I read on moparts that you can use a 4 pin ecu on a 5 pin system but not the opposite. Well, mine will run with either 5 or 4 pin ecu and has the single ballast..?

the 5th pin needs the secondary resistance on ballast. If you plug a 4 pins module will mean the secondary resistance source won't arrive anywhere, but still will be there available if needed to plug back a 5 pins module.

if you have single ballast and plug 5 pins module, will mean the 5th pin NEVER will get the secondary resistance source... NEEDING IT!!!

4 pins modules have internally the resitance what 5 pins modules use externally from ballast.

NOW... in your case what using the 5 pins on single ballast resistor set up could it be the 5th wire ( green traced red ) was hook on some source down the tape on harness ( danger, could get burnt I guess with time )

I never have tested if a 5 pins on a single ballast setup will work without power to the 5th pin... probably yes what won't mean, correctly

yes that is commonly written that the 5 pin ECU REQUIRES a dual ballast & wont work with a single. & as Nacho says the 4 pinners have the resistor inside rather than using the 5 ohm side of the dual ballast I would think either a reduced spark or early failure or both. & the 5th pin was on the 5 ohm side of the dual ballast & this pin is open in your wiring? Chad I dont think I answered your Q!

4 pin works fine as a replacement on a five pin system but the you can't use a 5 on a system designed for the 4 pin.

Try this: Turn the key to the run position, (make sure that teh transmission is in park), now go under the hood and try to start by jumping the relay and operating the carb by hand. If it runs or tries then the problem is in the start circuit wiring.

I'm more confused now,
What is the difference in 4 versus 5 pin.
Where is 4 pin used and where is 5 pin used ??

logan426

Not too many "5 pin" ECU's made that need the dual ballast anymore. If you find an aftermarket 5 pin unit it's internally the same as the 4 pin unit, the 5th pin is generally a dummy pin, for looks only so the customer won't complain that the new ECU is "wrong".

That probably explains while both work in your system.

I didn't believe it when I first heard that there were 5 pin ones with a dummy 5th pin but that would explain how a 5 pin unit could run OK on a 2 terminal ballast. I'd ohm it real quick & find out for sure & do the test the other poster said to find out if the start/run wiring is crossed.

I heard that the 4 pin is internaly grounded, and the 5 pin has the fith pin for a ground. There is/are drawings here on Moparts in the tech archives.

The fith pin is an auxilary circuit to the ECU when the key is turned on. It is supplied 12 volts to a 5 ohm ballast that then drops it down and sends it to the fith pin. It does not need resistance in the 4 pin unit as the fith pin circuit is just an auxilary circuit to turn on a transister that is eliminated in the 4 pin unit. Tecnically the 5 pin ECU needs the fith pin powered to work but I can say I have never tried a 5 pin box on a 4 pin circuit. Ron

I carry a spare 5 pin unit off of a 74 Fury that still has the Electronic ignition sticker on it from Chrysler so I doubt its 5th pin is a dummy. Last spring my ecu went out on my road runner (converted single ballast with no power on the 5th pin wire) and I had to put the old Fury 5 pin box on beside the road. I didn't think it would run but it did run quite well. It was about 70 miles the rest of the way home and had no problems. I went to Napa the following Monday and got a new 4 pin box. I don't know how long the 5 pin would have lasted but that day it made it 70 miles fine.