Educate me on ballast resistors?

Whoever converted that "was" a fair amount of time ago

The earlier Mopar box had to have a dual ballast, it's actually two (different) resistors in one, and is known as a "5 pin box."

The later box looks exactly alike but the "5th" pin is a dummy, and it only needs a 2 terminal resistor. This is known as a "4 pin" box

Only way to tell the boxes apart is to check resistance of the 5th pin to see if it's connected to anything else.

Having the older 4 pin ballast on a newer box doesn't hurt anything, it's simply that the "other half" of the resistor does not do anything.

There has been some change in resistance over the years, but in essance there are only two resistors all these years. The "coil side" of the 4 terminal resistor is the same resistance as a 2 terminal resistor.

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Also, "for you Chevy guys," Chevies ALSO had ballasts before the advent of HEI. It was a resistor wire run in with the harness instead of a "block" resistor. 55-57 Chevs did use a block resistor, I'm not sure when Chev went to the resistance wire anymore.

Ford, AMC, and GM all had a separate "I" terminal on the starter solenoid to BYPASS the resistor for starting.

"Ma" does this in the ignition switch, that's the brown hooked in there to the coil + side