On 28/03/2019 8:07 pm, John-Del wrote:
On Wednesday, March 27, 2019 at 9:33:46 PM UTC-4, Rheilly Phoull wrote:
On 28/03/2019 8:23 am, Rheilly Phoull wrote:
On 28/03/2019 6:20 am, wrote:
Does anyone know the correct resistance of the ballast Resistor on old
Points ignition? I have an old Farmall M tractor. That resistor (which
is a large ceramic power resistor), only measures 3 or 4 ohms

I was expecting it to be at least 100 ohms, if not 500 or 1k.
I measured this with nothing connected to it, using an analog
multimeter.

This resistor is between ignition switch and the ignition coil and
points. It drops the 12volts to about 6 volts. Some information I got
about this tractor says the voltage should be about 8 volts after the
resistor. I dont know how much tolerance is allowed, but I'd rather see
a higher spark plug voltage than a lower one.

The tractor now runs, after it failed due to what appears to have been a
bad condensor (capacitor). But it runs rough after replacing the points,
condensor, plugs, dist cap and rotor. The spark on a test sparkplug
seems kind of weak to me.....

Kind of makes me wonder if this resistor needs replacement?

(The sparkplug wires were replaced about 2 years ago, so they should be
fine, since I dont use the tractor all that much).


Presumably you know ohms law, just calculate the value to give the 8
volts after starting.

I also assume you know that the ballast resistor is used to apply a
higher voltage to the coil by bridging it on start then opening the
bridge when running.

Back in the 70s, I wired a momentary switch across the ballast resistor of my car and mounted it on the throttle shaft of my carb. At full throttle the switch bypassed the resistor.

Between that and turning the air cleaner cover over, the car broke the sound barrier...

Seriously, it did seem to help at high rpm though.


Hmm, the thought of Mr Tubeguy doing rooster tails on the tractor would
be a sight you dont see every day.