I have a car with the orange box set-up. It has 4 pins and the harness has 5 pins. The problem I am having is the car will only run if the two wires on the Ballast are jumped together. Could the difference from harness to ecu cause this? The fuse links are all good. It only appears to be on the run side of the ignition. Can I get a 4 -pin harness from somewhere? Thanks Tod

You can use a 4 pin box with a 5 pin harness.you can,t go the other way.your ballast resistor is bad.buy a new four terminal one and you should be fine.

Do you have a 4 prong ballast or a 2 prong ballast? If you have a 2 pronger & the ballast has continuity then it's a seperate wiring issue from the # of pins on the harness as a 5 pin harness is OK for a 4 pin box if used with a 2 prong ballast. If the ballast is "open" grab a new one & several(2 prongers) are available(I think 1 for race & 1 for street) just get the right ohm one for your app. EDIT P5206436 street(1 ohm) & P2444641 race(1/4 ohm), both are 2 prongers.

The 4 pin ECU will work fine on a 5 pin system and is the replacement part for a 5 pin unit. The 5 pin will not work on the 4 pin system.

Here is a photo of the circuit card from an orange box. The large black resistor at the top of the circuit card replaces the one side of the dual ballast resistor that goes to the 5-pin box.
Bacause the 4-pin box contains this resistor, the box gets 12-volts directly from the ignition, and the single ballast resistor is only there to limit current to the ignition coil.
There is one instance when the 4-pin box gets power through the ballast resistor, and this is when the key is in the start/cranking position on older cars. because the ballast resistor is very low resistance it should not be a problem, but if your worried about it you can put a diode accross the ballast resistor.

From the archieves: http://www.moparts.org/Tech/Archive/elec/5.html
And experience.

You are fine with the 4 pin box. The 5th pin was to provide full 12v power to the coil while cranking to give a hotter spark, then when the engine starts, it runs just on the 4th pin, which provides voltage stepped down to a level the coil will survive at. Extended periods at 12v would kill a stock coil. Eventually they figured out this made no difference so just started making the boxes with 4 pins.

Wow! Thanks for all the info guys. As per the norm I left out some info. Ok here it goes. The car had a normal 5 pin ecu. I went to a four pin. I also changed ballast from four pin to 2 pin. This problem has been there before and after the Ecu change. Car will run all day with two terminals jumped with a wire. Question is does this cause long term problems with ecu or ballast? I would like it to be right. Where does the power come from on the run side of the ballast. It will start without wire as long as engine is cranking. When I put the wire back walla no more issues. The ballast is good and new. My stock wiring diagram appears to show the one wire on the ballast appears to be getting power from one of the field wires on the alt. Could this be? Thanks for all the help.

On bypassing the ballast, our stock coils wont take 12V forever(what coil do you have) but the 4 pin ECU's take full 12V. the ballast does not care if it is being used or not. See if you have batt voltage at the upstream side of the ballast, the main feed wire at the ECU, which is the (pointed) terminal on the pentastar ECU connector & a certain amount(the ballast reduces it) of voltage at the coil positive terminal when the ign switch is in "run" position & this also feeds the blue alt field wire. In "crank" check if the coil positive terminal & the above mentioned ECU terminal are getting batt voltage. You can pull the NSS wire off of the starter relay during your checks to keep the starter out of it. EDIT the 'run" (& crank) battery voltage comes straight from the ign switch thru the bulkhead in 2 different wires

The 5th pin on the 5 pin ECU is just an auxilary circuit to the ECU that goes thru the 5 ohm ballast. It only has power in the ign run position. It has nothing to do with the start circuit. Thats why when you go to a 4 pin ECU you can go to the 2 terminal ballast because the 5 ohm auxilary circuit to the ECU is not used on the 4 pin. The 4 pin ECU has 2 pins for the pick-up coil and one pin that comes from the coil and the last pin is 12 volts at key on. The coil gets fed thru the .75 ohm ballast with the 5 and 4 pin ECU. Ron

Also, when you go from the 5-pin to the 4-pin ECU, you can leave in your original 4-wire ballast(half of the ballast will be used, the other half will just sit there unused but it can still stay hooked up), you don't need to convert to the 2-wire. If you put the original ballast back in does it run fine?