Coil questions.

[blueosprey90]

It seems that my MGA and PV444 have similar ignition systems, so I look to the MGA Guru for suggestions.

He has a whole long explanation of how to test a coil which, of course, makes my eyes glaze over and soon puts me to sleep. (On an MGA, it takes six seconds to swap in a known good coil!) But he did offer, and I seized upon, this. Does anyone see why it would not work for the 444 or 544?

"With all parts in place in the car, you can do this test with a single jumper wire. Leave all wires in the car connected. Remove distributor cap, and rotate engine until the contact points are open. This leaves the grounded capacitor in series with the coil. Turn on the ignition switch. Short the off side coil terminal (the one not connected to ignition switch power) to ground with the jumper wire. When you disconnect the jumper wire you should get the spark. This of course assumes that the primary wires inside the distributor are properly connected and are not shorted to ground. As a point of clarification, the two wires connected to the contact points must be in contact with the spring arm of the points and must not be touching the mounting screw."

[Pigeon]

That's fine - it's electrically the same as the test method I posted earlier; of course, with the parts still installed in the car, it saves the fiddling about of setting it up on the bench.

[arcturus]

Done some more research on this. At the end of the day these tests only show the condition of coil when static. Under load, plug gaps to wide for example will cause the coil to over work and over heat and in time the internal insulation of the windings will break down and cause problems.

[Ron Kwas]

arcturus;

Semantics...

We can call testing an Ign Coil with an Ohm meter a "Static Test", as it is done with the application of very small test Voltage (and insignificant Current). A ST does not generate high ignition voltages.

Applying full, normal operating power (Primary Current) and breaking that Current (by some means, as in bo80's procedure), generates the High Secondary Ignition Voltage, in the same manner as if it were operating normally, with points breaking that current, so we can call that a "Dynamic Test" (the engine is certainly static, but the ign sys is operating no differently than if engine were running). As this generates the high ignition voltages, the DT is a very good test, as it also checks for internal insulation breakdown.

"plug gaps too wide for example will cause the coil to over work and over heat and in time the internal insulation of the windings will break down and cause problems." ...I will certainly agree with the "insulation breakdown" part...at some (huge) Sparkplug gap (or maybe even a disconnected sparkplug wire), the high voltage might find an easier discharge path inside the coil at an insulation breakdown, since it has no normal discharge path down wires, across gap in Dist Cap, eventually at gap at Sparkplug ...but can't agree that this causes "overheating" of the coil.

Remember, the heating of coil occurs due to high power within the coil primary...secondary high voltage ign spark has very little energy (area under the curve for the scientists) to cause heating...that is why it hurts, but little else when we inadvertently become the discharge path. Coil Heating is caused by the high power of the coil primary as it is charging for the (repetitive) big event...the discharge. A wide sparkgap only delays (minutely!) the point at which HV jumps across the gap, but does not result in coil heating, that is strictly due to primary...!

Hope that clarifies a bit.

Cheers

[arcturus]

The leading cause of premature failure of an ignition coil is due to a worn or bad spark plug ignition cable. A bad spark plug ignition cable will have a much higher than normal resistance. This high resistance causes a very high amount of voltage to be generated from your ignition coil's secondary winding.Sep 23, 2016

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.

The leading cause of premature failure of an ignition coil is due to a worn or bad spark plug ignition cable. A bad spark plug ignition cable will have a much higher than normal resistance. ... This excessive voltage creates a high amount of heat which consequently melts the coil's wire insulation.Sep 23, 2016

So the moral of this is to always keep you plugs and HT leads in good condition if you want your coil to last?