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MG MGB Technical - Overheating coil

Hi, I have had problems with my GT cutting out when hot. Fault finding found that the coil was not producing sufficient power to give good plug spark, if left to cool then all would be well. I replaced coil with a Accuspark Coil Red and the car has given good service for some 6 months. Today, queuing to get into a show for some 15 minutes the engine cut out and would not start. Checked engine and no spark at plug. Allowed the coil to cool down and it started first time.
Question: Why do coils overheat? What can I do to overcome the problem?
Many Thanks for your time.
Pete
Pete Dyble

1. High Ambiant temperature.
2. Running a "Ballast coil", i.e., a 6/7volt coil on a non ballast (12volt system.
3. Coil with internal short.
4. Crap coils.
Allan Reeling

Too long of a dwell time - check points gap. Cheers - Dave
DW DuBois

If you are checking the spark at the plug, it isn't necessarily the coil that's at fault.

You need to check for a spark at the king-lead from the coil.
Dave O'Neill 2

Dave O'Neill has an excellent point. The coil is tested with the coil lead (king lead). Testing at the spark plug introduces three more variables, the distributor cap, the rotor, and the spark plug lead. If fire is checked for by holding the spark plug to the block while cranking, it becomes a fourth variable. Check the coil lead and, if that is bad, check for voltage on the coil input terminal, then the output terminal. If you have input voltage, but no output voltage, the coil is shorting internally. If you have output voltage but no spark, there is a problem within the distributor, points or a bad ground wire.

Les
Les Bengtson

The heat in a coil will vary with ambient temperature. In tests I've seen non-ballasted and ballasted running at about 40C in an ambient of 10C, but at 60C+ at an ambient of nearly 30C. A coil should continue to produce a spark at temperatures much higher than that, they do run in desert states after all.

Having said that it does seem that coils will stop producing a spark for some reason if they do get too hot, as I found when inadvertently leaving the ignition on with the engine not started, not once but twice. Particularly annoying as it was a relatively new coil, fitted because the rivetted spades on the original were giving an intermittent bad connection. I was relieved to find that cooling it down with a wet cloth wrapped round brought back the spark, and it has continued to work correctly for many years since.

A coil in a ballasted system will run cooler than one in a non-ballasted - as long as it has the correct ballast resistance in circuit from the harness. But both will run hotter if the dwell is too high i.e. contact gap too small, and it should be noted that some electronic triggers do run a few degrees more dwell than correctly adjusted points.

As well as checking the king lead for the spark as Dave says, you also need to check that you have voltage on the coil +ve or SW terminal (varies according to whether you have a ballasted or unballasted system) and that your points or trigger are doing to correct thing to the coil -ve or CB terminal. It could be something else getting hot and breaking down.
Paul Hunt

Pete,
Had the same problem a while ago,
Was advised to bin the top seal of the engine bay
Never looked back
Bill
W M Griffiths

Hello Pete
Do you have a photo of the coil in position in your engine bay? I did read somewhere that they are meant to be mounted upside down, the terminals at the bottom, so the oil cools the windings etc. I should imagine yours is already mounted the like this. Just a thought.

Dave
dave bignell

The coil on my V8 has been mounted upright (terminals on top) for the last six years and 25,000 miles and has never given any trouble, not even in 36 degree temps in France this summer. And boy, does it get hot in a V8 engine bay!
Mike Howlett

"And boy, does it get hot in a V8 engine bay!"

Quite, my ballasted measurements were in a V8, but then as mentioned they do run cooler than a 12v coil anyway.

If any mod such as removing a seal appears to solve such a problem, then it is merely getting round the real problem, it is not the root cause of the problem.
Paul Hunt

My V8 doesn't even have a ballasted system. However, the coil is up the front by the radiator mount and there is an air inlet hole beside it because the air filter for the injection is in the same area.
Mike Howlett

This thread was discussed between 23/08/2015 and 25/08/2015

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