Not sure when farmall went to a coil, my 1950 farmall C has a mag. So the old mags don't have a ballast by-pass circuit. A by-pass circuit shorts out the ballast resistor when starter is cranking. This applies the full battery voltage, minus a few volts, because the battery cranking voltage is usually less than 12v. This is how all the old cars were wired back in my day. The ign switch was responsible for applying full voltage to coil when starting.
I used a diode to apply 12 volts to coil when the starter is on. I connected the anode to the starter wire, the cathode to the + coil wire. Some starter solenoids have a terminal for this purpose, mine didn't. For me, the diode was the simplest way to get the job done.
Sometimes I think 12v conversion is spinning the starter too fast. I slowed my farmall C down by using a 185 cca lawn mower battery. I like the way it starts turning over slower. Just my opinion, but I think the old tractor were designed to start using a crank, lot slower than even a 6v battery. I hand started my share of old tractors and kick started old motorcycles. Nothing fast about that.
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Today's Featured Article - New Life for an Old Allis - by Tyler Woods. My friend Jon, has an old '39 Allis Chalmers B. He thought it a marginal tractor that had long since served its time. She smoked terribly and never had much power but he couldn't afford another so he was limping along with what he had. Jon's Allis has a small front loader and though it doesn't carry much, it serves his needs. It was the hard starting and low power that made him think it was time to replace the old girl. Jon called me to help him discover why his tractor wouldn't start
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