The Stovebolt Site Forums The Shop Area The Electrical Bay 39 GMC positive ground

I'm working on a 39 GMC and the battery is connected - to ground. The manual says positive ground. The engine starts, runs good, dash lights and all gauges work, taillight, horn, the only thing I've noticed is the headlights are dim,which I thought was probably a bad ground.
I'm scart to change the terminal connections around. Experience, advice please.

Whoa pahdnah!

Let's start at the beginning. You said "I'm working on a 39 GMC and the battery is connected - to ground."

Do you mean that the positive terminal is connected to ground, and the negative terminal is connected to the starter? If yes, this is correct.

The first thing to do is to replace both cables with "double ought" cables to carry that 6 volts. If the cables were replaced at one point, they were likely replaced with 12V cables which are only half of the guage required to carry the juice needed for 6 volt systems. These new cables will have to be made up by a truck garage for you and will be as big around as your finger...or more. They are big honkin cables. Typically, the positive and negative terminal posts are different sizes and most "positive" cables are set up for starter install. You need the negative cable to go to the starter and it should be at least a two ott cable.

This will make a world of difference right off. Take (00) the positive cable from the battery terminal to a bolt on the back of the bell housing. Then take another (00) cable from the back of the bell housing back over to the frame to set up the ground side correctly. Then take a third (00) cable from the negative terminal to the starter. This will make the engine spin fast on starting and will get the old electrons a zooming along a super highway instead of down a windy cart path through the woods.

If you reverse the polarity, you'll create a host of new problems...all unnecessarily.

or if ya mean it's "-" [as in: "negative" ], then prolly someone switched it around sometime. I had a 46 GMC that was "normal" neg ground when I got it, and didn't pay any attention till someone mentioned that it should be pos ground - don't know what was done, but it worked fine - starter didn't run backwards! [imagine the main diff would be the coil connections and some gauges, but, hey - if it works don't "fix" it!]

dim lites could be poor ground [batt to engine or frame, or body to frame, or lights to body], but, like Kook sez, it is important that the cables and the wiring be heavy enuff, and ALL connections on a 6 volt need to be good . . .

Bill

Thanks for the response guys.
The negative terminal of the battery is connected to the transmission by a 3/8 OD wire. There is no electrical connection from the trans, bellhousing, or engine to the frame. The positive terminal is connected to the starter by a 7/16 OD
wire. I want to restore this truck to original condition.
My concern is what changes, if any, may be necessary to the electrical sytem to change the battery polarity. Generator polarity ? Coil and gauges as Squeeze mentioned.
I will change to the larger 00 gauge wire and ground the frame. Thanks Kook. I'm not finding much information out there on this vintage GMC.
Thank you for your help.

This page (on Tech Tips page) has a checklist of what is required to switch 6v(+) to 12v(-); you should be able to simply do the reverse of what is described.

Thanks Hedgehog
Thats what I needed to know.