Welcome to our resource for MG Car Information.



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 - Coil problems - again!

I have posted before about coil failure. I have completed my engine and gearbox overhaul, all seems well apart from driving about 10 miles and miss-firing occurring until finally stopping and refusing to start. Brand new Coil is hot to touch and no spark at plugs. This has happened many times before. All the usual things have been replaced - plugs, coil, leads, cap, points, condenser. Had Accuspark coil and modual fitted before and blamed them, but still have same result from original set-up.
Do you think that the ign switch is to blame by supplying inconsistent power to coil or does anyone else have any ideas?
Many Thanks
Pete Dyble

Have you checked for a spark at the coil, rather than at the plugs?

Have you changed the rotor arm?
Dave O'Neill 2

My first thought was that you have a 1 1/2 ohm coil, instead of the 3 ohm one, i.e. a 6V rather than a 12V. Are the points burning?

H J Adler

I have those red see-throu plastic connectors that fit between HT lead and spark plug which light up everytime that cylinder sparks. Easy check and looks pretty when running. Only fit them when first starting and remove them when I am happy car is running OK. It is an easy check when there is no power from the coil.
I have ordered new rotor, condenser, points and cap to make sure that area is working OK. I have checked voltage on coil and it is a 12v one.
It will be a couple of weeks before I will be able to work on car again, but will keep informed of progress.
Thanks for comments.
Pete Dyble

You say you have checked voltage at the coil and it's a 12volt one
As Herb mentions you need to measure the coil----
Primary wires off at least one side and measure the resistance accross the coil primary terminals
What have you got there---
It's easy to get caught out with the wrong coil as a lot of the resistor type coils have 12v marked on them like the non resistor coils and the electronic ign.ones
Trust nobody's word, measure it yourself
William Revit


When you say ďhot to touchĒ is it too hot to keep your hand on? All coils will generate a certain amount of heat. The spark from a coil will diminish as it overheats but you need to be sure this isnít a red herring, you need to check whether youíre getting a spark or not directly from the coil when it fails as Daveís says.

I had a similar issue and traced it to the rotor arm which I fixed by replacing it with a red one.
The basic ignition system is pretty simple and there are only a couple of things that can cause it to overheat. As mentioned a wrong coil is one possibility, to high dwell angle is another. If youíre running points, setting the gap too close will cause this but I guess youíve checked that.

I had an issue with a coil getting hot and had a letter published in Enjoying MG, only to get into a bit of a spat with Mr AccuSpark (I canít remember his name) who claimed it couldnít possibly be his products (not that I was insinuating that anyway, I think he was just trying to get some publicity). As a consequence I built a distributor test jig, the interesting results (amongst others) was that the Magnetronic unit I was running had a dwell angle of 70 degrees (standard points 60 degrees). The AccuSpark Stealth variable dwell unit performed quite well, it has a fixed on time of (from memory) 5mS, this gives it the effect of variable dwell. Iíve been using this until recently and itís performed reasonably well keeping the coil cool but itís started to play up after about 30 minutes causing a hesitation when accelerating so Iíve gone back to the Magnetronic unit. This works really well particularly at high RPM, I just need to keep an eye on the coil temp.

R.A Davis

I think that it would be helpful if those who have a hand held IR thermometer to take a temperature measurement of a normally functioning coil.

It would be interesting to see the temperature of coils running with the various triggering systems.
Steven Rechter

I took these measurements in 2014, both standard points systems, roadster is a 12v system, V8 is a 6v ballasted system:

"May 2013: On one day with an ambient of about 15C the V8 coil was 52C, and on another with an ambient of 21C it was 58C. So with each increase in ambient there is a similar increase in coil temperature, as expected.

"July 2013: In the midst of this heatwave I've been checking both cars again. The V8 at an ambient of 27C saw the lower part of the coil at 62C (the upper was a little cooler), so again a correlation between the increase in the ambient temperature and the increase in the coil temperature. Whilst 40C (10C ambient) is only warm, 62C is very much hotter to the touch. The engine compartment temperature varied between 40C bowling along the M6 round Birmingham at 9:30am, 45C coming back at 1:30pm, and in some stop-start traffic round Solihull with the fans on it got up to 58C. With the roadster at 26C ambient the upper part of the coil was at 67C (in this case the lower part was a little cooler). Higher than the V8 as before, but a slightly smaller difference than at lower ambients. The engine compartment in stop-start traffic round Solihull got up to 50C."

The temperature that a coil in a given ignition system will reach is totally dependant on the ambient temperature. It always has to be hotter than the surrounding air before it starts dissipating heat, and only then will it start to reach a stable temperature.

Some electronic triggers that are not variable dwell have a slightly higher dwell than points so the coil will run hotter (dwell is the proportion of time that the points are closed rather than open, the coil will heat when the points are closed and cool when they are open).

I've not had the opportunity to measure one but variable dwell electronic triggers SHOULD run with significantly cooler coils, at anything other than peak rpm.

Typical heat generation of 12v fixed dwell (including points) systems is 37 to 44 watts. For 6v systems it is 20 watts, as only half the power is being dissipated in the coil, the other half is in the external ballast resistance. For variable dwell systems it is typically 4W at 1000rpm, 18W at 4k rpm, and 24W at 6000rpm.

Firstly as others have said the only true way to know what coil you have is to measure its primary resistance, wires disconnected. Ignore whatever it says on the label, or the box, or what the supplier says.

Secondly the only way to know what ignition system you have is to connect the white (chrome bumper), white/light-green (rubber bumper), or white/light-blue (V8) to the coil +ve, connect an earth to the coil -ve, remove all other wires from the coil, and turen on the ignition. Measure the voltage wrt earth on the coil +ve.

If you see 12v you have an unballasted system and need a 12v coil i.e. one with a 2.5 to 3 ohm primary.

If you see less than 12v then you probably have a ballasted system, although it could be bad connections. If ballasted, then you need a 6v coil i.e. one with a 1.4 to 1.5 ohm primary.

If you see 6v, and measure 1.4 to 1.5 ohms, then your ballast is matched to the coil as it should be.

If you see about 9v then you probably have a 12v coil with a ballasted system, which will give a weak spark.

Finally you need to measure the dwell, and compare that to the book figures. A high dwell, as caused by the points gap being too small for example, will make the coil run hotter.

But if you are having ignition problems now, i.e. with the ambients we have at the moment barely into double figures, then something is seriously wrong, that may be nothing to do with coil temperature.
Paul Hunt

This thread was discussed between 06/04/2016 and 08/04/2016

MG MGB Technical index

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