VCCA Home Forums TECHNICAL - GENERAL Electrical - 6 volt Generator putting out negative voltage

This post is longer than it probably needs to be, I'm adding the details to establish I'm not a nutter. I have 2 '54 generators, 1 from my truck the other from a Corvette, that needed work to get running. I have confirmed their provenance not based on just their tags but on their characteristics, researching both online and from local club members. I am not an electrical engineer nor been professionally trained, but I have been working with 6 volt systems since I bought my first car when I was 16 back in 1976 so I am comfortable with my skills (it's a 1948 Chrysler Windsor, positive ground but 6v is 6v).

Anyway, the generators needed very little in the way of repairs, mostly cleaning up and installing repair kit parts; the truck's generator needed a new armature, sourced and replaced. Each came with its matching voltage regulator, in this case I did rely on the part number stamped into their bases to confirm. The Corvette is in the paint shop and the truck is running on an alternator that I stuck into it so I can drive around while taking my time on the generator, so I set up a bench testing station using a drill press motor and pulleys to supply the power. Yes, I checked and double checked, running clockwise when looking into the pulley of the generators. I pulled the battery from my truck, put the multimeter on it for confidence and got 6.54 volts, no surprise but no harm checking. I polarized the truck generator by grounding to negative on the battery and swiping a jumper wire from the battery's positive to the Field coil (confirmed and reconfirmed it was the Field and not the Armature). I grounded the VR to the body of the generator and did some minor tinkering to the VR. I put the multimeter on first the Gen and then the Bat terminals at the VR and got 7.5± volts on a sustained basis at each of the 4 speeds from the drill press pulleys. Same story with the Corvette generator only the VR needed no tinkering. I was temporarily pleased with myself.

Then it occurred to me that I hadn't gotten 7.5 volts from the Gen and Bat terminals, I got -7.5 volts (I think). I hooked everything back up, ran through the same setup (I even polarized the generators again even though I was sure the residual would be sufficient). Sure enough, negative voltage from each generator. Even though I know there is nothing in the VRs that could reverse polarity, I swapped them (Corvette's VR to truck genny, truck's to Vette--they are both rated for 45 amps so I was not concerned) and got the same result. I tested the voltage at the generator instead of at the VR (like that could matter but what the heck). I turned the truck generator around so it was running backward, got no voltage but my curiosity was satisfied. Out of ideas, I pulled a couple of spare VR's off the shelf, both worked to put out the same voltage but still negative. Nearly blind with fury, I got out my spare generator for my 37, added a bulb for resistance to the Field coils, and sure enough, -7.5 volts. One more thing, during this process, I got out my old multimeter just in case there was a gremlin in my new one, double-checked at each stage with the same result.

So now I firmly believe that the output is not random, but unless there is something about DC current that is new to my understanding, it can't possibly be right. What am I missing?

Hi Eli,

This one has me scratching my head also. Many thanks for all the details. I agree that it sure seems like you are doing things correctly.

Here are some random ideas to consider. Please don’t be offended if they seem too basic.
- You do not mention if you have a battery connected in your test circuit set-up. I wonder if the results are different if you connect a battery that is properly grounded to the generator and VR.
- What voltage are you getting directly from the generator with the field coils energized and the generator body grounded to the battery?
- Where are you putting the negative lead for your meter when you take your readings?

Hopefully the real experts will weigh-in and offer the solution.

No offense taken, when you think about it, it HAS to be something basic.

-The battery positive is not connected, I used it only to polarize. Right now I am hesitant to put it in the circuit until someone gives me the go ahead. My turn to apologize, it doesn't sound like you've had experience with the same situation and you are advising me to do so. I feel like I could really fry something (probably the cutout) if I do it.
-It's the same voltage at the generator as at the VR, 7.5± volts.
-I alternately grounded to either end of the ground jumper from the VR to the generator.

Appreciate the thoughts and suggestions, but I think it's going to take one of the experts to talk me off the ledge, I just cannot understand this.

Eli,

You are correct that I have not had this experience. I agree with your conservative approach.

My suggestion to add the battery is based on the test rigs I have seen at various real auto electrical repair shops. The ones I have seen have a battery in the circuit. Otherwise there is no power to energize the field. My thinking is that the voltage you are reading is just due to the residual magnetism in the field coils from the polarization.

Let's see what the real experts offer.

Well, I put the battery into the circuit but left the positive disconnected until I had everything running. I tapped the positive to the Bat terminal and the generator noticeably lugged, which was no surprise since it would obviously spin much more freely with no load. After trying it a few more times, I connected the positive instead of just tapping it and it put out the proper voltage for about 10 seconds and went kaput. I put on another VR and the generator is now putting out positive voltage, but I'm not going to put the battery back into the circuit until I hear a few more opinions--I hope.

Somehow or another, if it was truly putting out negative voltage, the generator was polarized backwards. That is the only answer. A regulator cannot do it. Many generators can work fine with no modifications in a wrong-polarity car if polarized backwards.

I am not near any books right now, so can't look it up, but I would verify the polarizing procedure for Chevrolet. It is probably different that the Chrysler. If it's backwards it still needs to be re-polarized

P.S. you cant expect it to regulate properly without a battery. Be absolutely sure to polarize it right before starting the engine.

A silly question so dont be offended but are you sure the meter leads are connected correctly???? If the meter is connected reverse it will read reverse is why the consistantly getting negative reading.
I have been caught with a similar situation.
Tony

This text is from a 1931 Chevrolet Repair Manual. It is older, but probably same problem, same solution.
Mihaly

It is good practice to polarize the generator whenever the generator, battery or regulator has been replaced or disconnected so you know the polarities are correct. Connect everything properly and then polarize.

In general, the car should not be ran without a battery as it acts as a ballast. Without a battery the voltage can run very high and damage other components and the generator itself. Less risky with a regulated system, but I'd be tempted to ignore everything you've measured without a battery in the system and start fresh with a battery in there.

It is very simple, you polarized the field backwards. The filled terminal is grounded when it is charging. Ground the field, touch the armature with the positive battery lead and everything will be good.

Thanks everyone, I'll get back to it next weekend.