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MG MGB Technical - Ignition Coil Wiring?

I have a 73B and am replacing the dual points in my Mallory Dizzy w/ pertronix Ignitor II. I don't trust anthing the PO did and The wiring to the new coil doesn't look right to me.

A B/W pair of wires are connected to the negative side of the coil. My Complete Official MGB book says that the B/W pair should be connected to the CB side of the cable.

Question: Is CB the negative side?

A W wire is connected to the positive side of the coil, the book showes the W wire connected to the SW side of the coil.

Question: is SW the positive side?

It seems to me that CB means contact breaker and should be pos.

It seems to me that SW means switch and should be pos.

The igniter diagram shows white to pos and black to neg termenals of the coil.

Do I eliminate the condenser?
Ken Knize

Ken. The coil might be an old one from a positive ground car. My 68 has a Sports Coil, not the stock coil in it, so I am not sure of the markings used on the early cars. The later model cars, using a resistor wire, were marked with a plus and minus sign.

If this is an early coil, the SW terminal was hooked to the switch which was negative. The CB was, as you suggest, the contact breaker circuit which was positive.

I am told that a coil will function best when the connections are made to the proper terminal--i.e. the negative to the terminal designed for the negative lead and the positive to the terminal designed for the positive lead. Others have stated that while the coil will fire connected in either direction, correcting it in an incorrect manner results in a reduced potential output.

Assuming that you have an older coil, the connections you show would be correct for the current flow of a coil used with a positive ground system.

Unfortunately, I do not know sufficient to give you an absolute answer. The Moss catalog suggests that the positive ground coils (part number 543-020) had a screw in terminal on the nose of the coil. The 68-74 coil had a nose designed to be used with a push on HT lead.

I would suggest that you examine the nose section of the coil and, if the interior is threaded, you assume it is a positive ground coil and leave it hooked up as it is currently. If there are no internal threads, it would be the later coil and the connections should be swapped.

Perhaps someone will have the number that should be on the bottom of the proper coil and can post that.

Les
Les Bengtson

Les, Thanks for the input. The coil is new from Moss. I replaced the old one wire for wire unfotunatly I didn't keep the old one it's possible it was an old pos gnd. My neighbore who sells used cars stopped by and also said it would work either way so I gave it a shot and started it. Never ran so good but it does backfire when I let off the gas. Perhaps it is reversed.
Ken Knize

Ken, the backfire when letting off can also be caused by an air leak in the exhaust or intake system.
John H

John, Thanks for the back fire note. I reversed the wires on the coil and as my Air Force buddys say " No Joy" wouldnt even start. Put the wires back and she fires right up. I'm not much of a mechanic so this is the best she has run in 5 years. She's been sitting in the garage while I was playing golf. I'll look for air leaks but I doubt it Just had a Peter Burges design exhaust installed and a new gulp valve.
Ken Knize

An original coil for a 73 should be marked +ve and -ve, with the +ve being connected to white for the 12v supply and the -ve to the points. There was a second wire on the -ve of the 73 and later which goes to the tach, which was voltage pulse operated from 73 on. Prior to 73 the tach was current pulse operated from the white wire and there was only a single white/black on the CB or -ve going to the points.

The CB (Contact Breaker) and SW (ignition SWitch) markings on the coil refer to earlier cars with positive ground electrics, the labelling changed at about the same time the polarity changed.

The coil should work connected either way round, there are said to be only very small differences in HT spark if reversed. However I've read elsewhere that these changes in HT actually occur only with old plugs, it is the plugs that develop a 'polarity' which makes them fire better one way than the other. On modern cars with 'wasted spark' ignition it is quite common to have two plugs fired with a negative pulse and two with a positive, and the recommendation is to keep the coils and plugs to the same cylinders and not mix them up. The backfire is almost certainly nothing to do with the coil connections, this is usually caused by a faulty gulp valve (new one noted!) on anti-emissions equipped cars.

You remove the condenser when fitting a Pertronix or other electronic trigger. However keep it and the points in the car and be able to fit them at the roadside, for when the Pertronix fails. If you reverse the polarity to the *Pertronix* it won't run, and will probably burn it out, it seems polarity protection is only a relatively recent addition.
Paul Hunt 2

Paul Thanks for the detail. It has been so long since this car has run this well I have new vigor to sort out this small detail (I hope). My problems started 5 years ago when I put in the Mallory Centrific Dual point dizzy to replace one that wabbled. This car is in really good shape but has had many changes made. Almost nothing is stock.

I think that I shall start a full tune up, everything has been tweaked so much I should do a by the numbers tune up rather than strike out at whatever isn't working at the moment.

The carbs were way too rich hence the black smoke with the backfire.

Rebuilt to solid state fuel pump
New Mallory Dizzy
New Pertronix Ignition
New Coil
Tomorrow will get new plugs
Rebuilt Carbs (will install new throttle adjusters)
New Air cleaners K&M with matching needles
New fuel filters
New Peter Burges design exhaust from manifold to Peco pipes and flow through muffler (sounds great not too loud)
Do I own Moss Motors yet?

The Petronix was such an easy do, I don't know why I waited so long making the change. The improvement was truely remarkable. I'll keep you posted.
Ken Knize

This thread was discussed between 31/05/2006 and 02/06/2006

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