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: Winterizing Engines - To Drain or Not to Drain? - by Russ Berry. What is this strange attraction I have to equipment and machinery? How did I get this way? I came from the suburbs and own a small horse farm in rural Loudoun County, Virginia. You can call me a "weekend farmer." The local farmers do. Does it bother me? No. I am just happy to have their friendship. At least the word "farmer" is in my title. But what is the attraction? How can I explain the sensation and exhilaration I feel when I turn the key and hear the engine come to life (most
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