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MG MGA - Coil Failure

I fitted a new coil about 2 months ago, after the earlier one failed, and I'm now about to do the same thing again as the new one has failed too.

The new one was supplied by the MG Owner's Club in UK (who are sending me a replacement under guarantee) so I've got no doubt about its 'provenence', but I'm now wondering if I might have another problem that is causing these failures? The technical guys at MGOC have suggested a new 'red' rotor arm, which I've now ordered, but has anyone got any other ideas, or have I just been unlucky?

Robert Sinclair

Check the coil primary resistance to see if you need a ballast resistor. If it is 3.5 ohms you don't need one.
(I mean check the new one)
Art Pearse

Thanks Art. I'll do that test when the new one arrives. In the meantime I've been searching the web for information on reasons for coil failures, and one site ( says "the number killer of ignition coils is voltage overload caused by bad spark plugs or plug wires". Has anyone had experience of that? I've recently replaced all HT leads with silicon ones, but the plugs haven't been changed for a while so maybe I should do that too?
Robert Sinclair

Robert. As Art notes, using the wrong coil will eventually lead to coil failure. There are two types of "12 Volt Coils". The first is the one having about 3.5 ohms internal resistance (standard Lucas coil) which is designed to operate with a full 12 volt input all the time. The second type of coil has about 1.6 ohms internal resistance across the two primary terminals and is sometimes marked "12 Volt for use with an external resistor". The later model MGBs used this type of coil and Lucas never used the external warning that other brands of coils used. Hence, with a standard Lucas coil, the resistance test is important in diagnosing coil failure.

In addition to using the correct coil, how it is attached to the car plays a part. The MG coils were mounted with the nose downwards, unlike most other cars, or on top of the generator, again unlike most other cars. With the coil mounted to the inner fender and facing nose down, people tend to over tighten the clamping bracket, to keep the coil from slipping downwards, often sufficiently to short out some of the internal wiring. With the coil mounted on the generator, people can over tighten the coil and the heat from the generator adds to the heat produced by the coil itself and, over time, may lead to coil break down.

Excessive resistance in the spark plug wires and in the spark plugs may, under certain circumstances, lead to a coil operating at higher voltage output that its normally rated load. If the required voltage becomes greater than the rated voltage (20K volts on a standard coil, higher on Sports coils) you will have excessive strain on the coil. You will, also, have excessive strain on the points, condenser, rotor, distributor cap, and distributor wires (both the coil and spark plug wires, and the wire from the dizzy to the coil and the internal ground wire within the dizzy).

How have you determined that the coil is bad? What form of testing is being used in your diagnosis process? You replaced a "bad coil" and now have a "bad coil" again. Perhaps the root cause of the problem is not the coil but something else? If the coil is actually going bad in use, it is either a bad coil (old or poor production quality) or it is one of the symptoms of your actual problem. Simply replacing coils, perhaps many times, is an expensive method of not finding the root of the problem.

Les Bengtson

Thanks for the info, Les. My coil is mounted on the generator and has a screw-in connector for the HT lead. Soon after it failed it was so hot that it was almost impossible to touch. which I assume means that it failed because of overheating.

As this isn't the first time it's happened, I think you're right when you say that it might not be the root cause of the problem, but what else might it be? As I said at the start, I've already replaced the plug leads and rotor arm, so I'm now not sure what else to try.
Robert Sinclair

Looks like too much current and you need the ballast resistor.
Art Pearse

It seems common for the repro coils to fail. It happened recently to a friend of mine with a Morris Minor. He had just fitted the new coil and was on the way to show at which the vendor was present. The supplier in question duly exchanged the unit without quibble, but made the following comment. He said that these cheap repro coils didn't contain enough oil and would commonly fail when mounted horizontally and that mounting vertically solved the problem. Sounds a bit like b/s to me and in any case totally unacceptable. My friend has more patience in such matters than I and duly mounted the coil vertically without further incident.
N McGurk

Robert. Most coils are mounted with the "nose" upwards and on the inner fender. Mounting it on the generator (dynamo) means that the heat produced by the generator is added to the heat produced by the coil with the added stress of engine vibration thrown in. There is a reason that the MG Car Company changed the mounting location of the coil and that is a great part of it.

Your current coil has the screw in type terminal. This is not a requirement that need apply. The more modern style of coil, which has the push in lead, can be used by simply using that type of coil lead. It is also more moisture resistant than the older style of screw in terminal.

When the Brits went to the inner fender mounted coil, they mounted it nose downwards, unlike most of the rest of the world. I suppose that this was to allow shorter wires? But, the oil in the coil is then moved to the opposite end of the coil than is commonly encountered and a small air space, seen as windings no longer covered in oil, may be possible. The same thing happens when the coil is mounted horizontally. While original coils may have taken such mounting positions into account when they were manufactured, I doubt modern makers do.

You have, unless your system has been modified, a system designed to work at a full time 12V input (which is a system voltage of 13.5 to 14.5 voltage with the engine running). Thus, you need either a standard coil having a 3.5 ohm internal resistance or a Lucas Sports Coil which has about a 2.7 ohm internal resistance but is designed to work at full time "12 volt input". Avoid any coil saying "12 V for use with an external resistor" or any coil showing about 1.6 ohms resistance when measured across the two primary terminals. The latter will overheat and fail in use. The former should work for quite a number of years.

I would suggest, however, that you move the coil to the inner fender using an MGB bracket and, if necessary, extend the wire from the coil to the distributor and the input line from the wiring loom. Mount the coil nose upwards and you should not have any problems for quite a number of years. The only reason that one would not change the mounting point, as the factory did during the MGA production, would be on a show car. With a show car, you can always push it onto the trailer, off the trailer, and into is position of the show. But, for a driving car, changing the mounting of the coil and switching to the more modern style of coil, is a good idea.

Les Bengtson

It is a statement about the quality of many replacement parts currently available. My first coil lasted 45 years. I've had three new ones since the original one failed.
No change to wiring, or plug leads, or mounting position, or other "factors". I'm pretty sure the MGCC knows which coil to supply for an MGA, i.e. no ballast resistor required.
There are many reports on this, and other forums, of modern coils failing, some even exploding. Rubber parts are also of poor quality now, unfortunately.
P. Tilbury

So does any company currently make a reliable, fit-and-forget 12 volt coil? Anyone got a recommendation?
Bruce Mayo

This is what I got for mine a couple of years ago No trouble so far. The same sort of coil ought to be available from your local Auto shop, but I doubt you'll find it as cheap as the eBay one! At that price you could buy two and have a spare in your boot. The key word is "Standard" which denotes a coil that runs without a ballast resistor.
Lindsay Sampford


I was recommended to Aldon Flamethrower coils. I have one and has been working well for the past 3 years.

However, If you or anyone else is running or is intending to fit one of their Ignitor 2 electronic ignition systems you need to be very careful about which coil you use. My system is just the standard Ignitor (UK branded Petronix) so I use their 3ohm coil (part 40511). It can also be used with points ignition.

I repositioned my coil to the inner wing, nose up when I changed to an alternator. It makes access to the terminals a doddle.

Steve Gyles

This thread was discussed between 08/09/2012 and 12/09/2012

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