Issue with blown coils

[verdirick]

I've been trying to help a new member with getting his "new" 30 Fordor. We finally got it running for a few minutes before the new coil burned out. WE have gone through 2 coils. With the key turned off!! I get power (sparks) when i attached to black wire to the neg pole on the coil. The car still has 6 volt positive ground with the battery in correctly with a positive ground 6 volt alternator.. Haven't checked out the voltage output as not running with a blown coil... Somewhere the wiring is not correct. Only thing I can think is to replace wiring loom with new wiring.
Apppreciate if anybody out there in Barn Land has any Ideas of why I get power to the coil with the key turned off
Thanks, Verdirick

[Kurt in NJ]

is it original “pop out” ignition switch?
wiring modifications?
originally the coil got power directly , the ignition switch was between the coil and distributor
what coil are you using?

[Tom Endy]

I suspect there is a partial short to ground between the bottom side of the coil primary and the ignition points in the distributor that is pulling current through the coil and overheating it since there is always battery power applied to the top side of the coil primary. The short could be at the ignition switch, inside the pop-out cable, or inside the distributor.

Tom Endy

[ronn]

It might be faster to chg out the loom then try and trace it.
especially if its intermittent.

eating coils can be costly and the problem may persist. one and done.

[nkaminar]

If you have a short somewhere, then changing out the wiring may not cure the problem. As Tom Endy said, it may be in the distributor or some other place not related to the wiring. Disconnect everything and test for shorts, either one at a time or all at once. Use your multimeter on the ohm setting to test for the shorts then use it in the volt setting to see where power is coming from. A logical approach will find the answer.

[Joe K]

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Model A Wiring Diagram. A similar one exists for "cowl light" version.

https://lesmodela.files.wordpress.co...ng-diagram.pdf

Coils generally don't like CONSTANT current. Its not their "thing." I would guess your problem somewhere around the cut-out/ignition switch. See that the armored cable is not too tight into the distributor casting. (Hand tight is more than enough.) Check armor cable for "shorts" created from aged wiring/subsequent close bend on install.

Joe K

[Bob Bidonde]

Ditto Tom Endy's advice, and also take note of JoeK's comments. There are Ford service bulletins about an ignition switch terminal shorting to ground at the gas tank gauge casting.

[rotorwrench]

The coil only gets energized when the circuit is complete. The pop out has to be ON with the breaker points closed to complete the circuit. Many a coil has died when operators forget and leave the pop out on when the engine stops with points closed. An energized coil is just an electromagnet if the engine isn't running and it gets way hot.

Modern replacement switches can be wired to control power to the coil or ground path to the points but either way is an alteration over the stock OEM system.

Check all terminals and terminal block wiring for condition and security. Loose terminals or damaged wire insulation can burn a car down. Check behind the instrument panel too. If the car doesn't have a fuse then one should be installed.

[verdirick]

Thanks to all who responded. Ran issue past Steve Becker today. Consensus is that Tom Endy hit the nail on the head,,,New ignition shorted out against tank...Problem solved.....