DIYbanter - View Single Post - Resistance of Ballast Resistor on old Points ignition

Rheilly Phoull[_2_]

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.