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MG MGA - HOT coil

Ok, here's one for all of you to try to figure out.
I working on a 1959 MGA w/ negative ground and a single battery. On hot days or when the temp in the engine bay gets pretty hot the car stalls. I've narrowed it down to the coil. The fuel system is working fine so, I know its not vapour lock. When the engine quits, the coil is way too hot to touch and the spark is very weak. I can replace it with a known good coil and the car starts right up. (so it is not fuel related)I leave the known good coil in place and run the engine for a while longer and it heats up to where I can't touch it and the engine dies with a very weak spark. I've tried new points, condensor, rotor, cap and wires to no avail. I've tried a ballast resistor (just in case) to no avail. I've checked and cleaned the ground cables on the engine and battery as well as connections at the fuse block. The generator and voltage regulator are working fine. I know this has to be something really simple but,I'm about to my wits end. Any help or suggestions are appreciated.

Thanks, John
john

Hi John. What voltage is listed on each of the coils? They should be 12 volt coils and NOT 6 volt coils. Are the coils you have been using meant to be used with or without ballast resistors? Have you measured the actual voltage right at the coil? Perhaps there is excessive voltage that is making the coils run hot? I can't think of any other things that would make a coil run hot, except perhaps if you had a bad ground connection or a loose connection somewhere in the ignition circuit. Could both coils be defective? An intriguing problem John. Please let us know what you find! Glenn
Glenn

John. Use an ohm meter to measure the internal resistance of the coil from one brass terminal to the other. You should see about 3.1-3.5 ohms resistance if you have a coil designed for full time 12V input. A 6V coil, or a coil maked 12V use with external ballast resistor, will show about 1.6 ohms resistance. If your figures do now show at least 3.1 ohms resistance, replace the coil with a proper one.

Examine the body of the coil. If the mounting bracket has been over tightened, it can result in crushing the exterior and shorting out wires inside. This will cause the coil to overheat and become unreliable in the manner you describe.

If all else seems good, run a jumper wire to the positive (input with a negative ground system) and recruit an assistant to ride along with a volt meter. Hook up the volt meter to the jumper wire and to a good ground and monitor system voltage as you are driving. If it goes over 14.5V, you have a voltage regulator problem.

When you did the negative ground conversion, did you reverse the leads to the coil? SW should be connected to the distributor and CB to the input.

If your coil is still mounted on the generator (do not remember when it changed), consider mounting it on the inner fender as the later cars and MGBs did. This removes it from the heat generated by the block and generator and allows more air to get by the coil and cool it.

Les
Les Bengtson

Les, The ballast resistor seems to be the solution so far. I'll let you know tomorrow. My next question is, without the resistor, Why didn't the points burn? Or should I even worry about this?
Thanks, John
John

John Try a different condenser, as some coils are more tolerent of a defective one. I have even seen engines keep running with the condenser disconnected, but only just. A defective condenser is affected by heat as well and may work well for a short time after startup. All coils get hot. As Les said if wou have changed the car to NEG earth you need to change the terminals on the coil. If you are using a newer coil made for neg earth app, connect CB or - to the distributor. Just something else to try. Denis
D M HILL

John. Points do not seem to be particularly sensitive to the input voltage (within reason). When I switched my daughter's B and my 79B over to a points type ignition system, the retailer (Brit-Tek) suggested that I convert the system to full time 12V input and included a Lucas Sports Coil as part of the package. I know that there have been add on systems which extended point life by using very low voltage to trigger a higher voltage system (Capacitance Discharge?), but, have no experience with these systems myself. I am, mostly, a driver and use my cars as my daily drivers (even in Phoenix summers). I tune the cars up on an annual basis, prior to their emissions testing. Since I still have points, I take the opportunity to clean and lube the dizzy while I have it out. Fletcher Millmore (hope I spelled his last name correctly) has a much better understanding of coils than I and, hopefully, will post soon.

As to the condensor, perhaps. I do not know myself. My limited experience is that a bad condensor leads to rapid points burning, but I have never had a coil over heat because of one. But, I am still learning. My coils get hot, but, I can put my hand on them and leave it there without getting a burn. The cars that I have seen with coils heated hot enough to burn either had the outside squished, due to over tightening of the mounting clamp, or were designed to run at lower voltage than they were being fed. One of the reasons I suggested you check for excessive system voltage.

Wish I could help more. Les
Les Bengtson

John - Check also, that your dweel or points gap is correct. The cars will run with a very small gap (or large dwell angle, but if you have such a situation, it could be causing the coil to over heat as theere is current running through it for a longer period of time (longer duty cycle). Good luck - Dave
David DuBois

Well, Les and Dave have pretty well covered it. The only way for a coil to run hotter than normal is for it to pass more current than designed for, or for a longer time. A coil with the key on not running will get too hot to touch comfortably, since there is constant current flow. As Dave says, if it will run it will get hotter than normal if dwell is too large, but probably not enough to hurt anything. The most common problem is a 6V coil for a ballasted system running on 12V. Check as Les says. It might be possible for a partially shorted condensor to pass enough current to heat the coil but still work - but this condition is not likely to last for long, and you say you replaced the condensor. You say that "the voltage regulator is working fine" - has this been verified with a voltmeter? High system voltage is the only other thing that will cause the coil to draw more current than normal. Typical good voltage is 13.5-14, with 14.5 about max.

You might also see my post on Robert Rushing's "How do I fix this?" on the B tech board. It is entirely possible that you in fact have a hot weather fuel problem, as I explain there. While you change coils, with engine off and bonnet open, it cools off enough to "fix" itself temporarily. Note that this is not the same as "vapor lock" which is caused by vapor formation on the suction side of a pump - not a common problem with electric pumps. This summer is a killer!
FRM
FR Millmore

I did not read the thread well enough in the first place and on doing so see that you have already changed the condenser. What I was trying to get across is that the condenser is also affected by heat, not that it will cause the coil to get hot.Denis
D M HILL

Sorry, that's Robert Browning's thread on the B board.
FRM
FR Millmore

Thanks for all the response.
There was never a fuel related problem. I forgot to mention that the point gap and dwell were set correctly. It seems that Les was right to try the ballast resistor. when I first tried the resistor, I tried it on the wrong coil. With the correct coil, everything seems to working fine. I still wonder why the points never burned or even looked like they never got hot.
Thanks again,
John
john

John It takes a fair while for the excess voltage to show on the points. You may find that you have hard starting when winter comes unless you also fit a starter solonoid with the 12V tapping, but you may get away with it. Denis
D M HILL

This thread was discussed between 25/08/2005 and 27/08/2005

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