Poor Ignition Coil Reliability (thoughts?)

[svenedin]

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

[KeithL]

I have a Pertronix Flamethrower coil from Martin Jay at Distributor Doctor as well, and it has never given me a problem in the 7 years I have had it. Martin is the man to go to for all ignition related products.

[geoberni]

Back in 2019 I posted this image of the broken solder joint that had caused my problems in 2018.


You can see how it had separated, which wouldn't happen if sufficient solder had been applied. It was probably a 'dry joint' when made, meaning it had not been kept motionless until the solder fully cold.

This is probably the cause of most modern coil failures, rushing the manufacturing and lack of quality assurance.

[michael4]

My coil is bolted vertically to the inside of the wing rather than horizontally on top of a hot alternator.

I have no idea if this makes any difference but it just seems more 'sensible'.

[svenedin]

I have a Pertronix Flamethrower coil from Martin Jay at Distributor Doctor as well, and it has never given me a problem in the 7 years I have had it. Martin is the man to go to for all ignition related products.

He really is and both Martin and his assistants are extremely helpful and knowledgeable. A great asset to the classic car community. I did wonder whether the resin filled coils would get hotter than the oil filled ones but there really seems to be no noticeable difference on a Minor. Apparently the resin filled coils are not very suitable for fast revving engines with large numbers of cylinders but in a Minor they are well under-stressed. Resin filled is supposed to withstand vibration better and this does seem to be true. Having said that, the old coils were dynamo mounted and they didn't fail for decades so something must have changed.

Stephen

[svenedin]

Back in 2019 I posted this image of the broken solder joint that had caused my problems in 2018.


You can see how it had separated, which wouldn't happen if sufficient solder had been applied. It was probably a 'dry joint' when made, meaning it had not been kept motionless until the solder fully cold.

This is probably the cause of most modern coil failures, rushing the manufacturing and lack of quality assurance.

As the failed (expensive Lumention, oil filled) coil is well out of warranty (it had been in a box in the garage for many years) I may do an autopsy to satisfy my own curiosity. On the other hand I might still send it back to Lumention so that they can do their own quality control. I will e-mail them and offer to send it back. I don't care if my postage is not refunded, I just want this awful failing coil saga to stop blighting classic motoring! It's dangerous to suddenly break down, inconvenient, upsetting etc.

Stephen

[svenedin]

My coil is bolted vertically to the inside of the wing rather than horizontally on top of a hot alternator.

I have no idea if this makes any difference but it just seems more 'sensible'.

It does seem sensible. I have no room there due to the screen wash bottle. I keep the coil on the dynamo for originality's sake.

Stephen

[mobylette]

I was going to ask where it was mounted. I'm pretty sure my own coil failure some years ago was vibration induced, the engine in that case was in dire need of attention, so the extra vibrations would not have helped. I do think it sensible to move the coil to the bodywork somewhere convenient away from engine vibrations, but of course modern coils should be better made. https://photos.app.goo.gl/EAxSzfrgrRGvoyxq6

[59Elva]

Could it be that the better manufacturers have dropped out of the single coil market as the auto industry has moved to individual coils at each spark plug leaving the “antique” coil market to second tier manufacturers?

[svenedin]

Could it be that the better manufacturers have dropped out of the single coil market as the auto industry has moved to individual coils at each spark plug leaving the “antique” coil market to second tier manufacturers?

It does seem so. "Lucas" are not really Lucas as they once were. Bosch blue coils had a very good reputation but were discontinued although I think they are being made again. Petronix in the USA are excellent. Modern cars with a coil per cylinder do not seem to have coil packs fail all the time. Those coil packs are mounted right on the engine and subjected to a lot of heat and vibration. Of course those coils per cylinder are resin filled and for a 4 cylinder engine each coil pack only does 1/4 the number of cycles as compared to a single coil. I recall my father's old DKW was a 3 cylinder 2-stroke with 3 ignition coils, 1 per cylinder. The idea is nothing new!

Stephen

[59Elva]

Our recently scrapped ‘08 Kia Rio had three of the four coils fail over 18 years and 64k miles. The saving grace of individual coils is that you can limp along with one bad coil as opposed to no spark at all.

