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MG MGB GT V8 Factory Originals Technical - Overheating Ignition coil?

My recently converted 73B left me stranded a week ago. Driving down a side road at maybe 40 mph and it simply shut off. No warning, no sputtering, no nothing. Its as if I turned off the key. Side of the road diagnosis finds 12 Volts at the coil but no spark. Its a 3.5 Rover w/EFI and pertronix ignition and Pertronix coil. The coil was installed according to the instructions at full voltage. This set up ran great for 1500 miles until this failure. I got the car back home, pushed it in the garage and walked away from it. A couple of days later, I turn the key and it fires right up on the first crank. I have not been able to reproduce the failure, but after driving it for a few minutes, I find the coil to be very hot. You can touch it but cant hold your fingers on it. I suspect this was my problem but cant confirm it. My first thought is that I maybe should have run the coil ballasted. So I installed the stock SD1 ballast resistor set-up. Coil is still very hot. Thinking that maybe I have a bad coil, I swap out the Pertronix coil for the original Rover/Lucas unit with the stock ballast resistor. Still hot. With the car running, the alternator is putting out 13.7 volts and I have 9.5 volts at the coil. Anybody have any suggestions as to why the coil is apparently overheating? I should add that the coil is mounted to the inner fender in front of the motor so I dont suspect that engine heat is a significant contributor. For now I carry a spare coil, but I really cant trust the car until I can pinpoint the failure.
Steve Mc

A few years ago I experienced that same issue with a new Pertronix coil - engine just cut out while on I-75 near Bay City, Michigan and stranded me high and dry. Coil is mounted to the left front cylinder head as per the V8 website gallery.
The Pertronix was replaced with an equivalent Accel coil mounted in the same position and has given me 4 years of trouble free motoring.
Whether the chrome plated Accel provides a cooler temp vs the black painted Pertronix is a matter for debate.
Graham Creswick

Had the same problem on my 4 banger. I put a resistor in to drop the voltage to the coil, but not to the pertronix. Never had another problem.

Good Luck

Coils do become temperature sensitive, particularly those with rivetted spades where the connections get loose over time. During MGB production the coils were changed to threaded studs and nuts to correct this.

Measure the primary resistance of the coil. If it is from 2.4 to 3 ohms it is a 12v coil i.e. it doesn't need a ballast resistance in series. If it measures about 1.5 ohms it is a 6v coil and needs a ballast resistance in series of approximately the same value. If it is lower than that, and some coils go down to tenths of an ohm, it is designed for specialist electronic ignition and should not be used with the Pertronix.

'Hot' is totally subjective, coils do get hot, and if the engine is stopped with the ignition left on and points, for example, closed it will get very hot. If the Pertronix gives extended dwell then the coil will get hotter. Standard dwell for a points V8 is 27 degrees, which is exactly what you are getting going by your voltages and assuming you have equal coil and ballast resistance.

Paul Hunt 2

Thanks for the comments guys. It turns out that the Pertronix and the stock SD1 are both 1.5 ohm coils, so both should be ballasted. It's still hot, though. Paul, I agree that "very hot" is subjective. In my original message, I though about how I would characterize that, but to keep the message from rambling too much, I just left at that. So think of it in these terms, consider a hot cup of freshly poured coffee. You can momenterily touch the cup but can't pick it up without using the handle. I'd guess about 200 Deg F. That, in my experience is way too hot for an ignition coil to be operating. So I'm still scracthing my head about this. I run either the stock or aftermarket coil ballasted, and they both run hot. Puzzled.
Steve Mc

Well, your voltages and resistances do seem correct, so I'd tend to ignore the heat and assume that the previous coil had a problem, possibly caused by *not* having a ballast in series, and just drive it now that you have the replacement coil *with* a ballast in series.
Paul Hunt 2

That's exactly yhr conclusion I've come to. I've been driving it for almost two weeks now with one or the other coil ballasted and have had no issues. I'm confident that the original problem was operator error, I should have ballasted the coil in the first place. I have the spare coil in the trunk, so I'm closing the book on this one. Again, thanks for all of your comments.
Steve Mc

This thread was discussed between 18/09/2007 and 22/09/2007

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