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A little more confusion to "help"… Firstly, both Jerry and Bob have valid points... There were many 12 Volt coils made in the 70s that did have an internal resistor, mostly imports, I think… and many others that were just more wire as Bob said. Secondly, the real idea of the resistor is that you deliberately design the coil to operate at a voltage lower than 12 V (or 6 V) and then use the resistor to get to that correct operating voltage under normal conditions and then try to use this temperature sensative resistor to compensate for the other conditions. And the ballast resistor, while often having the temperature affect that Jerry describes, has an operation a little more complex than just the initial heatup when you turn on the ignition. This heatup is actually quite rapid and unless the engine starts immediately, the resistor is already hot and voltage regulating. And when you hit the starter, your voltage drops to say about 75% of the full charge voltage. Your spark would then be much weaker except that it is mostly compensated for at the coil by the fact that the ballast resistor resistance will drop with the lower applied starting voltage (and therefore current). Keep in mind that the heating of the resistor goes like the square of current, so a modest reduction in current makes a big difference to the resistor heating. To further combat this low voltage situation, many cars from the same vintage as our tractors, had a bypass wire from the starter that applied the full voltage to the coil until you stop cranking. (These ballast resistors did not need temperature compensation.) This gives pretty much similar sparks during cranking and normal operation. Jim
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