Overheating Ignition Coil

[Wayne croxford]

Hi
On a recent trip to Wonnangatta after about 4 hrs driving 1991 3.9 R/Rover started to missfire at low revs getting worse untill it stopped started looking at ignition system and found coil to be extremly hot , luckily had a spare changed it bueaty problem solved ,not so next day after a couple of hrs driving same thing.
I've now tried 3 different coils ,the original a new Lucas & a Bosch all 12 v Electronic Ignition coils all overheat after a period of time seems to be worse at low revs
Have thought about fitting a voltage regulator to coil supply but shurly there must be some other reason
Hope someone out there can shed some light on my problem
Regards Wayne

[vnx205]

Some coils are (or at least used to be) designed to be used with a ballast resistor which was bypassed while cranking. They were really 10 volt coils.

That briefly supplied the full 12 volts to the coil for starting, but settled back to 10 volts once the starter motor was no longer operating. Actually it probably just compensated for a voltage drop caused by the current draw of the starter motor.

If one of those coils was used without a ballast resistor, the points became burnt pretty quickly and the coil became hot and eventually failed.

At least that is what happened with my Series III that had previously been fitted with a coil requiring a ballast resistor, but had no ballast resistor.

Is that possible on your vehicle?

[Reads90]

Yeah as said I was always lead to believe that the coils on rover v8's are 9v not 12 they have an inline resister to drop it down to 9v from the 12 v

[slug_burner]

Current runs through the coil while the points are closed, the points are closed for longer periods at lower revs, that might explain why your coil is getting hotter at low revs. I think that you have an issue with the ballast resistor not switching back in after starting.

I am sure a mechanic with experience in this area will come along soon and give you the fix.

[101RRS]

I thought a 91 3.9 is full electronic so no points ;)

I would be looking to the ignition amp getting hot and opening up a dry joint.

Problem with three different coils points to another issue as it is unlikely that 3 coils would have the same issue.

Garry

[bee utey]

As garrycol said, this is electronic ignition. They use coils which have a low primary resistance, much lower than 9V or 12V points coils. There is no ballast resistor. Unlike points, the transistors in the amplifier don't switch the negative side of the coil to zero volts, they switch a set voltage below supply. That is why these are known as 'constant energy" ignition systems in LR speak.

The most likely suspect is the Lucas ignition amp, they seem to be able to malfunction in the most amusing ways. I replace any I see with a Bosch module/amplifier, part number BIM024. There are some modifications needed to fit one as they have to be mounted remotely from the distributor.

Thread here: http://www.aulro.com/afvb/technical-chatter/96950-lucas-ignition-amplifier-replacement-bosch-024-a.html

Last week I had an '86 Rodeo come in on the end of a rope with failed electronic ignition. After multiple phone calls to track down a working dissy or replacement module (during the xmas break), I gave up and modified it to run an external Bosch module. The ignition now works great, another successful adaptation. I have long lost track of how many different vehicles that can get these modules fitted.

[vnx205]

I thought a 91 3.9 is full electronic so no points ;)



As garrycol said, this is electronic ignition.

It appears that there have been some advances in Land Rover ignition systems since 1973. :D

[slug_burner]

I thought a 91 3.9 is full electronic so no points ;)

I would be looking to the ignition amp getting hot and opening up a dry joint.

Problem with three different coils points to another issue as it is unlikely that 3 coils would have the same issue.

Garry

That might be the case however the principle is still the same, you have to run a current through the coil and then interrupt it to get the HV. Points or transistors both act as a switch to interrupt the current.

I knew the real mechanics would come along and sort it out.

[Wayne croxford]

Thanks for your replie 91 Rangie is electronic ign no resistor looking for something which is causing coil to heat up
Thanks Wayne

[Wayne croxford]

Thanks for your replie as you say no resistor in circuit ,can't work out if fault is in coil supply voltage or caused by ign module or amplifier
Regards Wayne

[Wayne croxford]

Thanks for your help you obviously know your electrics still not sure if my problem is not just too much voltage supplied to coil .ie alt voltage regulation ,when running with charged batteries voltage at coil is 13.5 is that too much for a 12v coil
Regards Wayne

[bee utey]

As I said the supply voltage does not affect the coil output. Electronic ignitions happily run from about 10 volts to 15 volts without any change in output. The "dwell angle" set by a timing capacitor inside the amplifier will be out of calibration, a much longer dwell time than necessary will cause excess heat in the coil. You need a new ignition amplifier, either genuine or adapt another type as I suggested before.