Ignition Coils - On and Off - Frustration Yes and No

One of the most insidious problems with older cars is a failure of the ignition coil. The symptoms are very often slow to replicate themselves so the source of the engine running problem is not always easy to identify, especially for someone not familiar with what makes the engine run in the first place. More often than not folks scramble hither and yon, replacing parts, adjusting this or that, moving stuff and generally wasting time and money.

Symptoms and fix

The short version is that when a coil operates properly and is able to withstand the heat that is generated as a natural part of its function and it goes about its business supplying high voltage current to feed the spark plugs. When it is on the verge of failure that very same heat can cause a shifting of some of the coil’s innards and this appears as an “open” in one of the internal electrical circuits or, in more rare cases, causes one of the internal wires to sort or go directly to ground. Either way, the coils ceases to do its job properly and the car won’t run or at least will not run well. The fix is to replace the coil.

You might get a better understanding of all of this if you examine the accompanying photograph of a disassembled coil. The photograph was graciously supplied by my friend Robert Schau of Reston, Virginia..

The long version: The internals of an ignition coil are composed of two electrical circuits. One, the one we call the “primary” circuit, is nothing more than a winding, or coil, of copper wire that encircles a central core, refered to as the secondary windings. This primary winding is connected at either end to terminals on the top of the coil that are generally marked positive and negative. Without going into the political correctness of whether electrical current “flows” from positive to negative or negative to positive, I will just use the commonly held thought that a potential current is supplied to the positive terminal, runs through this coil wiring as described, and exits VIA the negative terminal.

The actual flow of current is allowed or not allowed by a triggering system located beyond the negative terminal. This trigger can take the form of a set of mechanical “points” that physically open and close the circuit or some sort of electronic device that can handle that chore. Either way, the potential for current flow through the coil windings is alternately made or disallowed.

What happens when this current is allowed to flow and then stopped is sort of basic voo-doo electric stuff.

When the current is flowing through the coil, which is wound closely around the secondary windings core,
it creates an electrical potential in thos windings. This is referred to “induction” and it is the voo-doo part.

After a time of flowing current and the building up of this potential, the primary circuit is “opened”, that is it is broken, by the triggering device and the electromagnetic field created by the current that had been in the windings disappears.

The second part of the voo-doo is that as this happens the energy created and stored in the core decides that it just has to leave the building, so it makes an exit out of the larger central terminal in the top of the coil and runs over to the distributor cap.

From there, the distributor parcels the current out to the appropriate spark plug where it ignites the waiting volatile fuel/air charge.

The energy release of the explosion then drives that cylinder’s piston down, thus changing the energy stored in the fuel/air mix to the mechanical work that eventually drives the wheels of the car and causes sweet breezes to slip across the grin on your face.

Back to the coil problem. What happens in a failed or failing coil is that that those primary windings that I just described stop doing their thing. The wire either breaks in half, or it cracks in such a way that it is sometimes a complete circuit and some times not, or, a part of it contacts the inside of the coil casing thus allowing the current to “leak” out to ground all the time instead of being triggered and controlled by the stuff over in the distributor.

The last scenario is perhaps the least likely to happen and the first a little less likely. It is the middle one that happens most often and because it is not dependable in its failure it is the one most likely to drive a person nuts trying to pin it down.

In the case of this middle scenario, all things are just fine in the coil department most of the time, but when the coil is functioning and as the heat builds, that tiny crack somewhere in the windings is opened sufficiently to break the circuit. When that happens the coil “fails” and stays failed for a time until the whole unit cools enough to cause the break to contract and complete the circuit again. With the circuit complete again the whole circus starts its cycle and the engine will run for a time, only to quit again when tings get hot again.

There is one variance on this theme. That is when the coil windings don’t actually break and open, but instead “short” between themselves. When this happens the voo-doo of inducting the potential energy in the secondary circuit is short changed and less voltage is available to be delivered over to the distributor. Fewer windings means less voltage, simple.

It also should be understood that the combustion process in an engine varies in ease and efficiency because of fuel/air charge variances and the load put on it. In other words, if an engine is just sitting there at idle, there is very little load being put on it and the fuel/air is exploded easily and at the correct time, but when the engine is being called upon to “deliver the goods”, say with wider throttle opening while pulling a hill during peak torque production, the ignition system had better be ready willing and able to deliver a nice, fat spark to the plugs to ignite that more dense fuel/air mix under the more demanding conditions.

This is the point at which a semi-failed, that is internally shorted, coil will fall to its knees and not be able to deliver the needed spark. It may be coincidental with the heating of the coil t not necessarily.

When the going gets tough, the “weak” coil will fail in its duty, by either completely giving up the ghost or by delivering current to the plugs in a sporadic or weak way.

The results, according to the driver, will be spitting and sputtering, misfire, backfire, running on less than all cylinders and that sort of thing.

In the case of failure because of heating and the opening of the coil circuit, the symptoms are usually more pronounced. There may be a period of sputtering and misfire, but the real problems appears as the engine just completely shuts down. As noted above, this “shut down” period will last as long as it takes for the coil to cool and for the innards to again make contact with each other.


Testing a coil that has “sort of failed” is usually not worth the effort simply because, it in its cooled state those coiled windings will have joined once again and the circuit will show up as being just fine.

If the engine will simply not run, it is though a good idea to test continuity across the positive and negative terminals to see if the circuit is complete, hot or cold.

If a coil is failing in the sense that the windings are shorting between themselves, some sort of poor reading will show as a lower than optimum resistance. The shorter the effective length of the windings the less resistance will be encountered. Simple, right?.

Of course, a full short to ground through the case will show up as there being nearly no resistance when current is applied to one of the primary terminals with the case is held against a ground.

The fix for all of this, after checking the resistance in circuits with an Ohm meter, is to at least substitute a known good coil for a time and going for a test drive or the car is put under lead on a dynamometer..

If the engine returns to good operation you will have found your man and the bad coil can be replaced permanently.

If no improvement is enjoyed you should go back and make dead sure that all of your connections and wiring elements are indeed in good order before running your drivability tests again and making a decision to replace the coil or not.

You might want to review some of my other notes concerning points and electronic triggering systems.

Quote: This was a standard 6 volt coil as fitted to a 1974 XJ12. It was determined the coil popped because; either a tech or the owner had connected a green wire from a kick-down switch in the vicinity of the coil directly to the coil. It should have been connected to a resistor and has nothing to do with the coil. Because of said green wire, when running, the coil was receiving a full 14.5V and that raised the temp of coil above 250 degrees! The voltage should have been stepped down to no more than 8V at full tilt.   Robert Schau
In this photo you can clearly see both the primary and secondary windings and their connections.
Photos and quoted note below, courtesy of Robert Schau