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 MGB Technical - Spark Coil Resistive Ballast

I have a 1979 rubber bumper MGBGT for which I have purchased an electronic ignition. The existing ignition utilizes a ballast resistor in series with the coil which must be bypassed when installing the electronic ignition. I have located the two wires at the spark coil end, both of which have a light green stripe on white, but they disappear into the wiring harness. I have found the other end of one of them which is the one that goes to the starter solenoid, but I have no idea where the resistive lead (ballast resistor) comes out again. Can anyone tell me where to look for the other end?
Chris Barrow

Chris, go to www.advanceautowire.com and get a copy of the wiring diagram for your car. The resistance wire is in the loom and goes across in front of the rad. The diagram should tell you where it is connected.
Barc Cunningham

Chris,

Just to add to Barc's comments. The resistive wire is indeed inside the loom. It runs from somewhere near the fuse box, forwards and across the front of the car, in with the lighting/fan/horn wiring, and then comes back the same way, to emerge again near the fuse box. So look for the two wires going that way. On my 78 GT, one of them is white and pink.

David
D Balkwill

Thanks guys, I will have a look at it this evening.
Chris Barrow

If you bypass the ballast resistance to the *coil* you will have to replace the coil with a 12v type, the one you should have at the moment should be a 6v type. You will have to measure the coil primary resistances to be sure which is which as the labelling is often confusing if not downright incorrect. 6v coils measure from 1.2 to 1.5, 12v coils measure from 2.4 ohms to 3 ohms. If you don't change the coil it will overheat and fail.

But whilst some aftermarket electronic ignition systems need a full 12v supply for themselves, they will operate perfectly well with the coil connected as before. If you bypass the ballast as well as having to rweplace the coil you lose the boost function on starting, which can make the difference between starting and not starting under some conditions.

Whilst the resistance wire itself may be pink or white/pink, it has 'tails' at either end and it is those that come out of the harness, white/light-green at the coil end and white or white/brown at the supply end. It's further complicated on 77 models and later depending on whether it is North American or UK spec. North American spec is fed from the ignition relay via the front of the 2nd fuse up (not not *through* the fuse) on a white/brown and UK spec is fed from the ignition switch white.
Paul Hunt

Hi Paul, mine is a later model MGB, and to complicate matters even further it was assembled in South Africa. (During the late 1970's, British Leyland had an assembly plant in South Africa) I suspect that it has an ignition relay, but I don't know where it would be located. Do you perhaps know?
The people who supplied my electronic ignition advise that it will not work satisfactorily with a ballast resistor in series.
Chris Barrow

Chris, you need to replace the coil with one that has 3 Ohms internal ballast - yours likely has around 1.4 Ohm, with an extrenal ballast to bring it up to 3 Ohms. If you use the external ballast, you run the risk of dropping voltage into the coil.

I recommend getting a replacement coil designed for the earlier cars that used points. Then run a wire directly from the + terminal to the "green circuit" at the fuse panel, deleting the two green/white wires. Thgat will get you a full 12V at the coil any time the key is on, and the coil will be correct for almost any aftermarket electronic ignition.
Jeff
Jeff Schlemmer

Chris - 77 and later MGBs should have the ignition relay, it should be near the fusebox as is the starter relay.

You should be aware that powering the coil from the green circuit instead of the white or white/brown on the front of that fuse will mean the ignition will be fused when it wasn't originally, also that the voltage on the green circuit is usually slightly lower than on the white. If anything causes the green circuit to blow then it will cut the engine. OK if replacing the fuse is all that is needed, but if you have a permanent short on the green circuit such that it keeps blowing fuses, you will be stranded. If you want to fuse the ignition then you should use a separate in-line fuse powered from the white/brown coming from the ignition relay, or if you don't have the charcoal canister and anti-runon valve then from the white coming from the ignitions witch that operates the relay.

