Hugh, I can answer that. Engineers found out by testing and consumer input, that if a 12v system coil was designed to have neither an internal or external resistor, the reduction in starting voltage from the starter draw would reduce the coil output enough to compromise the starting capability. So automakers, and eventually tractor manufacturers. installed systems to compensate. They ran two wires to the ignition, one from the starter relay/solenoid directly to the coil, and one through the ignition switch to a resistor (Ford used a pink wire in the loom that was a resistor, while others used a block ceramic resistor)
The resistor bypass wire (that was connected to a terminal often labeled "I" on the solenoid or relay) allowed the 10 volts, of the remaining battery volts, to power the coil above the normal run voltage of the coil 10 volts on what is essentially a 6v coil. this raised the output at a time it was most needed, starting.
Another method of doing this is to use a thermal resistor that starts out cold and has low resistance, then gets hot and resists down to the 6v range. These can be internal (most) or external, allowing a simplified wiring system and about the same results. Seasons greetings from central MN Jim
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Today's Featured Article - Talk of the Town: What's The "Stuckest" You've Ever Been? - by Edited by Kim Pratt. Another great discussion from the Tractor Talk Discussion Forum. The discussion started out with the following post: "I was about 14 (part of the problem) when I got stuck. I was disking with a cab equipped IH 966. The window was dirty and I was driving into the evening sun. It was hard to see and it was my first pass down the field. I got the tractor so stuck that the underside of the tractor was resting on the ground. My uncle wanted
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