Originally Posted by Cab_Burge
The ballast "resistor" is a wire wound resistor design the reduce the voltage to the coil, as it gets hotter the resistance goes up and reduces the voltage to the positive side of the coil even more.
Mopar engineers design the wiring to the ballast resistor to have a 12 volt bypass on the starter circuit to provide 12 volts to the coil in the start position only, bypassing the ballast resistor to help the motor start quicker and better, hot or cold up scope
I always toss out resistors that failed in the past, but keep used older ones that were fine back then, if I happen to replace them with new ones while diagnosing other failing electrical components. I wonder if the older used ballast resistors have a "usefull" life expectancy. Every one I tried yesterday was one that I kept as a spare, not brand new......