Why Do Ignition Coils Fail?

J

JAKEFORD

Senior Member
I am trying to figure out the reason so many coils have failed and don't have an answer.

Assuming that the coil is in good condition and has not been exposed to excessive moisture or heat there is no reason for failure if the primary current is limited to about 5 amps on a 1 ohm coil in my opinion.

The coil is at the front in the rad air flow and gets good cooling air flow. Probably more heat transfer from the engine than electrical heating.

Can anyone shed any light on this?

Thanks
 
Certainly a good and fair question, as I had a decades-old Ford OEM "Mustard-top" coil crap out on me two weeks ago for no apparent reason. In my case I suspect that leaving my ignition key in the "on" position without the engine running (because I was troubleshooting my turn signals) was a contributing factor, although the car ran for about a half hour after that before the coil completely quit.

With no moving parts, theoretically a coil should last forever. I think that the varnish insulation eventually melts in spots, causing mini-shorts that actually shorten the windings, thereby causing more heat which results in more shorts. Kind of like a cascading failure.
 
I used to be a automotive instructor for Lincoln Tech here in Indianapolis. I taught electrical 1-2. We had a large Sears auto repair in one of the local shopping centers. I had a friend that ran the place at the time. He saved all the electrical failure items in a 55 gallon drum for me to use in class. When i taught the students ignition and coils etc , i would have them pick a coil out of the failure barrel, clean it up ID what it was from and test. Ohm the primary and secondary and then load test and scope test on a large Sun tester.
Maybe 1-2 in 50 coils would be defective.
Most were dirty ( spark will arc in grease ) damaged from abuse etc. Or the wrong application for the vehicle...

Think about it ( i call it blind technology) if the engine doesnt run . check the spark, if no spark it must be the coil ( based on ease of replacement).
I probably have 30-40 coils in our shop right now, i believe all of them would start a gasoline engine....

We also tested batterys by the truck load, same story clean , recharge and test . You dont even want to know what we found there..:D
Lets just say no one that worked with us ever bought a battery...:eek:
 
Sears auto centers were never known for having "good" techs, just parts changers. Anything beyond tires and oil changes is high tech stuff for them. Firestone, Pep boys are just as bad. I would never go to those places, nor recommend them.
 
Coils increase in temperature while in service. They don't get especially hot if things are running normally unless you leave the switch on while the breaker points are closed but they do go through heat cycles each time they are used. Eventually the fine copper wire gets fatigue cracking after a way lot of heat cycles with expansion & contraction going on. Once the wire cracks all the way open, then the coil will not meet specs and will get worse as it is heated up.

Now how long this takes is an unknown. Every one is different in manufacturers, design, wire quality, construction quality, etc. and each one get different use by operators, type of equipment, or environment it works in. Too many variables to predict.
 
This is just a note of stupidity (MINE).
Several years ago (over 25) when I first started working on my 39 Ford, I was determined to have a 12 volt system that appeared stock, that I could use a Pertronix (then Perlux) ignition. At that time there were no 12 volt original coils, or Perlux devices that would work on 6 volts.
In Hemmings, I noticed an ad from the late George Pounden for repairing Harmon & Collins & Pounden Magnetos. (George Pounden built his own Magnetos in the late 40's & 50's, and many were used by early drag racers on flatheads).
I called George and inquired if he could rewind an old Ford coil?
HE hadn't done one, but said come on over and we'll see?
I took (2) 39 coils and drove a couple hours to his home in Sebastopol, CA.
He rewound them correctly and tested them while I was there.
Off I go to try them.
Install 1 and the engine starts immediately. Runs for approx 5 min, then dies and won't restart.
No problem, I've got another coil. Install, start engine, fails..
NOT enough resistance thru original under dash resistor. Took me a long time to figure that out.
Had George redo the coils (both fried), got a modern ballast resistor.
 
An ASE master mechanic recently commented on my faulty diagnosis, "80% of the time the public is wrong" That includes me, and a good many of the "replacement technicians" out there that call themselves mechanics. Prescription without diagnosis is malpractice.
 
Kahuna said:
This is just a note of stupidity (MINE).
Several years ago (over 25) when I first started working on my 39 Ford, I was determined to have a 12 volt system that appeared stock, that I could use a Pertronix (then Perlux) ignition. At that time there were no 12 volt original coils, or Perlux devices that would work on 6 volts.
In Hemmings, I noticed an ad from the late George Pounden for repairing Harmon & Collins & Pounden Magnetos. (George Pounden built his own Magnetos in the late 40's & 50's, and many were used by early drag racers on flatheads).
I called George and inquired if he could rewind an old Ford coil?
HE hadn't done one, but said come on over and we'll see?
I took (2) 39 coils and drove a couple hours to his home in Sebastopol, CA.
He rewound them correctly and tested them while I was there.
Off I go to try them.
Install 1 and the engine starts immediately. Runs for approx 5 min, then dies and won't restart.
No problem, I've got another coil. Install, start engine, fails..
NOT enough resistance thru original under dash resistor. Took me a long time to figure that out.
Had George redo the coils (both fried), got a modern ballast resistor.
Click to expand...

