ECU/ Ballast resistor question...

If a car is wired for a 4 prong ballast and a 5 plug ECU, will a four plug ECU function ok or will I need to rewire it to a 2 wire ballast?

4 will work on anything. A 5 pin require a dual ballast. That is assuming it is factory wiring not Uncle Teds spaghetti special.

Yes & you can even parallel wire the ballast for more gain (add a blue wire loop on the other end). More gain is (sometimes) inversely proportional to longevity, its a tradeoff. if you have 1-1.5 ohms your good. what coil are you running? EDIT As Bee said OE 5 pinners require the dual ballast setup but some parts house 5 pinners do not (ohm the 5th pin (red/green terminal on MP harness) & if it is infinity then it is a dummy 5th pin) & will work on either a 2 or 4 terminal ballast system. MORE EDIT green terminal if OE harness

The best thing to do is hide a GM 4 pin module in Mopar ECU. Almost 3,000 miles $15 dollar Wells GM 4 pin module with a MSD blaster cannister coil.

How in the world did you manage to do that?

1971 Road Runner. Somewhere in the past it had a MP ignition distributor, coil and ECU wired up. It is an orange 4 pin ECU but was wired to a 2 pin ballast. I am COMPLETELY rewiring the entire car with an American Autowire update harness kit, that calls for a 4 pin ballast. I just discovered that the orange ECU is a 4 pin. Since everything should play nice, I am going to continue to wire it like the schematic calls for, with a 4 pin ballast.

The wiring in this car was so bad, it is a wonder it didn't ignite something.

Gutted the Mopar ECU put the GM module inside ran 4 wires to the Mopar ECU connector. The Mopar ECU shell works good as a heatsink just put some heatsink compound between the GM module and the Mopar ECU shell. Run a bypass wire thru the ballast for the needed 12 volts for the GM module. Look online there are numerous ways to run the GM 4 pin module. Most will say to run a e-coil and make a special mounting for the module but I wanted a stock look on this car.

that'll work fine & later if you decide to experiment you could temporarily add the blue loop for less ohms or sub in a 2 terminal less ohms MP ballast & if you like it then cut off or aesthetically tape up the 2 extra pigtails. I tried a rockauto furd E coil ran straight thru (no ballast) & it was awesome. Way back I had an HEI module straight thru with an Accel super coil (the big yellow beast) & it started instantly in the summertime but I had nothing to compare its performance to. EDIT Accel wants .7 ohms of ballast with that super coil. there is alot of leeway on what you can & cannot get away with.