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MG MGB Technical - Boiling the Coil

I have a '56 MGA with a '72 18V engine converted to negative ground. Negative lead from the coil goes into the D25 dizzy and the positive lead comes in from the harness (replaced five years ago). This system has a penchant for boiling the coil. I replaced the original system (screw in)with a plug in cap, silicon high tension leads, sport coil - gapping plugs to 38 and keeping dwell at 60 - I still boil the coil in just under a year. Have I had a few bad coils, does my set up self destruct or is the position of the coil (rides atop the generator) to blame. Should the points outlive the coil?
Bayard DeNoie

Bayard. I would certainly remote the coil to the right hand inner fender. I have been running the Lucas Sports Coil, on two vehicles, for something over four years with no problems noted. Unless you have always managed to get the 6v version of the coil (which some of the Brits have mentioned exists, but I have never seen one here in the US, only the 12V model), I can only assume that the position of the coil is the cause of your problem. I have seen coils damaged by over tightening of the mounting bracket, but this is obvious when you remove the coil and examine it. The sides of the case are depressed, shorting it out internally. The best person to give a definitive answer to your question would be Paul Hunt. But, my experience is that coils last for tens of years with no problems. Certainly, the problem you are having is unique. Les
Les Bengtson

The archives have tons of info on coils. Make sure you look under mgb and mga.
Fred Horstmeyer

All rubber bumper MGBs had 6v coils - UK, USA, and everywhere else. The primary on these measures about 1.5 ohms, the 12v coil measures about 3 ohms. Usually if a 6v coil is fitted to a 12v system the symptom is very rapid points burning. Coils *are* very reliable. In over 30 years I have only replaced one and that was down to loose rivets causing an intermittent connection after about 28 years service. What do you mean by 'boiling'? All coils get hot in use. What is the symptom and failure mode?
Paul Hunt

Dear Les - Thank you for the suggestion re: moving the coil will probably do

Dear Fred - Thanks for the tip - I've made a reasonable search of the archives and haven't found anything related

Dear Paul - The abbreviated versiion is that after a drive the engine died while the car was in motion. Upon opening the hood I could tell that oil from the coil had been forced out at reasonable force and through a series of checks determined that the coil had heated to a point sufficient to blister the paint on the generator below. I always carry spares and after changing out the fuses (the culprits I believe) and the coil, was on my merry way again.

The longer version includes a shorted fuse two days prior, (but without obvious cause) a second fuse which refused to burn out even at >50 amps and enough red herrings to qualify for a first rate Hercule Poirot plot.

My concern is that I may have only uncovered another symptom and not the true cause... I assume that a significant amount of amps passed through the fuse (which did not blow) causing the coil to overheat - but I am not sure why there would have been surplus amperage in the first place. I have a dash mounted ammeter which did not register anything untoward on the day the coil went out. I found no reason for the other fuse to have blown two days prior. No loose wires, no trace of electrical problems. They were very old fuses. The car runs great again... but I keep the fire extinguisher handy and continue with the electrical checks. If anyone could confirm that a faulty fuse at the A3 coil connection could cause the burn out, it would make me rest easier.
Bayard DeNoie

Bayard. I do not have access to all of my MGA manuals right now, hence cannot look at a wiring diagram. I do not remember the coil being a fused circuit. It certainly is not on the MGB. In any event, a bad fuse would not cause a coil to overheat and die. It can only prevent something happening if a part is bad or shorted, but cannot, to my understanding, cause such a thing. Both of my MGA 1600s had the coil mounted on the inner fender. Do not know if this was a modification or standard practice as both were puchased used 15+ years after they left the factory. As to damaging the paint on the generator, I wonder if the generator put out enough heat to damage the coil? Paul H is much better versed on electrical theory than I. But, I wonder if you could have had some form of voltage spike. Perhaps a temporary volt meter wired into the input line to the coil? If the mechanical voltage regulator was causing some voltage spikes, that could explain both the blown fuse and the overheated coil. At least I think it would. Perhaps Paul can comment on this. I have seen several of the mechanical regulators go bad over the years, normally through burned points sticking. Les
Les Bengtson

Dear Bayard,
Sounds as though your ballast resistor has given up the ghost. It allows the coil to get 12 volts whilst the starter is cranking but drops the voltage to 6 volts once the engine is running. Try putting a voltmeter across the coil whilst the engine is running. Not sure where to find the ballast resistor. I think it is somewhere in the loom. The Bently handbook shows it as a "resistive cable". You should have two leads to the positive terminal of the coil, one from the starter motor (12v whilst cranking) and one from the "resistive cable" (6v whilst running). If you have 12v all the time, buy a new "resistive cable" (Moss?) and connect in series to the wire which is presently giving 12v. Hope this is of some use. Live long and prosper, Greg
Greg Taplin

Oops! Did not read the MGA bit! Do A's have ballast resistors? If not, disregard my last post. Greg
Greg Taplin

Greg. The MGA, like MGBs prior to the RB cars, ran a straight 12V system with only one wire going to the coil.

