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Old 01-22-2019, 07:51 PM   #1
Don C
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Default Ignition Questions

I am working on a 39 Ford Deluxe. The car basically won't start once warmed up so did some troubleshooting on the ignition. I read on this site that the voltage across the coil should be 3.5 to 4 volts and that seems to be based on a supply voltage of 6 volts so assume the ballast resistor is supposed to drop the supply voltage by about 2 volts. The car I am working on has an 8 volt battery. I am measuring only 2 volts at the coil with the points closed. I thought that the ballast resistor might be bad so I bought and installed a new one. I again measured the voltage at the coil and only 2 volts still. I measured the voltage drop across the resistor and it was 6 volts. Could I be pulling more current across the coil than designed? Any ideas?

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Old 01-22-2019, 08:30 PM   #2
alanwoodieman
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Default Re: Ignition Questions

does the car have an original type coil, it could be faulty or does it have an adapter with a round coil? I would start off by cleaning all of the wiring connections, ign switch, the resistor, the coil connection. I have seen a big voltage/amp drop caused by a bad brush in the ign switch, so I would jump that also and then check voltage. last resort, just run a wire from the battery directly to the coil and again check voltage
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Old 01-22-2019, 09:00 PM   #3
Don C
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Default Re: Ignition Questions

The car does have the original coil. Thanks for the other tips. Thinking about this a bit more, the total voltage drop across the circuit must equal supply voltage. So if the coil is only dropping 2 volts then I am thinking that some of the winding's are shorted together?

Last edited by Don C; 01-22-2019 at 09:11 PM.
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Old 01-22-2019, 10:49 PM   #4
ford38v8
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Default Re: Ignition Questions

Don, By original coil, I assume you mean the coil mounted on top of the distributor, as correct for 1939. That being the case, we're on the same page. With open points, the coil wire will show what the battery has to offer. Points closed will show the drop realized from the resistor, as well as all other points of resistance, the greatest having been mentioned already as being the ignition switch. If you have only 2 volts with points closed, you will not get your engine running. 3 volts will run, 4 volts is almost impossible to obtain with stock configuration. From what you've said, there is no indication of your coil being shorted, but yes, that can happen with an old coil, but usually only when it gets up to running temperature. If you get your engine to run, check the spark for a good blue flame. Orange is weak spark. If it dies when warm, that would be your coil, check spark immediately to verify, and eliminate other causes.

Do not buy a new coil, they are crap. Send your old coil to Skip Haney in Florida for a rebuild. Also, the new resistors are not as reliable as the original. Go through your wiring connections, cleaning and tightening. Dismantle the ignition switch to clean up the contact surface, and to consider a replacement for that unit. 8 volt batteries are installed to mask the real problem of voltage drop, resistance from poor connections.
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Old 01-23-2019, 07:05 AM   #5
Don C
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Default Re: Ignition Questions

It appears I do not have the original 39 ignition system or coil but a 42 to 47 style coil and distributor? At least from the pictures I have seen appears to be. I assume these didn't have resistors in the coil?
This coil is producing spark with just 2 volts, but what I find happens once the car warms up it will start to just lose power at full load, engine cuts out off and one like it's starving for fuel.

Last edited by Don C; 01-23-2019 at 07:10 AM.
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Old 01-23-2019, 11:10 AM   #6
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Default Re: Ignition Questions

42 to 48 did have a resistor. They eliminated it in 1949. Check the coil primary resistance.
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Old 01-23-2019, 05:14 PM   #7
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Default Re: Ignition Questions

What terminal are you measuring the coil voltage at 2 volts with points closed ?? Neg or positive side of coil, is car pos or neg ground.....
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