21 posts / 0 new
Last post
Crossflow coil question

How hot should the coil get in a crossflow? It's running the Lucas electronic ignition supplied by Caterham in 1987.

Was out for a blat last weekend and the engine cut out without warning. Engine would not restart at all so towed home.. 

I traced the fault to the coil, which has run well for the past 2 years. When I fitted an old coil (possibly not the correct spec to run with the electronic ignition) the engine started immediately and ran well for 20 miles - when there was a loss of power and misfiring etc - the engine would not rev above 3000 rpm but fortunately I was able to drive it home like this. The coil was too hot to touch.

I have just fitted a Lucas DLB105, which I'm told is the correct one for the ignition. I've just been for a test and it all worked very well - however, again the coil gets very hot, perhaps not quite as hot as before but still too hot to touch really.

Are these temperatures normal? I'm concerned that there may be a fault which is causing damage / overheating to the coil. I've been careful to reconnect the +ve and -ve as before and checked the plugs and leads and distributor cap, all of which look ok.

The new coil seems to work perfectly, the engine running as it should but I don't want to risk going too far if the overheating is likely to cause it to fail again.

Any thoughts much appreciated.


I don't know how to tell if it's too hot.

I've been careful to reconnect the +ve and -ve as before and checked the plugs and leads and distributor cap, all of which look ok.

The new coil seems to work perfectly, the engine running as it should...

That's good news. 

Next step is to test the internal resistance. You need the values specified for your particular coil, which might be here, but it would be nice to find something official. 


Hi Simon

Trust all well. If the coil is getting very hot, I would look at the earth connection and the + connection and make sure it is good. It could be that a bad contact could increase current and resistance  and this may be the reason for it getting so hot. I don't recall my Super Sprint coil getting that hot.

By the way, Charles has  been in contact with a long email.





Hi Simon

I've been thinking about your coil. If the voltage is being reduced by a bad connection, then the current increases.  So for example,  a 12 volt system pulling say 6 amps, if you halve the voltage to 6 volts, you double the current to 12 amps. Your 7 is unlikely to go as low as 6 volts, but you get the point. So check every connection and make sure everyone is good.




If you have the plastic rotor that would be your first port of call, they know to fail. unfortunantly the only cure is a new dizzy and you will have to ditch the old Lucas electronic ign too but the new dizzy could have everything inside it.

Elie - please can you explain how does the rotor make the coil hot. Is it high resistance contacts inside the distributor cap ?

I would  expect that Simon is still on his original distributor and the shaft and bearings in the distributor may have worn out, causing bad contacts within the cap. Is this causing the issue ?



I also guess that it's the original ign system with big Lucas box. when a coil is to hot to touch that is not right and the reason is in the electronic box witch is triggered by the rotor in the dizzy. For these systems there are no replacement parts aldo I think I still have that electronic box somewhere lying around. Bottom line is that when there is any problem with the Lucas system replace the whole ign system.

I have two of those boxes somewhere... though finding them might be an issue.

Old school coils do run pretty hot as they are designed for 'fixed dwell' points. The inductance of the coil stores energy in a magnetic field, which is released as the HT spark. The inductance sets the rate at which the magnetic field is charged up when the points are closed, but after a few milliseconds the coil is magnetically fully charged and more electrical power just gets turned into heat. This is actually more of a problem at low revs, where the points are closed for longer. In extremis, leaving the ignition on with the engine stopped and the points closed will make the coil pop!

Modern ignition systems are 'constant energy' systems, where the electronics works backwards from the spark point to decide when to start charging the coil. The modern coil doesn't have to be designed to withstand overcharging as it relies on the electronics to protect it. Fitting a modern to coil to an old school system can lead to problems.

Having said all that, I would say that a coil shouldn't get hotter than 70C, which still feels pretty hot.

Thanks to all for your advice.

Jonathan - is the internal resistance measurement in order to check if the coil has been damaged?

Piers - yes I will check all the connections - I can obviously check the connections at the coil end but where else should I be looking? Are there any connections inside the distributor? I wouldn't be surprised if there are some poor earths etc as it's all 27 years old now... It was excellent to meet up at Goodwood on the Caterham stand! and I'm pleased Charles has been in contact with you - great news.

Elie - yes the rotor is partially plastic - it has a slotted metal component which passes the copper terminals on the cap. I've cleaned all the terminals and the metal surface which connects to the central terminal also. all looks ok. Yes, I've read about the "new" electronic ignition being part of the distributor now.

Thanks again to you all and I will let you know how I get on...





is the internal resistance measurement in order to check if the coil has been damaged?

Yes. they used to go off before they failed. I don't remember them getting hot, but it's certainly plausible... if it's passing the same current (and yours is running well) and the internal resistance is high then there must be extra heat somewhere...