Positive or Negative Ground

I have the riddle for the day........I'm stumped.
1937 Diamond T we're working on for a friend. Owner says both the starter and the generator have issues. So we pulled them out along with regulator and sent them to a local guy that has extensive knowledge of old electrical stuff. He says the starter looks like it has been rebuilt except the lug where the battery cable goes on was jacked, so he fixed that. Next he asked if truck was negative or positive ground? We look under it and see there is no battery in it. (this thing has not been started in years) I go online and see that these ole' Diamond T's were positive ground. The owner drops by and he says it's positive ground. Then the guy who's going through the generator for us stops by, crawls under it and says it's negative ground. I didn't get a chance to talk to him when he was here but all there is to see under this thing is a red cable that goes up to where the starter is and a black cable that goes the the frame. I contend that just because the cables are red and black and look like it's negative ground doesn't make it conclusive. I'm going to try hooking up a battery and see which way the amp meter goes. Of course that's not so conclusive either because someone could have change the polarization and never rewired the gauge?..........any thoughts?

Generator will work either way, just have to polarize it the right way. I think some starters will work either way, too. I think most vehicles were positive ground until the mid 50's.

Bamamav is correct.If I remember correctly you can tell what way it was run last by powering the gennerator to see what direction it turns, if it turns the same direction as the engine turns when the body is hooked to positive and armiture is hooked negitive. then it is positive ground. I think that is what I was shone by my Dad, but hat was a long time ago.

I usually figure the way with the most sparks and smoke is wrong.

Ya, I'm probably over thinking it but years ago I'd seen condensers and or voltage regulators fry when hooked up backwards.
I looked at how the coil and distributor are are wired to each other and the negative on the coil is going to the points, so that would indicate a negative ground.
I'm curious though, wouldn't a starter turn backwards if you hooked it up backwards. After all it is a DC motor, or do you poplorize it too?





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The starter will turn the same direction, pos/neg or neg/pos.

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Dr Crankenstein said:

The starter will turn the same direction, pos/neg or neg/pos.

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Not all of them. I accidentally hooked up a b&s lawnmower starter backwards and it ran in reverse.

I read where one of the reasons older cars were positive ground was because of the quality of the insulation on older wiring. Might be best to keep it positive ground (or change it back) instead of juicing up a bunch 80 year old wires?

Rolls, I agree with you that if you hook up your generator one way and the battery the other, then the ammeter gauge will read backwards. If you keep the battery in backwards, long enough, the generator will uncharge it and charge it up the other way. Don't ask me how I know this, OK.
Most starters will turn the right way which-ever-way you have your ground. My '36 Ford truck now has 12Vs negative ground running through a 6V positive ground starter. It really makes the old starter come alive, and turns the right way, ----- I think.

I hate to agree with Mac, but I've run starters like he said.

Bamamav said:

Not all of them. I accidentally hooked up a b&s lawnmower starter backwards and it ran in reverse.

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Permanent magnet motors will reverse like you say.

The old wire-wound motors require either the field polarity reversed, or the armature, not both at once. (The "north-south" relationship doesn't change with source polarity.)

offroadrolls said:

I read where one of the reasons older cars were positive ground was because of the quality of the insulation on older wiring. Might be best to keep it positive ground (or change it back) instead of juicing up a bunch 80 year old wires?

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Don't know if that's true, but I agree with your thinking!

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That makes sense Dr C. It probably is a perm magnet starter.

[P I have no clue so, I'm here for the education on this one [P

Neg on coil always goes to points, pos to switch.

If the horn blows it's ok...if it sucks it's backwards.







Ok........that's not true, I'm just goonin ya!

Sniper said:

If the horn blows it's ok...if it sucks it's backwards.







Ok........that's not true, I'm just goonin ya!

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[cl[cl[cl Thats a good one sniper I had my wires hooked up backwords and it didnt start but I had electronic ignition too .. I dont think the starter turned backwords I think it turned the same way , I just didnt get no fire on the distributor. Its a wonder I didnt fry all my stuff. when I figured it out I swapped the wires and it started up . I had the battery under the bed of the truck with two black welding leads as cable .. I have them marked now