[svenedin]

Our recently scrapped ‘08 Kia Rio had three of the four coils fail over 18 years and 64k miles. The saving grace of individual coils is that you can limp along with one bad coil as opposed to no spark at all.

Always carry a spare coil and ideally have it already fitted with a bracket so you can drop it straight in. 10 minutes or less and you are on your way again. It is good practice of course to test whether the old coil is dead so I also always carry a digital multimeter. I have had coils fail where the primary windings are open circuit or the secondary windings (which are much finer wire) are open circuit. Just because the coil tests at 3 ohms across the terminals does not mean it is working. You have to test the secondary as well. Sometimes as in Bernni's coil troubles a coil wire becomes detached internally but will make contact some of the time. That coil would test OK sometimes.......I had that too and the way to diagnose it is to connect the multimeter with crocodile clips and shake the coil. The loose wire shows itself as a wildly varying resistance.

Stephen

[Bill_qaz]

Rather have reliability than originality. I have original tudor screen wash bottle on offside wing and the coil is mounted next to it, so there's room. Must check what brand it is but the ignition is Petronix, 4 years in my ownership and probably the (kiss of death) but no ignition issues

[svenedin]

Rather have reliability than originality. I have original tudor screen wash bottle on offside wing and the coil is mounted next to it, so there's room. Must check what brand it is but the ignition is Petronix, 4 years in my ownership and probably the (kiss of death) but no ignition issues

I will have a look but in my car that would be underneath the elephant's trunk. I don't think there's room between the screen wash bottle (also Tudor) and the horn.


Stephen

[Bill_qaz]

I have the elephant trunk as the later type heater was installed.
But my horn is in nearside front corner next to the radiator.
Can't see coil only ignition wires.

20250905_141012.jpg

[svenedin]

I have the elephant trunk as the later type heater was installed.
But my horn is in nearside front corner next to the radiator.
Can't see coil only ignition wires.
20250905_141012.jpg

Ah. My horn is offside inner wing.

Stephen

[Bill_qaz]

I have the elephant trunk as the later type heater was installed.
But my horn is in nearside front corner next to the radiator.
Can't see coil only ignition wires.
20250905_141012.jpg

Ah. My horn is offside inner wing.

Stephen

But you don't have the lage DB10 indicator relay taking up space.
The coil is mounted bulkhead side of the washer bottle. I don’t have a picture.

20250213_151430.jpg (773.62 KiB) Viewed 2067 times

[svenedin]

I just went for a short drive as I wanted to check everything is working as it should be. Everything is fine after swapping out the coil.

On the subject of coils getting hot. They most definitely do. Although my drive was only a few miles the weather is rather warm today 32 C. On my return I popped open the bonnet. The coil was almost too hot to touch BUT it was no hotter than the dynamo it is sitting on which is also very hot and too hot to touch. Likewise the oil filter is very hot (as you would expect). Not only is the siting of the coil on the dynamo subjecting it to vibration, it also subjects it to hot air passing through the radiator. So really I have no idea what temperature the coil would get to just doing its thing. It is being heated up by heat from the radiator, bathed in a constant stream of hot air. Not ideal!

As an aside, I checked the temperature of the carburettor float bowl which has a heat shield fitted. Cool to the touch.


Stephen

[svenedin]

I have the elephant trunk as the later type heater was installed.
But my horn is in nearside front corner next to the radiator.
Can't see coil only ignition wires.
20250905_141012.jpg

Ah. My horn is offside inner wing.

Stephen

But you don't have the lage DB10 indicator relay taking up space.
The coil is mounted bulkhead side of the washer bottle. I don’t have a picture.
20250213_151430.jpg

Oh bulkhead side of the washer bottle. Yes it would fit there. It would make it a more difficult job to change the coil if the bracket had to come off. You'd need to be reaching in behind the wheel whilst undoing a nut on the other side.. A bit of a contortion. However, your point is to make it so changing a coil is a very rare thing to do and anyway, if it is the same diameter coil the bracket pinch bolt can be slackened and the coil changed without removing the bracket from the inner wing.

Stephen

[Bill_qaz]

Time for you coil to move home, to maintain you original esthetic
Leave a coil mounted on dynamo with 2 dummy wires lost in loom with no connection and the king lead to distributor cap taped to real one. See how many notice 6 leads

Next time I'm with the car I'll look how it mounted and send you a picture.