The Stovebolt Site Forums The Shop Area The Electrical Bay Pos or Neg ground

I wish there was a sticky created to clarify the polarity issue. We see people on here every day that are confused about this.These old trucks have had all kinds of shade tree work done and many of the present owners are not as old as the trucks so they are confused.Add in the difference between Chev and GMC and this adds to the problem.

Write it up and submit it to the moderator. Also send it to John & Peggy @ editor@stovebolt.com. They may want to add it to the tech tips page.

the simplest way to tell is is to look at the battery cables. since i own two gmcs and one chevy and i thought it was a no brainer. for my vintage its easy because gmcs are pos. ground and chevy is neg. ground. NOT! when i was removing the old wiring or what was left to it in my 40 i started at the wiring and not at the battery while tracing and removing each circuit. at the end of the process i noticed that a PO changed the polarity from original pos. ground to neg ground and its was at the battery is where i noticed it. why would someone do that?
since i had a 6/12 neg ground converter available i left it neg ground and installed the converter and hooked up my sound system to it. it saved me the cost of another converter.

a guy local to me has a 35 international and goes all crazy about the pos. ground thing. he'calls me all the time when there's electrical work to be done on his truck.

I will leave that to someone else,nobody beleives me anyhow.

Retired wrench is exactly right. While the accessories won't care if it's pos or neg. ground, the coil will. 6v. is difficult enough to deal with in cold weather, let alone losing a percentage of the sparking power on top of it.

The coil will not care either. It is basically a transformer.

Someone somewhere on this site said that in the 40's or thereabouts that there were "alternators" used to occasionally switch polarity to ignition.

If I recall correctly the biggest problem was higher wear on the points but the capacitor is supposed to take care of this.

Navy small vessels did not use grounded wiring.

Dan Bentler

Reversed coil polarity requires at least a 10% higher voltage to fire the spark plugs. It's easy to demonstrate that on an oscilloscope. The polarity is affected by the common connection point where the primary and secondary windings intersect, and there is a definite difference in performance if a positive ground coil is connected backwards. Theoretical guesses are great, but those of us who have dealt with the real world of working on these things for over 50 years have seen what actually happens if they're connected incorrectly.
Jerry

Some accessories do care if it is positive or negative ground. The power supply in a radio cares and a lot of heater motors run backwards on reversed polarity. I agree with Jerry except that I think its quite a bit more than 10%. I bought a heater for my 1950 GMC in about 1958 from a wrecking yard. It came out of a 55 Ford 2 ton. The wrecking yard had sold it to a guy with a Chev and he brought it back because the motor ran backwards. The guy at the wrecking yard didn't know that 50 GMC and 55 Ford both use posi ground.

How can you tell if a coil is in backwards?

Dale

Look at where the two low voltage wires attach to your coil. The terminals are marked + and -. If you have a negative ground system then the "-" teminal should be connected to the points.

So, what would be the issues or results if the coil was installed backwards?

Dale

After doing some research I think I found my answer... which is what I suspected.

"To answer the original question, yes it will work if connected backwards but it will not work correctly. The design is for the current to flow in 1 direction and for the circuit break to be on the negative side of the coil. If wired backwards in a points type ignition it will almost act like a flaky condenser."

How does one tell if its supposed to be negative or positive ground? I have a 48' big bolt and it has a positive battery cable going to the frame but the coil to points wire is on the negative terminal "-" and the posivite coil wire goes to a ceramic resistor on the firewall and then to the starter solenoid.