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MG TD TF 1500 - Coil Polarity

Quick question to confirm what I think I remember.

Am I correct that, on a positive ground car, the positive terminal of the coil should go the the contact breaker on the distributor. The negative terminal being connected to the white wire that carries current from the switch.

On a negitive ground car, the negative terminal should be connected to the contact breaker in the distributor. The positive terminal carrying current from the switch.

In working with the older coils marked "CB" (for contact breaker) and "SW" (for ignition switch) the assumption must be that the car is positive ground - unless the coils were manufactured as positive or negative ground (which I doubt since most cars of tha era in England were positive ground).

Just working on a few member cars and wanted confirmation on that " Positive ground = positive terminal to contact breaker" rule that I seem to recall. Just want that spark as hot as possible as I know it will run when incorrectly connected.

I'll write up some interesting pertronix experiences later today or tomorrow.

Thanks guys,



You are correct. The negative side on a positive ground car is the SW terminal which gets the white wire. The coil, as you said, will work wired backwards, but not as efficiently. Reverse the wiring for a negative ground car.

Dave Braun

Dave - thanks for the confirmation - always good to double check - I was always amazed that they never put "+" and "-" on those older coils.


Boy, this gets confusing!
My Positive-ground "TD", has the Lucas coil connected (+) to the dizzy (-) to the switch....
Hopefully that is correct, based on this discussion.
Thanks for further input...
I checked the other article in the Archive,with more technical explanation, and that left me more confused, than before....
E.B. Wesson

"The negative side on a positive ground car is the SW terminal which gets the white wire."

I'm not sure that the real world is as simple as the above statement. One would have to know which application the particular coil was manufactured for. Not all British cars used positive ground. Even the prewar cars, up to the TA were negative ground. Coils with terminals marked + and - are very straightforward, but those marked CB and SW and not so straightforward. Cheers - Dave
David DuBois

There was a way, years ago, using a wooden lead pencil to see if the coil was being run with the correct polarity. Seems to me you connected the spark plug wire to the lead of the pencil. Then brought the pointed end of the lead toward the block. The spark would then tell you if it was wired correctly.

Problem is I don't remember what the description of the correct spark and incorrect spark was. Anybody got some of the old "Motors Manuals" or "Chilton's Manuals"? Seems to me that's where I read about the procedure. Just before or just after the war would be the years to look in.

This getting old is for the birds!!
Bob Jeffers

Here's one link:
Gene Gillam

I'm about to "stir up a hornet's nest"....
Just recieved a printed article from a friend, that is the former president of a local car club...
He is one of the most knowlegable folks that I know, about antique cars....
The article is written by Donald M. Russell Jr., of Falmouth , Mass., o.k'd by the Technical Director of the Polytechnic Institute (not absolutely sure which State, sorry)...
Here's the quote that really throws me:

" Please note that the + and - coil terminal disignations, have nothing to do with the battery polarity. It has to do with the phasing of the coil magnetic windings. In transformer parlance, "-" designates the start of the winding, and "+" designates the finish of that winding.
....mechanics connect the (+)coil terminal to the distributor points, and the (-) coil terminal to the ignition switch, in a POSITIVE-ground car, result in having an INCORRECT phasing of the coil."
He goes on to say:
"Why is connecting these terminals properly so important? If they are properly connected in phase, so that the back EMF in the primary adds, rather than subtracts or blocks the back EMF in the secondary, a substantially better, hotter spark will be obtained...usually up to 25% better."
Now, I know this is going to throw some folks for a loop, it did , me!
Discussion is about to get hot.
E.B. Wesson

At best proper phasing of the coil will give about 15% increase in the spark efficiency. Bottom line, if the engine is in such a condition that a 15% decrease in spark efficiency, there is a lot more wrong than the wrong polarity (phasing) of the coil. Yes, it is good to have everything done correctly, but it will not effect the running of a car in decent tune. Cheers - Dave
David DuBois

I can't remember whether this has been covered before, but I have a Lucas BR-12 coil marked "Negative ground." There's no problem in reversing the wires and using it in a positive ground car, is there?

t lange

Tom - As long as the negative terminal goes to the negative potential on the car (i.e. negative terminal to the power wire on a positive ground car) it will work fine. Cheers - Dave
David DuBois


I think that the best method to determine if your coil is connected correctly is to use a meter to look at the direction of the spark.

You need an analogue meter.

This is how to do the test.

Connect the negative lead of the meter to any sparking plug cap.
Connect the positive lead to earth.
Set the meter to the highest voltage range.
Turn the engine over, with the ignition on, try you prevent the engine from starting.
While cranking, you should see the needle jump.

If it jumps upwards the coil is connected correctly, you donít need to read the voltage value.

If it flicks downwards, (against the backstop) it is incorrectly connected.

These instructions are apply to both positive and negative earth cars.


52 TD

J Scragg

I didn't quote the entire article, but the author claims that the difference will show up on long , uphill pulls, no matter the condition of the engine...
I'm going to try John's meter test, and see what the results are...since I have mine hooked up in reverse, according to the article.
E.B. Wesson

Well, this afternoon , I did the "meter" test described by J Scragg, and , sure enough, the meter pegged in the wrong direction...i.e. to the left.
I, of course, had the Lucas Sport Coil connected , with the (-) to the switch, and the (+) to the dizzy.
My car is a Positive ground TD.
I have now reversed the two wires on the coil.
Over the next few days, (when weather improves), I will be driving some long uphill roads, to see if I notice any improvement.
E.B. Wesson

OK...after being very confused, I think I understand...we are talking about positive ground cars with my hook up with a positive ground pertronixs is very different...+ to ground, - to ignitor/dizzy through B/W wire, Black wire from ignitor/Dizzy to ignition switch...
Bob Dougherty

If you are using a pertronixs you should follow this diagram...
Bob Dougherty

Sorry for any confusion...As usual, I made the incorrect assumption that it was understood that the discussion was about cars using points.
This does refer to points cars with Positive ground, and not to electronic ignition of any type, including Pertronix....
Having used Pertronix in both of my "B's", you have to hook up according to their instructions. (As stated previously).
E.B. Wesson

This thread was discussed between 17/08/2008 and 16/05/2011

MG TD TF 1500 index

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