ignition coil cutting out due to overheating

[mikeeskriett]

Hi,

I've just taken some readings from my Gold as the Bronze is flat.

Voltage at the battery with the ignition on is 12.2v I've had to measure mine from the +ve on the coil and the -ve to the battery as i didn't get much in the way of readings. At the coil with the ignition on I get 12.2v with the engine on fast idle I have 13.8v.

You could have a combination of the voltage drop and a coil being cooked giving you issues as suspect the ignition module would be out of voltage tolerance.

No the question is where in your loom is voltage drop. Where are the main power lines tapped to. On the Metro it's normally direct to the battery or to the solenoid. Mine are direct to the solenoid on the starter motor I also have a direct feed from the alternator to the starter solenoid hook up as I'm running a 72amp alternator.

I guess you could hotwire your car and test it that way do a direct wire to the coil.

Cheers,

Mike

[Hans Efde]

Thanks for the info Mike. I'd be interested to know how hot your coil is after a drive. 13.8V seems a lot to me for the coil. No doubt it will give a good spark!
You have the pre-84 loom with the ignition coil power taken from the starter motor. Mine is via the ignition switch which I prefer. Also the alarm is connected to the ingnition. Possibly that has an effect on the voltage going to the coil (it shouldn't be). I'll follow the wire loom to see for problems. I know from other problems I had (windshield wiper going slow) that the builder of my car was no soldering genius.

[mikeeskriett]

The guy who built mine didn't seem overly fussed if things are attached in the right place (spent today trying to get the exhaust and gear linkage to co-exist), so I guess each builder has their weakness!!

Mines a 1988 loom but for for some reason it's installed in an odd way so the connectors for the main power don't reach the battery...i did try too work out why but gave up and just used it as is. Mine doesnt get particularly warm but then again everything is hooked up as Rover intended so I wouldn't expect much. When I get it down off the stands to get the laser thermometer on it.

Cheers,

Mike

[Geoff Butcher]

Yes I just bought a 1.6 Ohms ballast resistor on Ebay. I couldn't find a local garage that has these things in stock anymore, so thank God for Ebay.

If the Aldon coil has a resistance of 3 ohm and you've put a 1.6 ohm resistor in series with it, then the voltage across the coil will only be of the order of 8 volts. It seems daft to me that they say you should use a resistor for longer coil life, they should design the thing properly in the first place! It's a wonder it will even cold-start with only 8v across a nominal 12v coil.

[Geoff Butcher]

You have the pre-84 loom with the ignition coil power taken from the starter motor. Mine is via the ignition switch which I prefer. .

I know I'm dim, but I don't understand this. Surely everyone's power to the ignition comes from the ignition switch? There's a clue in the name!

[mikeeskriett]

With a ballast resistor in the loom the voltage is circa 9v on the coil wire. To combat the cold start issue there is normally an overide cable which is switched from the starter solenoid which supplies 12v while the starter is cranking to give you a bigger spark. The coils themselves are designed to run at 9v, unless of course you have the more modern system which runs at 12v.

[Geoff Butcher]

Hans is saying that Aldon 'advise' using a resistor. So is it a 12v coil or a 9v? And if it's a 9v does he have the 12v bypass for cranking or not?

edit: I've been looking at the Aldon web site and they say their coils WILL OVERHEAT if used for normal road driving.

[mikeeskriett]

Hi,

The loom in Hans car appears to be a standard turbo loom or GTM copy so it's has only a 12v feed to the coil so won't have the bypass. The Turbo is setup from the get go to use electronic ignition however at some point in history the car has been changed to use a points ignition system possibly due to a custom dizzy or possibly because that was what was available to the guy who built it at the time.

My guess is that it's intended for racing where you run a 9v coil on 12v the theory being a bigger spark thus more power which is fine for short bursts but it'll get nice and warm in the process so for road use you'd need to drop the voltage to preserve the coil life. One concern with the inline resistor is the wiring gauge to the coil and making sure you don't melt the loom, I've seen that happen on my brothers MGB when he got sold the "correct" coil by hellfrauds.

Cheers,

Mike

[Hans Efde]

What I don't understand is the reading I got with the Aldon compared to the Bosch. With the engine running the Bosch measures 13.3V over the coil and the Aldon 7.7V. They both are 12V coils with (when cold) a resistance of 3 Ohms. It seems the loom doesn't have a ballast wire.
I would expect with 7.7V on the coil no problems at all. Possibly the Lumenition ignition has an effect on the coil. I have to find more info on that. Else I reinstall the breaker points ignition to see what it does for the coil. On another forum it was suggested that electronic ignitions have a different "closed" time (say 300degr instead of 220 degrees) so the coil is loaded for a longer time causing the heat build up. With breaker points it's easy to adjust the points gap, so the loading time of the coil is adjusted. Maybe that can be done with the Lumenition as well.
Cheers, Hans
(another beautiful day, so if the family don't have plans I'm off the the shed!)

[Geoff Butcher]

I can understand a coil failing totally through overheating, but not why it should recover when it cools down. After all. it's only two coils of wire.