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Hot ignition coil

on a 72 corvette, i went to start it barely turned over. so i charged battery. 3 hours later the coil was very hot, smoking & boiling over. had to disconnect battery. after cooling i tested voltage to coil 8-10v when swtch was in any position.

The voltage is OK. If you checked the ignition coil primary voltage with the ignition switch in START, it would be 12 volts.
Any electric circuit can get too hot if the resistance (ohms) is too low. So, if the spark plugs, and wires, are supposed to have a certain amount of resistance; but, the resistance is much lower, excessive current will flow through the ignition coil, and it will heat excessively. Check the ohms of the ignition coil, spark plugs, and spark plug wires. Replace if too low, or too high.

huh? You can’t tell much by a “too high or too low” rule.

The resistance of a good spark plug is infinite. Of wires is zero unless open. A coil has two connections, primary of about 1 ohm and a secondary of about 10kohms. But measuring with an ohmmeter tells you nothing about how the components behave under high voltage.

The 1 ohm value is low, there is maybe a resistor or something to limit the current when the engine is off and the ignition is on (otherwise you will dissipate 140 watts in the ignition coil, enough to smoke it).


Did you leave the ignition switch on while you charged the battery? Does this engine still have points?

Wait a minute,if the ignition switch is off, why are you reading voltage on the coil? It should be 0, nada, nothing there. Double check that, you may have another problem with your car that is keeps the voltage to lthe coil on. Remember, this is how an egine is turned off, you cut the 12 VDC to the coil.

If the coil was very hot this means the ignition switch was on or the ignition switch is messed up and providing power to the coil while the switch is off.
The coil will overheat (and can be ruined) if power is provided and IF the contact points in the distributor happen to be at the point on the distributor cam where the points are closed.

I was wondering about that. So there is no protection. Stop at the wrong point and turn the key, and in a few minutes, toasted coil.

OK4450 hit the head right on the nail! The ignition coil is being powered, even in ignition key OFF? It does look like the ignition switch is out to lunch. This (continuous power to the ignition coil) could still happen with a good ignition switch in the ON position, IF the distributor points happened to be closed. So, that’s something to be aware of, even after repairing the ignition switch.

The protection should be the ballast resistor. However, I wonder if the power may be coming from the starter solenoid. There is a contact in the solenoid that bypasses the ballast resistor and supplies full battery voltage to the coil. The problem with this diagnosis and the others about an ignition switch fault is that with continuous power to the coil the engine will not shut off

I had forgotten about the ballast resistor, knew there had to be something. And that triggered my memory about the bypass connection in the solenoid.

I have to respectfully disagree that a ballast resistor has anything to do with it.
A 6-volt coil gets just as hot as a 12 volt.

I have been involved with antique Harley Davidson motorcycles for years and they use a 6-volt, wasted spark system (1 coil, 1 lead for each cylinder).
The inside of the coil is full of stiff tar and the plug wires cannot be removed when the coil is cold. The recommended procedure is to turn on the ignition and allow the coil to heat for about 5 minutes or so. This moderately softens the tar inside and the wires can be pulled out. New wires are then inserted along with a new wire seal, which is held in placed by threaded packing nuts.

It is not recommended that the coil be allowed to heat for longer than 5 minutes or so because the coil will get too hot. This leads to the tar becoming too thin and when the plug wire is removed the thin tar will enter the cavity where the end of the plug wire is fitted. This then requires a homemade tool (steel tubing with teeth cut into one end) to cut out the tar after the coil has cooled a bit. JMHO anyway.