Overheating coil

[frad]

Hi
I switched to a 12 volt coil setup on my sprint engined spitfire a few years ago and have since had problems with the coil getting hot and occasionally failing altogether.
This happened again yesterday, after having the car running for a while on tickover setting up carbs etc.
When I turned it off it wouldn't restart. Checked the coil which was hot and had leaked cooling oil.
I stuck another coil on and it started and ran ok. On checking the voltage at the coil it was nearly 14 volts with the engine running on tickover.
After it happend the first time I pulled out the electronic ignition and went back to points as I thought that may have been causing a problem.
Is the alternator causing the problem or should coils be able to cope with the higher voltage.
Bit of a risk on a long drive at the moment stopping for fuel etc.

Any help or ideas appreciated

Thank you

Mick

[TedTaylor]

Is the coil a ballast resistor type?  

If it is, because it is designed to run at a lower voltage because the ballast resistor 'chops down' the voltage (usually to 6 volt) apart from when the starter motor is in operation it will overheat and stop working, often working again as it cools down.  This will go one for a number of cycles (stops working, cools down, starts working again, stops working, cools down etc.) but eventually the coil gives up the ghost completely.

However if your ballast resistor/resistor wire should ever fail so that no current is getting to the coil as once happened to me, substitution of an ordinary bit of wire will get you going again but remember that the coil will overheat as above.  The trick once got me the 15 miles to school and then back (when I worked as a teacher) though the coll was fooked afterwards.

MUT

[frad]

Hi

When I removed the ballast resister I changed the coil to a 12 volt one and am now on my third.

Thank you

Mick

[Dannyb]

It might say 12v on it but it could still be a ballast  type. The best thing is to measure the resistance. 1.5 ballast, 3 ohms non ballast.

[piman]

Hello Mick,

12 volts is the nominal voltage and generally it will be nearer 14 volts with the engine running. unless the generator is giving more volts than the battery it will not charge, so what you are seeing is normal.

Alec

[Toledo Man]

To get an accurate resistance reading you will need to disconnect everything from the coil.

[frad]

The last coil to pack in was a Lucas 12v non ballast sports coil I have now purchased an accuspark high power sports 12v non ballast coil. Just concerned I'm going to have the same problem.

Thank you Alec just what I thought, the alternator kicks out nearly 14 volts so the coils should be able to handle that. Maybe at higher revs when the coil is charging and discharging quicker 14 volts is manageable. Just thought the voltage regulator may be faulty and the output should probably be lower on tickover.
Would things like points and plug gap cause coils to get too hot or even a dodgy condensor cause problems ?

Thank you

Mick

[piman]

Hello Mick,

a wider than specified plug gap will take a higher voltage to create the spark, but is within the capabilities of the coil within reason, I don't think it takes any more current though or at least to affect how hot the coil runs. Too narrow a points gap means the coil is energised for longer so may increase heating?

Alec

[RobPearce]

Indeed, Alec, too much dwell will result in the coil running hotter. Whether you could make it overheat that way I'm not sure.

[Hogie]

Quoted from frad

The last coil to pack in was a Lucas 12v non ballast sports coil I have now purchased an accuspark high power sports 12v non ballast coil. Just concerned I'm going to have the same problem.



Thank you

Mick

Hi Mick,
           over the last couple of years there have been a few questions asked about the Lucas sports coil.
The biggest is that it is not a Lucas product. Many have been failing, probably a quality/price issue.

Roger

[frad]

Thanks for your replies

New coil fitted today and car run for a while coil getting warm rather than hot so will have to see how it goes.

Mick