Why Do Ignition Coils Fail?

There are a lot of people on here more experienced than me but here is my 2.5c. As an electrical engineer 5amps sounds high to me. The heat required to be dissipated by any device is the square of the current x its resistance. In this case this is 5x5x1 = 25W. That is a lot of heat to be dissipated by any device let alone by a sealed device like a coil.

The voltage created by the secondary winding is determined by the magnetic field collapse in the coil so you only need enough current in the primary to fully charge the coil magnetic between firings. Any more will just overheat the coil but not give a better spark. My 38 coil draws an average of 2 amps by using a 2.7 ohm series primary resistor which would give around 4w of heat dissipation in the coil. Quite a bit different from 25w. Just a thought.
Regards
David
 
A short between a a couple of turns in a transformer doesnt affect the the performance to bad in theory....turns ratio wise.
But the shorted turns is drawing a huge amount of current heating up instantly.
A transformer can only handle so much magnetic energy the shorted turns steal some of this turning it into heat leaving less energy left to create a good spark.
The primary winding charges the core with magnetic energy that is then released through the secondary winding to the sparkplug, if the shorted winding steals the energy turning it into heat you have a less good spark and a coil heating up. There is losses in a transformer from the beginning that makes the coil heat up slowly to working temp.
If it heats up fast running and to the point you cant put your hand on it you got an issue.
 
In a transformer its only the losses that has to be cooled of by the coil.
The rest of the energy is transfered to the sparkplug.
You cant calculate the primary as a resistor dissipating the energy put into it as heat.
 
Its hard to test a coil properly without setting it up in a testing device.

Testing it with a DMM just tells you if its an open winding or a real bad short in it.
 
Tom Walker said:
Can I test my nos ford coil to see if the thing is shorting inside? Maybe do it cold then after heatng in oven?
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Yes you can test a coil as you described. Almost every coil will look good cold but 99% of old Ford coils from 32 to 48 will have a VERY weak spark
when heated to about 180 degrees. I guess the resistance changes. I use
Ford~Heyer distributor machine which has a coil tester. The coil is heated by a full 6 volts applied directly to the coil for less than 10 minutes, to long and the tar will boil and split the shell. After heating the coil is hocked up just like being on a car. The spark gap on the machine is adjusted to see the color of the spark and how far it jumps. We are looking for a coil with a crisp blue spark that jumps 3/8 to 1/2" or more. The cold coils will all look
good but when heated the spark become yellow and only jumps less then a 1/8". They get down to a fuzzy little yellow ball. A weak yellow spark is blown out like a candle from compression. This can be observed in a spark plug test machine with a viewing window and compressed air. As the air pressure is increased the spark is blown out. In most cases this happens just above 25 lbs. I don't see why all of these people fool around with all these odd ball coils and as shown on this forum with the problems of
finding the problem. Why not do what 20,000 old Ford owners have done and have the original coil rebuilt by Skip and drive the car and leave the coil problem behind as reported on this forum by many that use his coils. With the original coil problems are easy to find. G.M.
 
G.M. that is a great explanation, as I do a lot of burner work that same scenario happens with the transformer of the burner.

Fires a nice spark when cool but as the boiler heats up and warms all the burner components on an off cycle, there will be no spark on the next call for heat to the burner. The old iron core transformers did not have as many problems as the new lighter/plastic ignitors. Scott
 
In almost 50yrs of being a mechanic I can on both hands when I ran across a bad coil (not counting the occasional carbon track). Worked on all the Ford systems, points, Duraspark, TFI plus three import systems. That doesn't count the COP sys. that would pop a coil just because. Had a lot of PO customers after their second coil went bad. At over 100$ a piece when they came out you could feel their pain.
 
LOWRIDER said:
In almost 50yrs of being a mechanic I can on both hands when I ran across a bad coil (not counting the occasional carbon track). Worked on all the Ford systems, points, Duraspark, TFI plus three import systems. That doesn't count the COP sys. that would pop a coil just because. Had a lot of PO customers after their second coil went bad. At over 100$ a piece when they came out you could feel their pain.
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I would guess that you are not dealing much with people that put something together out of parts and have no idea what they are doing! Big difference between OEM equipment and some cobbled together ignition system with mismatched parts.:) Think that is what G.M. is getting at, if you use original parts (and quality know good parts) not much likelihood of problems and easy to troubleshoot. Now, I like to mess with things and design my own systems, but you have to research and learn how things work!!
 
david.skinner said:
There are a lot of people on here more experienced than me but here is my 2.5c. As an electrical engineer 5amps sounds high to me. The heat required to be dissipated by any device is the square of the current x its resistance. In this case this is 5x5x1 = 25W. That is a lot of heat to be dissipated by any device let alone by a sealed device like a coil.

