it is in Amsterdam.

3) If no obvious shorts, other than the ones you are wearing in the Florida sunshine as you complete this diagnostic, then check to see that you have continuity along the blue wire circuit by connecting one end of a voltmeter to the positive battery clamp (Seriously, you HAVE disconnected the battery, right? The B+ cable laying in the engine compartment is an 800 amp arc welder waiting to cause Death or Grevious Bodily Injury or Property Damage if it contacts metal. If it contacts the magnesium engine case your car will do an impression of the firebombs of Dresden, no kidding.) With that warning out of the way, use a long jumper to connect the positive battery cable (the one that is NOT connected to your battery) with one pole of the multimeter, the other pole on the blue wire where it would connect to the alternator, then turn the ignition to "run" (not START) and verify you have continuity on the ohms setting.

If your blue wire circuit has shorted somewhere, check first the electrical console, then behind the gauge. But it's unlikely.

4) What is your battery voltage, to the nearest tenth of a volt? Exactly?

5) If all that fails, report back here. Also search for "blue wire" for more information about the charging circuit.

Good luck!

[cak]

John,

Thanks for the response. I've already posted some details at

73.5 charging light problem

so I won't repeat them here...

I'll try some of that non-welding tonight.

chris

[RoninLB]

also I've full fielded many simple ignition coil systems without problems. It was either to test the system or avoid a big swim.

also I'd cast an eyeball on the regulator points for temp solution.

[studeb]

Originally posted by john_cramer
Don't full-field the alternator unless you want to let the magic smoke out.

Magic smoke is what makes electrical circuits work. It flows inside the wires, making motors whirr and lights blink.

If, however, you do something like a full-field test, you will let the magic smoke out, and the alternator won't work!

BIG SNIP

5) If all that fails, report back here. Also search for "blue wire" for more information about the charging circuit.

Good luck!

Wow, i go away for a couple of years, check back in and find this.
That was a good read, made my day John.

You could take the VR to a shop that can rebuild older alternators, they can test the VR for you. If it tests good, then you have to check your car.

The light in mine come on faintly, can only see it at night, and only when it is off idle. Makes for real short trips.

[RoninLB]

Full Field Damage Control

[cak]

That's very very nice.

I've learned to be pretty good at British smoke control, especially Lucas (but I'm also adept with Clear Hooters). I didn't expect those skills to be quite so critical when moving to Porsche...

[RoninLB]

Originally posted by cak


I didn't expect those skills to be quite so critical when moving to Porsche...

many pelicanheads installed a volt gauge. I actually have a digital and analog v ga.

[cak]

OK, give ... where's that gizmo from? Very neat idea, I can think of several vehicles that need one!