View Full Version : TR4/4A Positive/Negative Earth Dilemma TR4A

06-07-2012, 08:45 PM
I recently got this 1967 TR4A IRS for restoration. I got it from a friend who is not a fanatic and knows very little about mechanical details so he is no help.
Now the dilemma: I don't know if the car is Pos or Neg earth!
The car has not run for 22 years. And you should see the look in my friends face when I asked him that question!. He has not the foggiest idea of what I'm talking about.
The facts:
1) When I got the car, the old shot battery was connected pos. earth with aftermarket autoparts generic store terminals.
2) The coil is also a store bought 3T made in Mexico with + and - marked terminals. It is mounted in the car and connected, + to ign and - to distributor.
3)It had a radio, a Clarion, "solid state" (transistorized) that I understand were almost invariably neg grounded.
My friend said that he never touched the battery and that he does not recall if the radio ever worked. The radio had a sticker stating the grounding method but the sticker is illegible. The radio came with the car when he bought it 40 years ago (it was a used car)
I don't know if it is possible to determine what is the present polarity of the generator, this could help.
I'm planning to take the radio to a technician to see if it would work and what is the polarity but he is located far away.
Who knows what happened here???!!!
Maybe someone can give me some ideas.

06-07-2012, 09:04 PM
I dont know if this will help

Its a Moss video on positive and negative grounds.

06-07-2012, 09:23 PM
If you got it with the battery connected pos ground, it probably is. Radio, if a neg ground unit, might well have been connected isolated from the chassis ground.

06-07-2012, 09:28 PM
Thanks for your help.
I have seen the Moss videos which are excellent!
Eventually I will find the real status of the car. I would like to know which way was the car wired from factory.
By what I read, TR4A's came in the two flavors. Once I sort this out , I can decide to leave it original or if to change to the other. Life with a pos earth is not too bad, I had british bikes and restored several, for many years.
The thing is that a car can start and drive with the coils with the wires reversed and the generator not repolarized.
I am assuming that the car was Pos earth originally. At one point the coil went bad and when replaced was wired for neg earth inadvertlently. The car would still run like this but not very well.
Knowing the grounding of the radio (if pos earth) would certainly confirm that the car was indeed pos earth (can not connect a radio to reverse polarity= danger)
It just occur to me to go and get a new battery, disconnect the radio, turn on the key and watch if the ammeter is not working in reverse .

06-07-2012, 11:58 PM
The owners manual shows TR4A as being negative ground. Although I know of at least one person who swears their TR4A was positive ground when it was new; most of the evidence I've heard agrees with the owner's manual. Certainly no one could say you are wrong for having yours wired that way.

My guess is that it was originally negative ground.

Then somewhere along the line, someone "knew" that all British sports cars are positive ground and wired the battery that way. My experience has been that it will run just fine with the coil polarity backwards. Frequently, the generator will even swap polarity all on it's own (although it's a bit hard on the cutout contacts). Whoever installed the battery may have noticed that the ammeter read backwards, and the radio quit working; but didn't realize why.

Radio is probably toast, but there is at least some chance it survived the experience.

06-08-2012, 06:45 AM

:iagree: with Randall.

Both of my TR4A's are negative ground and my second one is a very original with relatively low miles example (still has the original helmet battery cables).

I would agree that any TR4A that has positive ground was probably swapped by someone looking at TR4 documentation.


06-08-2012, 11:24 AM
Like Scott, I've got 4As. Both mine are negative ground.

06-08-2012, 11:59 AM
Agree mine was that way since I bought it in 1969. Built in May 66 as a reference.

06-08-2012, 12:18 PM
Another bit of evidence, the factory SPC specified negative ground:

06-08-2012, 12:38 PM
Beware! If you hook the battery up either way it will work....at first. Eventually the control box will fry if it hooked up the reverse of its polarity. Since you don't know, it would be best to reset the polarity and wire it up the way you want it. Mine is a 66 model, manufactured late in the model year, and it came with positive earth. After reading a Road and Track article in about 1982, we switched to negative earth by merely switching the terminals and it worked for a few days, then cooked the control box. The repair was a change to a Delco alternator.

Dan B

06-08-2012, 12:49 PM
Eventually the control box will fry if it hooked up the reverse of its polarity.

Repolarizing the generator will prevent that problem. My TR3 is still wearing it's apparently original generator & control box and has happily been negative ground since I put it together.

I did have to replace the bearings in the generator, and clean and adjust the contacts in the control box. But that's pretty darn good for such old components!

To clarify something I said above, I do recommend changing the wires at the coil & ammeter (if necessary) to match the battery polarity. The spark plugs will fire with either polarity, but they fire a little easier with a positive spark. And having the ammeter read backwards is just confusing.

06-10-2012, 12:28 AM
I agree with most all that has been said about switching polarity, would just add I bought a 4A in about '84 or so, it was positive ground when I got it, I thought that was correct (knew just enough to think "all british cars before 68 or so when they went to alternators were positive ground.")

I switched it anyway to put an am/fm cassette unit 9they were all the rage) in the car, But interesting to find out the 4As were negative. As far as the original question polarize th generator to the desired polarity and do the other switches discussed as needed and you should be fine. I am electrically challenged but have converted a half dozen or so cars, sometimes back and forth more than once, with no issues. Triumphs are easier than later MGs and Healeys with electric tachs, for those you have to get in and reroute a wire or two.