LOL, that's the way you learn! I leaned a lot when I started racing a B altered in the mid 60s. A lot of it was electrical. One was coils need a ballast resistor, burnt up a couple of them in the learning process.
 
The expansion/contraction on each operating cycle sounds interesting but should be the same problem with all transformers. I have some old radio transformers that I have taken apart and note that the enamel looked brand new on transformers that were 80 years old. I used the same wire when I rewound them to different voltage requirements and to date have had no problem

The secondary HV coil winding contains a lot of turns and operates at 10x the voltage of a radio coil so is probably more prone to short circuit breakdown. But the layers of turns are separated by insulation paper or tape so this problem should be minimized. Also the temperature ramp up and down is slow so the enamel should follow the wire and resist cracking.

I don't know the wire gage of a coil HV winding but presume its around 36-40 ga. If the enamel is too thick on the wire maybe its more susceptible to cracking with temperature cycles??? Likewise if too thin, then improper insulation and shorts might result???
 
I´ve wound a fair amount of transformers during the years.
First is you put stress on the insulation of the wire bending it from start.
Then adding potting to keep wire from moving and adding insulation.
The alternating current in a transformer will have the wire move rubbing on each other if they can.
The potting in older stuff was tar or wax today its resin often combined with vacuum treatment.
Reusing a wire for a highvoltage transformer isnt a to good thing it will die premature.
An ignition coil is up to 50kV so much higher then any tube audio transformer.
Heat isnt good for either wire insulation or potting and as soon as wire can move freely or there is a crack in insulation and arcover it goes fast.
 

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The layering and sectioning of a audiotransformer is for dealing with straycapacitanse affecting the audio bandwith more then insulation.
A ignitioncoil is rushwound no need for a nice layout just a tight winding.
 
All good answers. Heat, thousands of thermal cycles and insulation breakdown are the main causes. Leave the key on for a while and see how hot the coil gets. If you are looking to have a rare or obsolete coil rebuilt, E-Mail "Skip" Haney", skip@fordsrus.com. He can take a rare coil and make it new. He has done dozens for V-8 Club members and everybody can't say enough about his work.
 
There are many reasons why these 70-80 yo original early Ford V-8 coils fail, and in my opinion the best thing you can do with any of these old coils (used or NOS) is to have them rebuilt/rewound using modern insulated magnet wire and other modern insulating materials. It is also a good idea to pot these newly wound coils with modern potting resin or epoxy compounds that impregnate the windings and bond everything together. Using self bonding magnet wire may be a good alternative to bonding wound coils with resins and epoxys.

Many of these old original coils will start an engine, but the question will be how long will they keep the engine running once the engine reaches maximum operating temperature when ambient temperature is 90+ degrees F. This is when these old coils breakdown and leave you stranded on the side of the road, sometimes making you 'THINK' you have vapor lock, fuel pump problems, plugged fuel line, etc., etc.
 
JM35
I've also wondered something similar re old NOS coils, and maybe someone (Bubba or JSeery) can answer?
Old NOS coils work for a while, then quit. Why?
Is it the insulation that fails? It must be that because the old NOS coils have never been used, but why would time alone (50-80 years) make the insulation fail?
Is it the type of insulation used?
Just wondering?
Jim
 
The transformers I have rewound are not audio transformers but HV power transformers. One was for a linear amplifier power supply at 2000v center tapped. Also have rewound several filament transformers at up to 25 amp capacity.

I think the LV filament transformers cycle more violently than the HV ones as they seem to run hotter.

The current is somewhat self regulating as the wire resistance increases with temperature. Its like another ballast resistor in the circuit. The resistance increase can be significant.
 
I have never worked on the insides of an automotive coil, but I did work at an Electric shop that rebuilt electric motors. The insulation on the wire is a type of clear lacquer coating. Most coils and solenoids I have seen use the same type of wire. In electric motors the lacquer breaks down over time (mostly because of heat) and they have to be rewound.
 
One reason why a coil breaks down upon first start (same with stored electric motors) is that you have moist in them.
When they heat up and cracks in the insulation opens opens up you have arcover and failure.
 
A lot of ignition coils have windings immersed in transformer oil. Presumably this is for heat transfer as well as to keep the windings "dry". I had one on my old garden tractor and it failed. It was a single cylinder kohler with a lot of vibration. The HV secondary opened and the failure was immediate. The coil ran cool up to that point so I think one problem may be related to manufacturing techniques.

It would be nice to hear from some of the re-winding experts that are on this forum and get their opinion.
 
At our shop, we have a box full of old Ford coils. We buy them to use as cores for rebuilding with modern materials by Skip Haney. For years, failing coils caused the often discussed "vapor lock" when engines sputtered and failed when hot. Checking with an ohmmeter is mostly a waste of time unless a winding has opened. It doesn't reveal crumbling internal insulation or shorts between windings. It isn't expensive to have a rebuilt coil that won't leave you stranded on the side of the road.
 
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