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.
That is correct. The resistor is removed from the circuit while cranking so
the engine gets a hotter spark. The reason? Because while cranking, the
starter takes a huge amount of current from the battery, lowering the
available voltage to the coil. Removing the resistor from the circuit gives
more voltage (and current) to the coil, helping the engine start.
After the engine starts running, the starter is no longer activated,
removing the huge current drain on the battery, raising its available
voltage. Then the resistor is inserted into the circuit in order to reduce
the current through the points, thereby reducing arcing, which can eat up
the contacts rather quickly with high current. That's also the purpose of
the condenser... to help reduce arcing across the points.
Cheers!!
Dave M