I do wish people wouldn't talk about 'internal ballast'. Whether a coil has an internal ballast resistance or not, i.e. has a 1.5 ohm winding plus a 1.5 ohm resistance inside the can, or has just a 3 ohm winding in the can, is irrelevant. A coil is either a 6v coil measuring 1.2 to 1.5 ohms or a 12v coil measuring about 2.4 to 3 ohms.
Paul Hunt

Thanks Paul, I have to use the coil that was purchased with the electronic ignition, so I have no other options than to go with 12 volts. The car has two relays and three in-line fuses near the fusebox, but I guess that the wire colour coding will tell me which is which.
ChrisBarrow

Ah, if a coil came with the kit then indeed you should use that as it could be very much lower resistance than normal, possibly only tenths of an ohm.

If you have three in-lines I'm guessing it is North American spec with one fuse with green and white/brown wires, one with two browns, and one with slate and slate purple. The white/brown wire would be the one to use, but it is more robust to get a piggy-back connector (http://www.mgb-stuff.org.uk/images/piggyback.jpg) for the new wire, put that on the fusebox in place of one of the existing (if there are more than one) white/brown connectors, then put the displaced white/brown on the picgy-back. If there is only one white/brown connector on the fusebox you can put a standard spade on the spare spade for that fuse of course.
Paul Hunt

Thanks Paul,
I will be doing that this weekend.
Chris Barrow

Hi Paul, It has taken me the whole week to report back on this one. When I really started digging into the wiring harness I realised that the wiring on the positive side of the spark coil did not match the wiring diagram. Eventually I had to remove the tacho, and found that although it looks original for a rubber bumper, it uses an inductive loop on the back of the casing which is in series with the ignition switch and the coil positive terminal. Fortunately the electronic ignition that I have can live with this configuration, but it just goes to show what can crawl out of the woodwork when you really start working on a second-owner vehicle!
Chris Barrow

It's what makes living with these cars such fun - "So *thats* why ..."
Paul Hunt

"I do wish people wouldn't talk about 'internal ballast'. Whether a coil has an internal ballast resistance or not, i.e. has a 1.5 ohm winding plus a 1.5 ohm resistance inside the can, or has just a 3 ohm winding in the can, is irrelevant. A coil is either a 6v coil measuring 1.2 to 1.5 ohms or a 12v coil measuring about 2.4 to 3 ohms."

Paul, is this really irrelevant??? If you need to feed an electronic ignition with 12V, you can't attach it to the positiver terminal of the coil AFTER a ballast resistor, which is what most folks will do - given the opportunity. It IS imporntant to mention the voltage drop after a ballast resistor so they can interpret the wiring diagram and install the system properly. Even a 12V coil with power that's fed through a 1.5 Ohm ballast resistor isn't going to supply 12V to the electronic ignition. This situation occurs all the time with 76-80 US Spec cars that had the coil replaced and no wiring alterations.

And No, there are not only 2 coils that fit these cars as you describe. Take into account the work of POs and literally any of hundreds of aftermarket coils may be fit and function with points. Hundreds. I'v eseen 1.5 Ohms ballast resistors replaced with 9 Ohm ballast resistors. I've seen guys burn up 3 pertronix units in a row by attaching them to an unballasted 0 Ohm coil. There's nothing irrelevant about testing the entire ignition system to make sure its wired properly for the given components being used!
Jeff Schlemmer

INTERNAL BALLAST is irrelevant, which is what your post of 22nd October said, and what I was commenting on.

In this post you are talking about EXTERNAL BALLAST which is a completely different issue.

I didn't say there were only two coils, the range of resistances I gave cover a number of sport and standard coils suitable for both chrome bumper (unballasted) and rubber bumper (ballasted) factory MG systems. I have also said that there can be many other resistance values of after-market coils, see my post of 23rd October. These can't be used with the factory ignition systems but must be part of an overall replacement ignition system.
Paul Hunt

This thread was discussed between 20/10/2008 and 03/11/2008

MG MGB Technical index

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