Grounding

[RonR]

Folks,

Is there any way to tell if a car is positive or negative ground? The smart previous owner used a red postive battery cable for both the positive and ground cables.

Thanks.
Ron

 

[martx-5]

Look at the top of the battery. There's a + and - marked on it, usually right next to the battery posts. Just see which mark the ground cable is hooked up to. If the ground cable is hooked up to the + terminal, then it's positive ground.

 

[RonR]

I should have mentioned that the car did not have a battery in it when I acquired the car.

 

[TR3driver]

That can make it hard to tell, especially since just hooking the battery up the other way will usually work (more or less). So my suggestion, if there aren't any electronics (alternator, radio, etc.) is to just connect it the way you want it to be; then deal with any issues that arise.

However, there are some clues you can look at:

How is the coil wired? If it is marked CB/SW, then CB would go to the distributor for positive ground, to the wiring harness (ignition switch) for negative ground. If it is marked +/-, then + would go to the distributor for positive ground, - for negative ground.

Look carefully at the openings in the battery cables. The positive post is slightly larger than the negative.

What, if any, electronics are installed? Generally there are only a few items that will even work with positive ground, and they usually need to be wired differently or have a switch to be set.

Last clue would be to hook up a battery and see which way the ammeter indicates. If it swings to "+" with (for example) the headlights on and the engine stopped, then the battery is connected backwards to the way the ammeter is wired.

Perhaps too obvious, but your TR3 would have originally been positive ground. Lots of folks (myself included) opt to convert to negative, though, for ease of adding electronic components.

 

[RonR]

Thanks Randal

The coil has been changed to a later Lucas coil with + and - markings. The + terminal is going to the points. So, the car is still positive ground.

Ron