MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - Pertronix wiring

I just searched the archives for "pertronix" but could not see an answer to my specific question.

My 58 A is still positive earth. I have to complete the wiring to the coil. The somewhat pathetic instructions imply that I attach the B&W wire to the coild negative and the run the black wire from the ignitor to the ignition switch. It also states that I run a wire from the pos side of the coil to the battery or to ground. My questions:

Where do I run the ignition wire to - clearly I don't want to run it to the ign switch?

Where to I run the pos side wire? engine? chassis?

Any help and/or pics much appreciated.

Dan
Dan Barton

Dan. The first place to check would be with Petronix. They have a website with a telephone number. You can verify the correct conversion kit for the Lucas positive ground system and ask how it should be hooked up.

The Petronix systems I have used have all been for negative ground cars. One wire went to the coil output terminal (- sign or CB terminal) and the other attached to the (+ sign or SW terminal). The input wire to the coil, the SW or - terminal of your coil, is coming directly from the ignition switch and applies power to the coil when the ignition switch is turned on.

I would have a question about the wire to be run from the positive (CB or + terminal) to the ground or battery. Specifically, that would provide a full time ground through the coil (since you have a positive ground car) that would short circuit the points replacement system. At that point, you would have direct flow from the ignition switch, to the SW terminal of the coil, through the coil and directly to ground. Hence, there would be full time current flow through the coil and it would not be able to charge, collapse the magnetic field, then, induce a charge in the secondary windings. I am not an electrical engineer, but this does not sound like it would work to me. Certainly a question to ask of Petronix.

Les
Les Bengtson

Definitely check with Petronix. I ordered a unit from the local autoparts shop and when it arrived was clearly identified as for a negative ground application even though I had stressed that I have a positive ground car. The sales guy said I could just reverse the wires but being skeptical I called Petronix. They told me that had I followed the jobber's advice I would have fried the unit. The proper one works great and the car seems to run much more smoothly than with the old points.
Tom Heath

If I remember right, +/CB side of coil to ground. One wire of petronix unit to
-/SW side of coil. The other to the wire of the cars harness that previously went to the -/SW side of coil.
J.H. Hall

You don't want to guess on this. CB stands for contact breaker, doubt if that goes to ground.
Tom

Dan - I've been running a Pertronix on my positive-earth Magnette for a year or so. The instructions do make sense if you grasp that the pertronix is switching the live side of the coil instead of connecting the coil to earth as the points would have done. I used a Lucas bullet connector to join the wire that previously went to coil 'SW' (from ignition sw) to pertronix black. I added a wire from a convenient chassis bolt to coil 'CB' - this would have been the more 'earthy' side of the coil originally, so polarity is preserved.
stephen

Dan,
All three of my cars are positive ground with Petronix ignitions. Be sure that you have a positive ground unit and hook up the wires just as the instructions indicate and you will be fine. Have a good day!

John
John Progess

As always thank you gentlemen. I definitely have a pos earth unit and I will connect as per stephen's magnette. It will take a while before I test though; I'm still finishing off the dash wiring before re-connecting the battery.

Dan
Dan Barton

Further question: Stephen says "connect to a convenient chassis bolt". Can I assume that I can connect to a body bolt instead?

Thanks
Dan
Dan Barton

Dan,
I connected mine to one of the bolts on the removable pannel on the firewall. Works great. Have a good day!

John
John Progess

Sorry for the loose terminology - the Magnette doesn't actually have a chassis. I used a bolt on the inner wing which already had the headlight earth wires connected, but I would think any convenient fixing would be ok if you can get a good contact
stephen

I connected mine to one of the bolts hoding the coil brackett to its mount on the moter mount
J.H. Hall

Like Stephen, I have a positive earthed Magnette and have had a Pertronix unit fitted for close on two years. I followed the instructions which is exactly as Stephen has outlined. I even used the headlight earth connection on the inner guard for the CB side of the coil. The unit has worked beautifully over that period with mileage in the vicinity of 6,000.
Peter Daley

This thread was discussed between 12/01/2006 and 19/01/2006

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.