Why does a coil blow?

PS the tractor is a 1961 Cockshutt 550.

I can tell you its typical for an alternator to increase the battery voltage and resultant coil current a bit more then a generator (say 14.5 or over versus 13 to 14), which would make a coil run slightly warmer, but I dont see that as cause for any "extreme" premature coil failure, more like over a longgggg gggg period if even then. Another cause of premature point burning and/or premature coil failure could be a weak condensor. Those are current switching and energy absorbing passive devices which an increase in current could cause them to last less time. If unsure of the condition of the condensor I would recommend it be replaced !!!!!

You asked about internal ballast failures, but I would think the coil windings and/or their high voltage insulation withstand capacity would be more prone to failure then any pure resistive ballast portion of the coil. (Absent any specifications, however, I cant say for sure if there is a pure stand alone ballast resistor portion of your coil or if theres simply more DC resistance in its windings). Sure a coil can and does fail time to time, but I would bet failure occurs more often from high voltage breakdown versus any pure ballast failure (if it contains such).

You asked about increased ballast for an engine with less cylinders. If an engine has less (4) cylinders, the duty cycle of coil current increases cuz the points are open less time/cycles then they would be on a 6 cylinder engine. The 6 cylinder has the points opening n closing more often (shorter dwell time) so its sort of like theres less net effective current over time during which the coil can heat up. It may help if you consider the extremes of a one or two versus an eight cylinder engine. On a one cylinder the points are almost ALWAYS CLOSED versus on an eight cylinder when they are opening n closing rapidly. THEREFORE, you need more current limiting ballast resistance to reduce the net effective coil current and heat build up the less cylinders and duty cycles.

I have seen coils labeled "For use on electronic ignition" and the difference in design has to do with the fater and more positive switching of an electronic device versus the bouncing n ringing and necessity of a condensor in the points type ignition. The coil would likely work on EITHER ignition but may not last as long if used incorrectly.

If you go with an electronic switch I HIGHLY RECOMMEND the use of a high energy high voltage coil in order to realize the full benefits the switch has to offer. Make sure the switch is matched for the proper coil (the Pertronix techs can help with that) and the switch matches your polarity and one usually runs a wider plug gap also.

About measuring the coils output voltage !!! That takes a certain high voltage voltmeter that wouldnt arc internally and theres such a short time the voltage is present its NOT something you can do at home with a Radio Shack cheap meter. The voltage the coil rises to is dependant upon the medium (fuel and compression) and the distance across the plug gap and it rises no higher then necessary to jump that gap then it collapses.

Sooooo ooooo why does a coil blow????? ? I would say excess current caused by excess voltage and/or insufficient ballast resistance is a likely suspect (coil runs hotter) followed by a mis matched coil for the application (points versus electronic) and a bad condensor !!!!! !!!!!

Hope this helps, good luck n God Bless

John T Nordhoff in Indiana, retired electrical engineer

best wishes

John T

If the coil was only running warm, and not so hot you could fry an egg on it, I doubt the system voltage was too high for it.

Coils designed to be used WITHOUT an EXTERNAL resistor are simply wound with more wire eliminating the need for an EXTERNAL or INTERNAL resistor.

Modern alternators generally have their regulators set a bit higher than generators of some years back, but usually that is in the range of what a round-can coil will tolerate. You probably just got a defective coil. (A while back, I had a new one bad right out of the box.)

Another coil "killer" is a bad spark plug wire, or other poor connection on the secondary side. If the voltage is having a problem getting to a spark plug, it will take another path to ground, possibly inside the coil, shorting the windings,

The funtion of resistance in the primary circuit, whether an external ballast resistor, or extra windings in the coil's primary, is to limit average primary current to a level safe for the coil, and breaker points (or electronic ignition), typically in the 3 to 4 Amp range.

The reason for Pertronix to specify a coil with higher primary resistance for a 4-cylinder has to do with "dwell", (for this discussion,basically the % of time the points are closed).A 4-cylinder engine "fires" 2 cylinders per revolution, whereas a 6-cylinder fires 3, and an 8-cylinder fires 4. That means, for every revoltution of a 4-cylinder engine, the breaker points open (for roughly 30 degrees of distributor rotation) one time less than a 6-cylinder, and 2 times less than an 8-cylinder. This results in higher average primary current, and more heating of the Pertronix unit and coil. So, to be safe, they require a coil with a bit more resistance for the 4-cylinders.

No, you can't measure the coil output voltage with a regular voltmeter.

Take care now

John T