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Old 05-30-2020, 05:55 PM   #1
30-9
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Default Ballast resistor

1939 stock car. What would be symptom of an original ballast resistor going bad? Can it be intermittent? Work and then not work?

Is there a way to get test to insure itís working properly?
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Old 05-30-2020, 06:19 PM   #2
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Default Re: Ballast resistor

Could be. If it's intermittent, I would also check the ignition switch. I had one go bad on me.
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Old 05-30-2020, 06:50 PM   #3
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Default Re: Ballast resistor

Thanks I already replaced the switch.
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Old 05-30-2020, 07:23 PM   #4
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Default Re: Ballast resistor

How is the ballast mounted? I see two flathead screws but are there nuts or just screwed in?

The screws just seem to turn so I assume there are nuts. Are the nuts under the voltage regulator?

Last question what is the box (looks like transformer) next to the resistor?
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Old 05-30-2020, 09:36 PM   #5
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Default Re: Ballast resistor

Quote:
Originally Posted by 30-9 View Post
How is the ballast mounted? I see two flathead screws but are there nuts or just screwed in?

The screws just seem to turn so I assume there are nuts. Are the nuts under the voltage regulator?

Last question what is the box (looks like transformer) next to the resistor?
The Box is the Circuit Breaker for the lights. Should not have any relationship to the mounting of the Voltage Regulator.

Per the wiring diagrams 39s should still have the fuse, Ford went to the Circuit Breaker in 40. So not sure if it is a later model year change or if it has been upgraded along the way. Should look something like this.
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File Type: jpg Fuse 01A-12250-breaker-resistor.jpg (89.4 KB, 21 views)

Last edited by JSeery; 05-30-2020 at 09:43 PM.
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Old 05-30-2020, 09:46 PM   #6
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Default Re: Ballast resistor

Assuming here you are still 6v positive ground and using an original style coil mounted on the distributor.
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Old 05-30-2020, 11:17 PM   #7
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Default Re: Ballast resistor

Try cleaning all nuts and terminals both sides of the resistor. They get dirty/corroded and can cause problems.
And those mounting screws go right through to the engine side of the firewall, where the nuts screw on.
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Old 05-31-2020, 05:13 AM   #8
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Default Re: Ballast resistor

Thanks Guys I found the nuts. They were painted black and blended into firewall.
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Old 05-31-2020, 09:09 PM   #9
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Default Re: Ballast resistor

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Originally Posted by 30-9 View Post
1939 stock car. What would be symptom of an original ballast resistor going bad? Can it be intermittent? Work and then not work?

Is there a way to get test to insure itís working properly?
Symptom would be low voltage at the coil. With engine stopped should be about 3 volt. (the voltage drop thru the resistor is about 3 volt)

Test voltage at the coil. If low, work back to see where problem is. Should have 3 volt at the output side of resistor. Should have 6 volt at input side of resistor. Should have 6 volt output at switch, 6 volt input to switch. (all these voltages can vary a few tenths of a volt, depending on you battery voltage, connections, etc)

As you are working in the garage, you should minimize the time the ignition switch is on. Turn off switch after each test. With engine stopped, there is full maximum amperage running thru the circuit, is hard on the coil.

And don't touch the resistor while on; it gets very hot, very quick.
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Old 05-31-2020, 10:47 PM   #10
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Default Re: Ballast resistor

And to add to the testing confusion the timing contacts in the distributor must be closed so that the coil is drawing current. Otherwise no point in doing voltage tests if the points are open and no current flowing through the resistor and ignition coil. The longer the resistor is powered up the hotter it will become (engine not running) and so its resistance will increase with temperature rise, causing voltage to reduce. These voltage tests have to be done quickly otherwise its all a waste of time. Better to test with engine running. Regards, Kevin.
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Old 06-01-2020, 11:07 AM   #11
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Default Re: Ballast resistor

And to add further to the testing confusion:
If testing with engine running, input voltage at the coil should have jumped up to about 4 volts. (duration of points-open, the resistor is producing no voltage drop) I have read 3.5 volts or more is needed for good ignition.
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Old 06-01-2020, 12:00 PM   #12
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Default Re: Ballast resistor

Very informative. Not to hijack the thread but...I wasn’t aware a ballast could be intermittent. If driving what would be the indication of an intermittent resistor going bad and then working again?
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Old 06-01-2020, 01:42 PM   #13
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Default Re: Ballast resistor

Theoretically, if the resistor's coiled wire came loose from it's housing it could produce an intermittent open circuit. Symptom would be like turning off the ignition switch, and maybe it would make contact again when cooled. But in the real world, the resistor almost never fails. They are very robust. Most original resistors still function just as when first installed.

For an intermittent ignition failure, I think the condenser, coil, loose wire connector, switch, distributor internals would all be more likely suspects.
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Old 06-01-2020, 02:38 PM   #14
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Default Re: Ballast resistor

Makes sense - Thanks Jay.
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