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MG MGB Technical - Hot (to touch) coil

I am trying to help out a friend with a 1977 North American spec MGB. The car appears to be, for the most part, unmolested or modified. However, the factory electronic ignition, that was on the distributor, failed and an early MGB points distributor was used to replaced the original distributor. The car run and drives fine. However, the ignition coil gets so hot you cannot touch it. I have tried replacing the coil, with a new 12 volt non-ballasted coil, and have replaced the point, condenser and lead in. Still the coil gets hot and when it get too hot, or something happens internal to the coil, the car stops running until the coil cools off. Then the car starts right up and runs fine. It also seems that the coil is more prone to failing when the car is driving down the street, versus idling in the driveway. But again, when the coil cools off a bit, while still hot to really touch, the car starts up and drives fine. The only other modification on this car is a Weber in place of the Zeinth carb. I realize the car failing could be any number of other issues but I am assuming that the coil getting too hot, to touch, is the cause of the car not running.

I am going to swap out the distributor with another points distributor, another coil and pick up 12 volts from the fuze block but do not really expect any of these items to help (but I’ll try anything!!!).

Any suggestions or help would be greatly appreciated!!

Thanks in advance!!

Fred
Fred Wright

The ballasting for a 12 volt coil, on an MGB, would have been located in the wiring harness. This car doesn't have the ballasting because it had the early electronic ignition system. I would try installing a 3 volt ballast in the power line going to the the coil. It appears to be getting too much current and is overheating. The ceramic ballasts cost about $6. RAY











rjm RAY

*All* rubber bumper cars had ballasted ignition, regardless of whether they had points or either of the factory electronic ignition systems. It was always a length of resistance wire contained within the harness which measures about 1.5 ohms (not 3 ohms or 3v). The earlier 45DE4 electronic system had an *additional' ballast resistance feeding the distributor, which was a separately identifiable component (white and white/blue wires) as well as a direct 12v feed (white). Depending on which of the two factory electronic ignition system you had there are various ways to mess things up when removing it, particularly this 45DE4 system.

Have you measured the resistance of the coil(s) and measured the voltage at the coil +ve with the ignition on and the points are closed? This area famously causes confusion, even to the labelling on the components and packaging, the only way you can be sure what you have is to measure it. 12v coils measure about 2.4 to 3 ohms between the spades (take the wires off first) and 6v coils 1.2 to 1.5 ohms i.e. about the same as the ballast which is why there ends up being about 6v across the coil, the other 6v being across the ballast.

6v at the coil +ve (should be two white/light-green) indicates you have the harness ballast in circuit and a 6v coil, which would be correct, but you could equally have two ballasts in series and a 12v coil. OTOH if you measure 12v there is no ballast in circuit, but you could have either a 12v coil or a 6v coil! Hence the need to measure. Even if you have two white/light-greens on the coil +ve which implies the ballast is in circuit, but measure 12v, there could be a fault at the solenoid such that the white/light-green there is connected to 12v somehow. It should go to its own spade terminal (different to the white/brown which should be on another terminal) which only gets 12v while the engine is cranking,to bypass the ballast and give a boost voltage to the coil.

But if you really do have a 12v coil i.e. about 3 ohms resistance, then none of this should make any difference, you can't get more than 12v fed to the coil (unless the alternator is faulty and putting out too high a voltage). Which makes me think either the coil is faulty, or is *not* a 12v i.e. 3 ohm coil. Too high a dwell/too small a points gap will also result in more heat being developed in the coil, but not so much it stops the engine from heat alone.

At the end of the day it is better to keep the ballast and a 6v coil, and the bypass circuit from the solenoid as it gives improved starting.
PaulH Solihull

Ray/Paul,

Thanks for your reply, response and comments. I will go back (again) and check voltage and resistance measurements as you suggested and you indicated. However, I must admit that I am a little confused right now.

I have worked on several MGBs over the years. Both early and late model MGBs. Mostly all North American spec cars and lots of them were “played” with before I did some work on the cars. Only recently did I have a chance to work on a factory MGB-GT V-8 car that my friend brought over from the UK. With that as my background, I have always been of the opinion that the ballasted ignition was on all the later model North American cars and that the ballast was bolted to the inner fender with one of the same bolts that held down the coil. This ballast has since been removed since, as I indicated, the car is currently using/running on a standard 12 volt circuit ignition and a new coil for a 12 volt system. Om my friends B-GT V-8 I noticed the ballast wire in the harness as part of re-wrapping the harness. Still, I will go back and double check both voltages and resistance measurements as you indicated and suggested.

In closing, assuming the new coil is a 12 volt coil and if I did have >12 volts going to the coil off a ballast wire, don’t you think the coil would be cool since it was not working at its intended working voltage?

Between this cars coil problem and my C blowing coolant out the expansion tank for no apparent reason, I am going nuts!!

Again, thanks for your response and comments.

Fred
Fred Wright

"Assuming the new coil is a 12 volt coil and if I did have >12 volts going to the coil off a ballast wire, don’t you think the coil would be cool since it was not working at its intended working voltage?"

Do you mean <12v i.e. less than 12v? If so then that would be exactly the case, a 12v coil would either run at the correct temperature with no ballast (unless the alternator was putting out too much voltage, or the dwell was much higher than it should be), or at about 2/3rds normal temperature if there were a ballast present. Which is why I think you must have the incorrect coil.

Incidentally, the engine *must* be running to get the correct coil temperature. With the engine stopped but the ignition left on the coil *will* overheat (points and the early electronic system at least will almost certainly leave the coil powered continuously). I have found that inadvertantly leaving the ignition on means the car won't start afterwards until the coil has cooled back down again.
PaulH Solihull

Check the coil resistance. If it is only about 1.5 ohms it needs a ballast. If it is about 3.5 ohms it doesn't.
Art Pearse

Paul,most cars in NA used to use a 9 volt coil along with a ceramic resistor. When the car was started, the starter terminal would provide 12 volts to the coil to make starting easier, very similar to the system used on the MGB. Today, when you go to buy a coil, not very many salesmen know what a coil even looks like, since all or most new cars have a single coil per cylinder. It's very easy for Fred to have been given the wrong unit, as the auto parts suppliers don't stock as many different parts as they used to. I just overhauled a starter on a '93 Chevy 350 and had to move heaven and hell just to locate a new, not rebuilt, starter drive. This could explain the coil getting so hot since the local store wouldn't have likely stocked a standard coil for an MG. RAY
rjm RAY

Exactly why I and Art say the only solution is to measure the resistance and the voltage.

Chrome bumper cars didn't have the 6v coil, the ballast or the boost for starting, rubber bumper cars did. So even if you get a coil supposedly correct for an MGB you have to check it is the right one for your car.
PaulH Solihull

This thread was discussed between 14/09/2010 and 17/09/2010

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