The Ford Barn

ford38v8 · 06-30-2025, 06:14 PM

Mike, the '40 Ford coil has no internal resistor. You need the dash mounted resistor for this coil, which will deliver 4 volts (target) to the coil. If you use an aftermarket coil, odds are it will have an internal resistor, in which case you simply bypass the dash mounted one.

I'm no electrical genius, but as i understand, the ballast resistor is smart, delivering close to 4 volts whether it receives the correct 6 volts or not. It does this by way of heating or cooling, heat in an electrical circuit is resistance in itself.

Now, with that in mind, remember that the starter motor is greedy, grabbing lots of lightning away from the ignition circuit. This is why with some cars, you crank and crank, it won't start until the moment you release the starter. I f you experience this phenomenon, simply create a momentary resistor bypass from ignition switch directly to coil. Never run with this bypass, use it only to start the engine.

More info than you asked for, but good to know for your rewiring project.

06-30-2025, 06:14 PM	#3
ford38v8 Senior Member Join Date: Jul 2011 Location: SF Bay Area Posts: 7,282	Re: Resistor Coil Mike, the '40 Ford coil has no internal resistor. You need the dash mounted resistor for this coil, which will deliver 4 volts (target) to the coil. If you use an aftermarket coil, odds are it will have an internal resistor, in which case you simply bypass the dash mounted one. I'm no electrical genius, but as i understand, the ballast resistor is smart, delivering close to 4 volts whether it receives the correct 6 volts or not. It does this by way of heating or cooling, heat in an electrical circuit is resistance in itself. Now, with that in mind, remember that the starter motor is greedy, grabbing lots of lightning away from the ignition circuit. This is why with some cars, you crank and crank, it won't start until the moment you release the starter. I f you experience this phenomenon, simply create a momentary resistor bypass from ignition switch directly to coil. Never run with this bypass, use it only to start the engine. More info than you asked for, but good to know for your rewiring project. __________________ Alan