Poor Ignition Coil Reliability (thoughts?)
Posted: Sun Aug 09, 2026 11:14 am
I would really like to know why coils are so unreliable these days. During Minors on Tour there must have been at least 3 coil failures across the participants.
My car broke down yesterday on the way to relative's 60th wedding anniversary. Unfortunately, I was not driving or with the car and had to advise over the telephone which was rather difficult when speaking to a non-mechanical person.
It seemed there was a spark using a neon tester or so was told over the phone but in the end the car was recovered back home from Warwick to South London on an AA low-loader. This was a pity as there was a spare coil in the car all along.
In the early hours of this morning when the car arrived back I took one look at the coil and identified it as the culprit. Somehow the bracket clamping screw had stripped its thread so the coil was loose in the clamp. It had been chattering away as evidenced by the paint being significantly damaged. It still had oil in it and there were no signs it had leaked. Immediately after breakdown I asked about how hot the coil was and I was told it was warm but not that hot. So it seems it did not overheat but was probably wrecked internally from vibration exacerbated by being loose in the bracket. Testing the coil showed a normal primary resistance (3 Ohms) but open circuit in the secondary windings. An old coil put on the car and it started instantly.
The other coils I have had fail definitely overheated. The symptoms were sudden onset of misfiring causing me to immediately pull over. Very hot coil which would work again when cooled down but would never work reliably again, ultimately failing within a short distance.
I believe this affects the whole classic car community and it isn't something limited to Morris Minors. It seems to affect bulkhead mounted coils as well as dynamo mounted coils.
I wonder whether there is a difference in the quality of the wire that is used for the windings in the coil, a difference in the varnish, different types of oil or the actual physical internal construction. Older coils almost certainly contain PCBs (polychlorinated biphenyls) in the oil. These were banned as very nasty toxic substances that do not break down in the environment. They were used in oil cooled electrical equipments such as coils and transformers and although nasty chemicals they did a very good job.
My other thoughts are, is there something else going on that kills coils. As it seems they are less robust for the reasons speculated on above, are they more sensitive to over-voltage? In my case I am going to test the RB106 regulated voltage by measuring the voltage between D (dynamo output) and earth whilst raising the engine revs. I will be checking that the voltage does not rise too high. If the regulator allows the voltage to rise too high it could cook any electrical component.
It is also possible to overheat a coil if the points gap is too narrow which causes the dwell time to be too long. The coil remains in circuit for too long each cycle and so over time, with the engine running it gets hotter than it should. We all know a coil can be cooked by leaving the ignition on with the points closed. A spark gap that is too large needs a higher voltage to jump the gap. This can also unduly strain a coil but if it is rated as "sports" type coil it should be able to cope with 30 thou rather than the standard 25.
Incorrect coil primary resistance will rapidly kill a coil. In my case this is not relevant. It may be relevant when buying old coils of unknown specification. They should be tested for the primary resistance which is 3 Ohms or very close to it.
The only modern coils I have found to be truly reliable are the resin filled coils made by Pertronix. Fingers firmly crossed but I have never had one of these fail. I wish that this had been the coil on the car (it would not have broken down) but I had taken it off to repaint it. Needless to say it is back on the car. Scruffy or not it works! These are the coils sold by Martin Jay (Distributor Doctor).
Anyway, just my musings and perhaps a ventilation of my frustration. Any thoughts or experiences to share?
Stephen
My car broke down yesterday on the way to relative's 60th wedding anniversary. Unfortunately, I was not driving or with the car and had to advise over the telephone which was rather difficult when speaking to a non-mechanical person.
It seemed there was a spark using a neon tester or so was told over the phone but in the end the car was recovered back home from Warwick to South London on an AA low-loader. This was a pity as there was a spare coil in the car all along.
In the early hours of this morning when the car arrived back I took one look at the coil and identified it as the culprit. Somehow the bracket clamping screw had stripped its thread so the coil was loose in the clamp. It had been chattering away as evidenced by the paint being significantly damaged. It still had oil in it and there were no signs it had leaked. Immediately after breakdown I asked about how hot the coil was and I was told it was warm but not that hot. So it seems it did not overheat but was probably wrecked internally from vibration exacerbated by being loose in the bracket. Testing the coil showed a normal primary resistance (3 Ohms) but open circuit in the secondary windings. An old coil put on the car and it started instantly.
The other coils I have had fail definitely overheated. The symptoms were sudden onset of misfiring causing me to immediately pull over. Very hot coil which would work again when cooled down but would never work reliably again, ultimately failing within a short distance.
I believe this affects the whole classic car community and it isn't something limited to Morris Minors. It seems to affect bulkhead mounted coils as well as dynamo mounted coils.
I wonder whether there is a difference in the quality of the wire that is used for the windings in the coil, a difference in the varnish, different types of oil or the actual physical internal construction. Older coils almost certainly contain PCBs (polychlorinated biphenyls) in the oil. These were banned as very nasty toxic substances that do not break down in the environment. They were used in oil cooled electrical equipments such as coils and transformers and although nasty chemicals they did a very good job.
My other thoughts are, is there something else going on that kills coils. As it seems they are less robust for the reasons speculated on above, are they more sensitive to over-voltage? In my case I am going to test the RB106 regulated voltage by measuring the voltage between D (dynamo output) and earth whilst raising the engine revs. I will be checking that the voltage does not rise too high. If the regulator allows the voltage to rise too high it could cook any electrical component.
It is also possible to overheat a coil if the points gap is too narrow which causes the dwell time to be too long. The coil remains in circuit for too long each cycle and so over time, with the engine running it gets hotter than it should. We all know a coil can be cooked by leaving the ignition on with the points closed. A spark gap that is too large needs a higher voltage to jump the gap. This can also unduly strain a coil but if it is rated as "sports" type coil it should be able to cope with 30 thou rather than the standard 25.
Incorrect coil primary resistance will rapidly kill a coil. In my case this is not relevant. It may be relevant when buying old coils of unknown specification. They should be tested for the primary resistance which is 3 Ohms or very close to it.
The only modern coils I have found to be truly reliable are the resin filled coils made by Pertronix. Fingers firmly crossed but I have never had one of these fail. I wish that this had been the coil on the car (it would not have broken down) but I had taken it off to repaint it. Needless to say it is back on the car. Scruffy or not it works! These are the coils sold by Martin Jay (Distributor Doctor).
Anyway, just my musings and perhaps a ventilation of my frustration. Any thoughts or experiences to share?
Stephen
