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Bad ballast resistor on 12V converted H

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Dave in CT

05-24-2004 20:38:28

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This is a bit of a newcomer's report but I thought it might help as an archived account of a solution.

My '47 H would die or not be able to restart after it ran for about five minutes. It had a ballast resistor in line with a 6V coil. The resistor got hot but the coil did not - which I would assume would be normal. The no-start condition would also appear if the battery was left connected (I usually disconnect the negative ground when not in use) and the shutoff switch was pulled out to the 'on' position. This would heat up the resistor which, again, is to be expected. I pulled the spark plugs and grounded the outside of the plug to cylinder one. When the engine and resistor were cool, I'd get spark; when the resistor was hot, no spark. So I replaced both the resistor and the 6V coil with a 12V coil and now it doesn't die and can be restarted hot. Because the no-start condition appeared with just the resistor heating up, I'm assuming that it was the resistor that was bad. What would be the Volt/Ohmeter test on the resistor to verify this?

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riverbend

05-25-2004 17:07:32

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Re: Bad ballast resistor on 12V converted H in reply to Dave in CT, 05-24-2004 20:38:28

You can use your VOM to check the resistor cold and hot, as long as there is no power going through it at the time. Any voltage could be bad for your ohmmeter.

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Bob

05-25-2004 21:01:46

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Re: Re: Bad ballast resistor on 12V converted H in reply to riverbend, 05-25-2004 17:07:32

If you have the PTC resistor, the value will drop within seconds of removing power from it, as it quickly begins to cool. The only accurate way to measure it's hot resistance is to measure the current flowing in the circuit, and the voltage dropped across the resistor, and calculate the resistance.

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Jim in michigan

05-25-2004 07:01:39

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Re: Bad ballast resistor on 12V converted H in reply to Dave in CT, 05-24-2004 20:38:28

I have found that the best resistor is the kind used on Dodge vehicles, older ones.. I get mine from old junkers, they are also cheap to buy one at NAPA or any other parts place,,Jim

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Bob

05-24-2004 23:07:31

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Re: Bad ballast resistor on 12V converted H in reply to Dave in CT, 05-24-2004 20:38:28

There are 2 types of ballast resistors. One type is relatively constant resistance as it heats up, and the other has a positive temperature coefficient... it's resistance goes up rapidly as it heats up. If you don't get the machine started almost as soon as you turn on the switch, you begin to loose spark voltage.

Some electrical systems have a "boost" setup, where the resistor is bypassed during the time the starter is engaged.

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Dan

05-24-2004 21:13:52

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Re: Bad ballast resistor on 12V converted H in reply to Dave in CT, 05-24-2004 20:38:28

I have an inline resister on my SMTA. It would just die on me then restart ok. Checked resister and it literally fell apart when I took it loose. Sounds like you have solved your problem. I used a 12 volt coil with internal resister on another conversion and have had no problems with it.

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