Bayard. I managed to dig out my MGA workshop manuals and examined both the 1500 and 1600/1600 MK II wiring diagrams. In both cases, the A-3 terminal on the fuse box is power input directly from the ignition switch. All circuits from that terminal are hot whenever the ignition switch is turned on. These are unfused circuits. The A-4 terminal is connected to the A-3 terminal by the fuse and wires connected to A-4 are fused circuits. Hence, the actions of the fuse are only marginally relevant to the problem you are having. The marginal relevancy is that a blown fuse could have indicated a voltage spike. This goes back to what I said about possible voltage regulator problems. I have seen one case similar to yours. That was when the coil kept getting power, after the engine was shut off, due to a bad solenoid. (This was on a 65 Mustang.) The constant power to the coil caused the coil to overheat and burn out. But, in this case, the engine could not be shut off using the key switch and I had to pull the coil lead to stop the engine. This is not a characteristic that you describe. But, with the ignition off, check the voltage input to the coil. If you are showing voltage present, that is part of your problem. Les
Les Bengtson

I didn't realize boiled coil was such a delicacy, but does it really have to be boiled for so long? a year seems so excessive.
Jim Blackwood

Jim - Boiled coil with 3% truffles is to die for. The coil is only boiled for three minutes but you need to marinate for a year for full flavor! Serve over grits with ocra

Les, Thanks for the tip. I checked the coil with ignition off and there was nothing - 8 volts with ignition switch on. It is possible that when I was checking valve clearances the day before(using the starter motor to rotate the engine) that the solenoid plate (remote on the MGA) was locked close to the contact point by the cable housing being wedged against the outside of the cable housing flange (it has happened) possibly causing an arc (?). Would the voltage regulator spike intermitantly?
Bayard DeNoie

Bayard. Yes, if the mechanical voltage regulator has points sticking intermitantly, it could have an intemitant spiking problem. Back when American cars had mechanical regulators, it was recommended that the voltage regulator be changed everytime a new battery was purchased. The Brit mechanical voltage regulator was designed to be user serviceable, but I have never seen anyone service one. They use a points type set up, just as the fuel pump and ignition system does. The points are not cycled as frequently as the fuel pump or distributor, but they are cycled, can and do burn and can stick. Once again, adding a temporary volt meter to the line going to the coil would be an easy method of trouble shooting to see if this is part of the problem. Les
Les Bengtson

I agree with Les and the others - no ballast, no fuse, neither would a bad fuse cause the coil to burn out, but I reckon it would take a huge voltage to overheat the coil to the extent described, and that would almost certainly show up on the ammeter. What does the primary of the coil measure and what is the condition of the points like?
Paul Hunt

Paul and Les - Thanks for your continued support. The coil primary fluctuates between 2.4 and 2.6 ohms. The distributor points are six months old (4K miles), appear gray and smooth. The points in the voltage regulator appear brand new (replaced when I replaced the battery in 2000). No evidence of burning/sticking/pitting anywhere. The coil,which yesterday measured 8 volts with the ignition on, today measured 11 volts with the ignition on - (engine has not been started since the first measurement)- still zero with the ignition off.

On the day the coil boiled I had an incident in the morning. I had started the engine and allowed it to warm to 160 with the choke out a little. I heard the engine running rough and thinking it to be the choke went out and closed the choke.The engine has been so reliable I don't even have to get in the car to start it. I went to a store and stopped the engine. When I returned - I had no ignition. I opened the hood - saw oil - but thought it must have come from the dip stick port - (I remove the dip stick when adjusting the timing) - adjusted the coil wire (did not feel loose) and the car started slowly as though the battery were worn down. I let the engine run and watched the ammeter. When the meter returned to zero, I cut the engine and restarted again. This time the engine zipped over twice and started right up. I thought it might be the coil but decided to take the car for a drive. No sign of a bad coil. Really enjoyed the ride along the Delaware River and before I knew it I was 75 miles from home. The car ran perfectly. I turned around and upon acceleration felt the coil fudge. It continued to fudge at higher RPM's (above 3k)and fudged under load going up the many steep hills on the return home. Thought I had it made - but as I came down the last hill into town, the engine died. You know the rest. Since this has happened before, I assume it cannot be a fluke. Thoughts?
Bayard DeNoie

(Place smart a$$ed comment here) Truffles? I'd never have guessed.
Does the coil mount nosed down? Often the seal is less than perfect and allows the oil to gradually escape until the coil basically melts down. If 100 pt originality is a concern, simply carry a spare, otherwise invert it and perhaps replace with a Jacobs or other quality performance coil, it can be painted semi-gloss black to match better.
Jim Blackwood

Bayard - 2.4 to 2.6 ohms sounds like either a 12v 'Sport' coil (mine measures 2.4) or a bad standard coil. My Sport secondary measures 6500 ohms whereas the standard measures 5400, it would be interesting to know that too.

On your era of car I would expect to see full battery voltage i.e. 12 point something on the coil +ve with the ignition on but the engine stationary, and not 8 volts or 11 volts. However that just points to another possible problem (since when did our cars only have one?) that would be nothing to do with the coil overheating. Unless the primary resistance dropped so low that it tried to pull more current than the ignition switch could supply, of course ... But then slow cranking implies a dodgy connection in the heavy current circuit, which again would be nothing to do with a boiling coil.

Paul Hunt

Jim, Thank you. The coil is mounted horizontally over the generator so half the time its pointed up and half the time down (lots of hills in NE PA. As Les noted, BMC apparently moved the coil to an upright position on the right wheel well wall and now I know why. I will do the same. I suspect that the oil just leaked out over time and then roasted the sucker when it ran dry (I'll neve say 'boil' again). While I don't discard anything original, I enjoy and demand much more from the 18V than I ever did from the 1498 A engine.

Paul & Les, thank you for your input. I learned a great deal.

PS - forget the coil recipe, I've got better ones
Bayard DeNoie

I suppose roasted coil might be even better, given that roasting generally seems to impart a more savory flavor to most things compared to boiling. Can't wait to see those recipies.

Jim
Jim Blackwood

Bayard - cooking from heat generated internally from the windings is boiling, being cooked from external sources is roasting. We need to establish the source of the heat ...
Paul Hunt

This thread was discussed between 08/09/2002 and 16/09/2002

MG MGB Technical index

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