The voltage created by the secondary winding is determined by the magnetic field collapse in the coil so you only need enough current in the primary to fully charge the coil magnetic between firings. Any more will just overheat the coil but not give a better spark. My 38 coil draws an average of 2 amps by using a 2.7 ohm series primary resistor which would give around 4w of heat dissipation in the coil. Quite a bit different from 25w. Just a thought.
Regards
David
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David,

I think our models here are different from yours.
I show the 1938 Ford as having a underdash resistor at .6 ohms and the coil tests with a .6 primary as well . Thats a total of 1.2 ohms primary working resistance driving down the road with a 6 volt battery. ( actually 7 volts when charging).
Using the ohms law math thats 6 volts, 5 amps, 1.2 ohms primary and 30 watts.
The heat energy of the current flow would then be divided using two resistors the coil and the resistor. Or approx 15 watts each.
I agree coils have different laws but thats the potential at the very least...
:confused:
 
I can honestly say, knowing what I know now, I would not leave the driveway of my home, in any of my old Fords, without having a Skip Haney rewound coil on my distributor. Also, I carry a spare just in case I do something stupid like forgetting and leaving the ignition switch in the 'on' position.
 
I did some quick research into the types of wire insulation available today and was amazed at the selection. There are Polyurethanes and Polyesters for core temperatures right up to 250 deg C.

There are Bondable polyurethanes and Polyesters that are wound and then heated to melt the Bondable into a monolithic assembly. This looks like a good idea as the wire wouldn't move around.

What type of wire do our coil Re-Winders use? As they re-use the old coil the only variable left would be the type of wire to enable the coil to be so reliable.

Knowing the type of wire would give some confidence in the rebuild.
 
JAKEFORD said:
I did some quick research into the types of wire insulation available today and was amazed at the selection. There are Polyurethanes and Polyesters for core temperatures right up to 250 deg C.

There are Bondable polyurethanes and Polyesters that are wound and then heated to melt the Bondable into a monolithic assembly. This looks like a good idea as the wire wouldn't move around.

What type of wire do our coil Re-Winders use? As they re-use the old coil the only variable left would be the type of wire to enable the coil to be so reliable.

Knowing the type of wire would give some confidence in the rebuild.
Click to expand...

Any of these modern magnet wire insulations you mention are far superior to what was available and used in these coils back in the day. That's why it is so important to update these old coils with modern insulation systems. In this case, old is NOT better.
 
Using the fancy wires with multiple layer insulation has the disadvantage that it takes space.
And with magnet wire being thin and lots of turns the thickness of the insulation starts to count...
Next step on the fancy scale is square wire ;)
 
You know, I was thinking the same thing. You send your coil in for re-wind and there is only so much room available for the wire. If you use "modern" wire insulation the wire is probably thicker and so the turns will be less??? So you might go smaller in gauge which might work as the secondary current is only about 0.5ma max. But going thinner with the wire seems going in the wrong direction.

Sure wish we could get a comment from the coil re-winders.
 
I have a Chinese 4 wheeler Quad fun vehicle that my neighbor gave me. The wiring harness burnt out. But after a couple of weeks I fixed the harness.

The Chinese Quad uses a CDI ignition system that throws a dynamite spark from an ignition coil about the size of a flashlight battery. The unit sells for about $6.00.

I was thinking this might be a solution for our flatheads. Probably multiple coils triggered from the distributor. Maybe even those coils could be adapted. The CDI units could be Chinese. The CDI units in the Quad are triggered by a winding in the Magneto.

I have to say these things are rugged. They operate through mud, water and snow.

Remember the Model T used multiple coils when things were not "modern".

I am not suggesting we go out an experiment but maybe an aftermarket company might be interested.

Comments
 
Many years ago I worked in a copper Wire & Cable factory. One product was magnet wire. The wire was spooled off one reel, took a turn through a lacquer bath, up a drying tower, over a pulley and down to be wound on another spool. With multiple wires on multiple pulleys, it happened fairly often that a wire would break, which required a man to go aloft to restring it, or them, actually, maybe a dozen at a whack. The lacquer room was well ventilated, as laquer vapor is extremely strong and intoxicating. Oxygen masks were required to go up in the tower, and a second man for safety. Once, during my time there, a man went up alone, and didn't come down. It was a wake up call for us all.
 
I have a dead coil (open secondary) that I am going to modify by splitting it into two pieces. Ths lower piece I am going to use to connect to my 11A distributor. I am going to use NAPA or equivalent external coils at about $20 each and carry a spare. I will check and make sure the primary current is OK.
This is identical to the 1946 ford and works like a champ.

This system will give piece of mind at a fraction of the rewind cost of an unknown coil. I would dearly love to get my coil rewound but if I don't know what is going into it, I don't feel comfortable.

The publicity is to get a rewound unit with "modern" material but nobody wants to say what these modern materials are. I believe commonly available coils are the answer. Just make sure the primary current is OK to avoid overheating.

Thanks
 
A cranktrigger 8 sparkplugmounted coils and a computer later you have a modern ignitionsystem JAKEFORD :)
But as many said before...do we really need it ???
 
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