I've been having consistent ignition problems on my 1979 400. Early last summer, it cooked an ignition box. In the fall, it cooked the coil. I haven't taken any action on it this winter, but in the spring, I'm going to drop a new coil in it and start it up. In preparation for doing so, I decided to take some measurements and see if I could figure out why I keep having ignition woes. It turns out I found the problem - on battery voltage (12.6 volts measured between the posts), I get 11.9 volts at the coil connector with the key in the Run position, which implies to me that the ballast resistor wire has been bypassed. My question: What's the best approach here? Is there an easy place to splice in a ballast resistor under the hood, or should I look for a high-resistance (3-ohm) DSII-compatible coil? Does such a thing even exist?

 

In the 80's the DS-II ignitions had a 1.1 ohm resistor in the wiring, as shown here: Start, Ignition, & Carb Circuits - ???Gary's Garagemahal. I'd look for a stand-alone resistor of about that value.

 

You have a few options here:

Option: You could run an HEI distributor and take the coil, ICM, ballast resistor wire and accompanying wiring variables out of the start/run equation.

Option: You could replace the OEM ballast resistor wire.....D7AZ-12250-A, or Motorcraft #DY-213. RED/green stripe, 4' 1" (49") long, 16 ga wire, ~ 1.5 ohms. It runs from the ignition switch (two R/g wires connected together) - one goes to the resistor bypass on START, the other ties into the coil (+) providing < 9VDC in RUN. You could chase this down...under the dash....and splice it in/replace the old, shorted out one.
What happens is the OEM coils and most aftermarket ones have ~ 1.5 ohms resistance built in. You'll fry the coil if it gets battery voltage. It's meant to run on < 9 VDC and gets an initial 12 VDC only in START. The DSII is a good unit when properly maintained.

Option: You could obtain a Mopar ceramic ballast resistor.....1.5 ohms.....chase down the old wire and splice it in. It usually mounts on the firewall..... gets pretty warm so mount it away from sensitive wiring.