Burning Out Ignition Coils
 

My Model "A" has a 12V electrical system with a negative ground. In the last few hundred miles, one Petronix and one Bosch coil have failed. Both coils are epoxy filled.



Another peculiarity was the carbon button on the underside of the distributor cap was totally gone, so the rotor was arcing to the brass ferrule that surrounds the button.


Could there be a relationship between the coil failures and the missing distributor cap button?



I am now running a 12V coil with internal resistor intended for use in a VW, and I just bought another new one from Parts Player on eBay.


The failure symptoms were random minor cutouts of the ignition followed by more severe cutouts until a total ignition failure, all within 5 miles.

Re: Burning Out Ignition Coils
 

If the contact on the rotor doesn't touch the carbon button the carbon button will be burned away, also there is increased spark gap which increases coil voltage-- too much voltage stress can short out coil windings

Re: Burning Out Ignition Coils
 

I agree !!!!!!!

Re: Burning Out Ignition Coils
 

Found this elsewhere on the web,

IGNITION COIL FAILURES
Ignition coils are very rugged and reliable, but can fail for a variety of reasons. Heat and vibration can damage the coil's windings and insulation causing shorts or opens in the primary or secondary windings. But the number one killer of ignition coils is voltage overload caused by bad spark plugs or plug wires.

If a spark plug or plug wire is open or has excessive resistance, the ignition coil's output voltage can rise to the point where it burns through the coil's internal insulation causing a short. The insulation in many coils can be damaged if output exceeds 35,000 volts. Once this happens, the coil's output voltage may drop causing ignition misfire when the engine is under load, or the coil may cease to put out any voltage preventing the engine from starting or running.

Re: Burning Out Ignition Coils
 

So in words that fit my experience, excessive resistance to the ignition coil's output voltage, due to the missing carbon button on the distributor cap, made the coil's voltage rise to the point it burned through the coils internal epoxy insulation and suffered a short circuit.



The epoxy insulation inside of the coil failed when the coil's output exceeded 35,000 volts. When this happened, the coil's output voltage dropped causing ignition misfire when the engine was under load, and eventually the coil's hi-voltage output ceased.


To me, this explains what took place in my Model "A."

Re: Burning Out Ignition Coils
 

Any gaps , at the rotor tip , spark plugs or the carbon button in the upper distributor cap cover that is over thirty five thousants can strain the coil .More gap to a certain extent increases spark intensity and can improve performance , way too much gap will shorten coil life or cause complete coil failure . A performance coil such as the Pertronix flame thrower 40.000 volt coil can jump larger gaps with no problem . Pertronix recommends increasing the gap at the spark plugs .I have had excellent service from my 40.000 volt Pertronix coil for the past 12 years and never had a problem., I get instant starts and improved power that I can feel !!!
A person needs to know what to look for when ignition problems arise . Checking and adjusting gaps in the ignition is always part of a tune up for me . Any time that my engine sputters , the first thing that I do is check gaps at every point that spark jumps in the distributor . The coil produces the spark and has a noticeable effect on how the engine performs . Too wide of gaps can kill any coil if allowed to run that way for very long .

Re: Burning Out Ignition Coils
 

A big Yup! High voltage of a coil or any device putting out high voltage, more so, requires the load resistance of the plug wiring, the distributor rotor air gap to the distributor cap and then the gap of the plugs to load the coil with a gap resistance to hold the spark to a reasonable given value. The more gap, the higher the voltage allowed and when the voltage over and extended period is allowed, the coil insulation materials tend to break down. And with High voltage, once something starts being a gap and a place for voltage to cause burning or allowing non wanted gaps to get wider and wider, the output voltage of the coil keeps going up and up and then the damage occurs!

Re: Burning Out Ignition Coils
 

I will stick to the stock ignition,very dependable,since 1961!

Re: Burning Out Ignition Coils
 

Nost probably don't know the size of their gaps . Most of the distributor cap bodies that I have checked have gaps that can be all over the place . Rotor tip lengths can vary . Some replacement coils are not near original specs . Locating a coil that has the correct OHM to work with the model A can be difficult if you don't know the correct OHM number unless you are dealing with a model A parts vendor .

Re: Burning Out Ignition Coils
 

Make sure you don't have anything else connected to the coil wire that goes to the ign switch and down to the dist. Don't let that wire feed anything.

Re: Burning Out Ignition Coils
 

The only coil failures that I have had in over 50 years with my model A's was caused by an incorrectly mounted OIL filled coil . Oil filled coils need to be mounted with the tower of the coil and wires at the top . If the internal windings are not submerged in the oil the coil will over heat and fail . I only use epoxy filled replacement coils or original tar filled coils. These coils are often called solid coils. Solid coils don't allow the internal cooling material to flow by gravity and cause the upper windings to become uncovered and over heat

Re: Burning Out Ignition Coils
 

Good to know!

Re: Burning Out Ignition Coils
 

Don't ever use worthless carbon fiber spark plugs wires or coil wire if it has a non stock cap..use only metal conduit type wires..start your engine in the dark and check spark arcking and manifold color..very interesting..

Re: Burning Out Ignition Coils
 

Agree also; tried 3 new 12 volt oil filled which got really hot and primary ohms varied. Went back to an old tar filled / internal resisted out of a wrecked car and even turned it with leads up. It tested at 3 ohms (12 volt) exactly. The 6 volts' should read 1.5ohms across the primary, secondary per mfg. rating.

Re: Burning Out Ignition Coils
 

Could the generator be over charging, causing the problem? Or a bad ground causing a feed back.

Re: Burning Out Ignition Coils
 

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I did have a few coil failures before I learned what was causing it !!!

Re: Burning Out Ignition Coils
 

You might also check to make certain that the 12 volt coils you are using contain an internal resister. Before I learned that there were also external resisters, I had issues with coil failure as well. The ones with an internal resister cleared that problem up for me. Good luck.

Re: Burning Out Ignition Coils
 

That’s interesting. I cut an oil filled coil open and the “ bottom “ of the coil has a 2” spacer so the windings are off the bottom by that much, so when upside down, they stay immersed in the oil. Don’t remember the brand off hand, but it’s an older unit. Don’t know if all oil filled coils are that way either. It sure seems that a coil built that way shouldn’t overheat??

Re: Burning Out Ignition Coils
 

I'm curious... why would you go through all the trouble of going 12v. So much hassle to have problems. if it ain't broken why fix it???

Re: Burning Out Ignition Coils
 

The 3 OHM coils are sometimes called internally resisted and is the coil that I use on my 12 volt converted model A . If an original model A coil is used on 12 volts it will be necessary to use a resistor . As a general rule I use a 3 OHM coil for 12 volts or a 1.5 OHM coil for 6 volt use on my original type model A .