Ballast Resistor

BIGR · Nov 17, 2013

What does a ballast resistor do? I am wiring up a Worthington Tractor that I picked up. It has a Ford Model "B" engine which is similar to the 9N. I'm using a Ford Model "B" Car and a Ford 9N wiring diagram as a guide. Both diagrams show a resistor before the coil. The Worthington's old wiring harness did not have a resistor. Do I need one? I am adding lights and it's still 6V Positive Ground. If I need one can I use one from a 9N?

Bruce (VA) · Nov 17, 2013

a true ballast resistor, as opposed to a fixed resistor, has minimal resistance when cold & increases resistance as it heats up. The purpose is to allow max current to the coil for start up & then reduce current while the engine as running.

It depends on the internal resistance of the coil you are using.

For example, a front coil (2N & 9N) had a very low internal resistance (.3 to 1.5 ohms) and requires a ballast resistor. An 8N w/ a round coil does not need a ballast resistor because the internal coil resistance is greater than 3.25 ohms.

The A model Ford used a round coil, no ballast resistor. AFAIK, the 4 cylinder B model Ford engine also used a round coil & probably does need a resistor. But, to be sure, just measure the internal coil resistance.

And BTW, a B model Ford engine is only similar to an N engine in that it's a 4 cylinder flat head.

BIGR · Nov 17, 2013

Ok, how do you test the resistance? Also, is there such a thing as using the resistor and not harm anything if you don't need it. Can I use one from the 9N? I have one sitting on the shelf?
Thanks for the help!

Bruce (VA) · Nov 17, 2013

'how do you test the resistance?'

With a multi-meter. Set it on resistance.

' is there such a thing as using the resistor and not harm anything if you don't need it.'

Too much resistance in the ignition circuit will make it hard to start & run poorly because of a weak spark.

' Can I use one from the 9N?'

You just don't stick a resistor in a circuit w/o knowing what the coil will handle. The 9N OEM ballast resistor is about .3 ohms cold & 1.7 ohms hot. It might be too much or too little resistance.

What does your coil look like?

BIGR · Nov 17, 2013

It's a round coil and it says Delco-Remy 6V on it.Looks like it's the original? At least simlilar to what the original was.

HCooke · Nov 17, 2013

A 6 V round coil needs no resistance as long as you are using a 6 volt battery.

Royse · Nov 17, 2013

As said, the round can doesn't need the resistor @ 6V.
The 9N/2N and early 8N did not have a round coil, they used
the square coils so you will see it in all of their wiring diagrams.

Ballast Resistor

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Bruce (VA)

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