The Stovebolt Site Forums The Shop Area The Electrical Bay 1939 GMC positive or negative ground

Is a 39 GMC POSITIVE OR NEGATIVE GROUND

WHEN I GOT IT IT WAS HOOKED UP POSITIVE GROUND

HAS AN ALTERNATOR AND 6 v battery

I believe it was originally positive ground. No reason to keep it that way, but whatever you want. Only there things are affected, coil wiring, battery cables and ammeter wiring.
So you have a 6V alternator?

Good question I don’t know

Hopefully get it started tomorrow and check output of alternator

Thanks for replying

BTW after researching some more looks like GMC POSITIVE CHEVY NEGATIVE

GO FIGURE

Running no output from alternator

Sooooo

Can I run a standard 70s style (2 wire plug in plus battery wire) alternator with a positive grounded 12v battery?

Or just switch the battery to negative ground?

I was always told DC stuff wired backwards spun the other way but apparently this isn’t true

But isn’t that how power windows are controlled by reversing the current?

And as long as we’re on the subject I still use my grandpas ancient bench grinder

It needs help getting it to spin and depending which way you spend it is the direction it runs

We may need to consult Nicola Tesla on that !

It needs a starter cap or the switch that controls the cap.

Slow down a bit Jack. First things first. We can't start helping you before we know what parts you have and your goal.
1. IMO Change to negative ground. very easy
2. IMO Change to 12V system.
3. Post picture or part number of the alternator you have.
4. Do you have a voltage regulator?
5. Do you have a ballast resistor?
6. Do you have a foot start starter? Is it 6V or 12V?
7. What motor is in truck?
8. Early GMC's having positive ground is a well known fact around here. They went to neg in early 50's.

It seems to me you are between making up your mind on 12V or 6V....and Neg or Pos ground and have some kind of "alternator" not generator.
So need to get it all sorted out, then we will help hook it all up. I do recommend 12V negative ground and using a Delco 10si alternator. Many have gone this route and we have helped them do it.

The grinder is for later. You can continue to start by hand. As Bill says the winding with a capacitor in series will magnetize slightly earlier than the winding without the capacitor. The old single phase motor on it should have been wired to start in the correct direction. And yes switching wires will casue motors to change direction.

A bit more details in my posts would be helpful

Sorry

Yes going to 12v negative ground

It has the 10si alt internal regulator on what I’m thinking is the original 216? My assumption is that it’s 12v not one of those 6v aftermarket alternators

I’ll post a pic

Have access to lots of spare alternators so if this one doesn’t kick in I’m good

Not worrying about lighting etc just needs to run for now

No blower motor etc

Was going to install a 12v no resister needed coil

Foot starter switch

I’m guessing where the confusion came in is that the person who brought this to me put in the 6v battery and grounded it to positive not realizing it had an alternator on it

I guess the biggest question I have is will the starter spin the right way?

Many thanks

The original 6 volt (positive ground) starter in my '52 GMC runs the right way after changing the electrical system to 12v negative ground with the battery + connected to the stomp switch and battery - connected to the engine block.

DC permanent magnet motors turn in the opposite direction when you change polarity. Our starter motors (and most [all?] of the other starter motors in the world) are NOT permanent magnet motors and don't care what polarity they are hooked up as.

Many thanks!

That makes sense

Pics

Should I put on the taller cap and rotor

I think someone said the those came about with 12v and higher voltage coils to prevent carbon tracking arcing to ground

Much appreciated everyone!

WOW! Jack...that is a museum piece!!!!!!!! What a really neat truck. Very rare.

I don't know about the cap............the AD and pre war guys do.
Stamp indicates a 270 block. Internet says produced 1941-1963.....lots of swapin' going on from 39 up on all the straight 6 series of GMC motors.

Thanks

Was having trouble decoding engine numbers

Very helpful

My friend who owns this has lots of “interesting” stuff

He owns a wrecker business so lots of trucks he’s purchased get antique wreckers installed

Appreciate all of you

J

GMC (and Chevy I think) used the short cap distributor in '52 and earlier and the tall cap in '54 and later. I can't remember about '53. But you can't just put the tall cap and rotor on the '52 and earlier distributor, nor the short cap and rotor on the '54 and later distributor. You have to swap the whole distributor.

I have a '54 Chevy distributor in my '52 GMC.

...and what is the idea of this cap change? Does it do something you need?

One of the forum discussions I had about 6 months ago someone suggested that the taller cap came about with higher energy 12v coils to prevent arcing

Anyway I’m good I don’t need it

Thanks

Are 4ga battery cables ok for 12v?

Yes

ok? As long as everything else was perfect.

Yes, I forgot........if the creek don't rise.

Thank you

Bartamos

What makes this rate

I know because they’re aren’t ma y if them

Was 39 low production or is there something unique about this model

You did say a 270 engine?

It is pretty unique. It's old, has original looking paint, signage and lettering. It's a GMC, it's wrecker equipped. It's complete, good condition and the no. 1 thing with old trucks is quantity of rust. If it runs and drives, more value points.
If the truck, logos, lettering and paint is actually original, it is possibly of interest to gas station/service station collections. Albeit the Internet says 270 came in 1941. Pretty era correct though.
The Freeport and AAA look add on.

Thanks for the reply

It’s pretty rust free

My friend that I’m working on this for bought it years ago

He owns a wrecker business and had the lettering done

His brother owns a sign business

He also dropped the wrecker in the back

Not sure what it looked like when he got it

Under the hood is dark green

Runs needs brakes

Tried to post more pics earlier

I’ll try to add some more

It’s being used as part of the opening of the stock car hall of fame in Rockford il next month