Another Farmall 100 question

Charlie M

Well-known Member
Tried starting my 100 after sitting over the winter and I have no spark. Its a 6 volt system and I only have 3-4 volts at the coil. I thought maybe the wire to the coil was the problem as there were some bare
spots on it but it didn't change anything. Shouldn't I have 6 volts at the coil or is there that much voltage lost normally. I get 6 volts on the battery and it turns over the engine real well for a 6 volt
system. I'm measuring voltage without cranking the motor.
 
the switches go bad. measure the voltage there. if 6 on one side, and 3 on the other (turned on) the switch is bad. Jim
 
Try a simple hot wire from the ignition side of the battery to the ignition side of the coil. If it runs that way then the problem is in the wiring or the switch. If it doesn't run hot wired then the problem is in the distributor
 

You can do a voltage drop test across different components to see where the problem is. To see if the switch is the problem, turn the switch on and use a voltmeter (set to DC volts) to measure the voltage drop across the terminals, it should be very very close to 0, if you get much of a reading, then you have found your problem.

You can do this to each piece (switch, wires, etc.) in the circuit to see what the culprit is, if you find the that switch is not the problem.
 
If there's only 3/4 volts on the coils input with ignition on and points closed, I suspect a faulty resistive ignition switch or a loose resistive connection somewhere in the circuit from ign switch down to coils input, assuming there's a good 6 volts getting to the ignition switch input CHECK VOLTAGE TO IGN SWITCH BAT INPUT on and points closed ????? maybe there's a voltage drop BEFORE the switch ???

Over on the coils other output (to distributor) terminal you should have battery voltage with points open but near zero volts when closed. If slowly cranked a test lamp there would flash ON points open and OFF points closed

Rig a temporary a "hot wire" direct from the battery down to the coils input and see what voltage you get then with points closed, if the battery is good I would expect battery voltage there NOT 3/4 volts.

Those old switches can develop resistance through dirty loose burned carboned contacts

John T
 

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