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MG MGB Technical - replace coil

I believe my coil is failing on my 79B
After about an hours drive I noticed the odd hesitation and misfire which gradually got worse and worse
When I got home I checked under bonnett and coil was very hot
I let car cool for about an hour and car ran perfectly
The substance of my question is what type of coil do I need
I appear to have an external ballast unit and the coil is stamped....16C6 45251A 05 79 Plenty of 12v coils but not many 6v for sale



Do I assume its a 6v coil 1979. What can I replace it with 2 pics attached
Thanks
Chris

C Vine

You cannot assume anything - measure the voltage at the coil - someone may have bypassed the resistance wire in the loom.
Chris at Octarine Services

I have had the car for 2 years using it most weekends and this is the first ignition problem
The coil is the only ignition item I have not replaced since purchasing the car
attached is a pic of the ballast unit
The car is ex Californian imported to Oz by PO about 10 years ago

C Vine

Chris is correct, you cannot be sure unless you actually measure the voltage at the coil. In theory I believe your assumption is correct in that a 79 'should' have a 6V coil - but anything could have been changed, even though the 6V wire system appears to be in place.

One suggestion I would make though is that you first try running your present coil inverted from the present mounting of terminals uppermost. The coil type you have is oil filled and should be mounted so the oil supply is lying against the terminal inner ends. The fact your coil is mounted as it is could be causing it to run too hot.

I'd also offer that on my 69 the coil is mounted on the inner fender wall, a good deal further removed from the motor and oil filter sources of heat (and more exposed to cooling air flow) than is yours. Might be worth making that change at the same time. On my car there are two captive nuts to mount the coil clamp through to, located inside the wheel well.

Regards
Roger
R Taylor

Chris. Where is the coil actually mounted. I presume that you disconnected the coil from its mounting and placed it near the engine for photo purposes. But, if the coil is mounted somewhere other than midway on the inner fender, that may be part of your problem.

The "ballast resistor" for the ignition system is not a physical resistor (as you show) but a resistance wire in the wiring harness. The piece you show in your second photo is part of the wiring to the distributor, but not part of the wiring going to the coil, as I remember it. Thus, more information on what ignition system you are currently running would be of use.

Yes, the voltage to the coil should be measured and the coil suited to the input voltage. But, with the engine running, the "12 volt" input is 13.5 volts to 14.5 volts while the "6 volt" input runs closer to 8 volts than to six. Measure what is present and post the information. That will make it easier to advise you.

Les
Les Bengtson

How hot a coil gets is almost entirely down to the ambient temperature. Having said that the 12v coil will get hotter than a 6v under the same conditions, as both coils are carrying the same current but the 6v coil only has half the voltage, and Watts equals Volts times Amps.

The coil will always (engine running for a while) be hotter than the surrounding air in the engine compartment, which is being heated by the engine and the radiator. In January this year i.e. a UK winter with about 10C ambient mine measured about 40C i.e barely warm to the touch. In May with 15C the V8 coil was 52C and at 21C it was 58C. In our July heatwave and 27C the V8 (6v coil) was 62C and the roadster (12v coil) 67C.

That ballast is quite possibly the one for the 45DE4 electronic distributor, not the one for the coil. The coil ballast should be a length of resistance wire contained within the harness - white or white/brown at the fusebox end, white/light-green at the coil (2 wires) and solenoid (1 wire) end. The coil ballast should measure about 1.4 ohms, the electronics ballast about 6.6 ohms. However if that resistance has been PO provided for some reason (that blue wrapping doesn't look original to me) then all bets are off as to what you presently have, and as Chris says you can't assume anything.

As for measuring the voltage it all depends on what ignition system you have. If it has been retro-fitted with points, then you can take a voltage measurement. With the points closed, and the ignition on, you should see about 6v on the coil +ve and 0v on the coil -ve. However even then you could still have a 12v coil in series with an incorrect ballast, so you need to measure the resistance of the coil as well, between the spades, all wiring removed.

Originally 6v coils measured about 1.5 ohms when used with points, but North American cars with 45DE4 electronic ignition measured 1.4 ohms. This small difference is to compensate for the fact that electronic systems have a higher 'contact' resistance and hence deliver a lower current when charging the coil than a decent set of points, and not lower as many believe. Later American 45DM4 systems used a 0.8 ohm coil, as that system only gives a very short charging pulse at all revs which means the coil stays cool even though the charging current is higher. 45DE4 systems give a longer charging pulse at a given rpm so the coil gets hotter, and points gives longer still making it hotter still. You can't measure the voltage with either electronic system - with a meter anyway - as they only give a pulse to recharge the coil when the engine is running, and not a steady current at certain positions of the distributor like points do. With those systems you will have to do a resistance measurement again, between the fusebox white or white/brown, and the wires at the coil +ve, again with the wires removed from the coil.

So if you have points, then you need a 1.5 ohm coil, although you would probably be OK with a 1.4 ohms coil. If you have the 45DE4 system you should have a 1.4 ohms coil although a 1.5 ohm would be OK, and if the 45DM4 then a 0.8 ohm coil although again either a 1.4 or 1.5 ohms would work but with a lesser spark. A 0.8 ohms coil with points will run too hot. All need a 1.4 ohm ballast in series with the ignition supply.
Paul Hunt

PS. With points and a stopped engine, points closed, 12v supply, then you should see about 6v as I say. However on a running engine, you will see about 9 or 10v, because the voltage is switching between typically 14v with the points open and 7v with them closed. That's with points. With either of the electronic systems you will see significantly less than that with the 45DE4 system, and less again with the 45DM4 system, as they are giving the shorter voltage pulse to the coil that I mentioned earlier.
Paul Hunt

This thread was discussed on 29/09/2013

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