Mopar Action On-line – Tech Questions


Tech Question

Steve Whelan, High Wycombe, Australia, 1973 Dodge Challenger 440

HI Rick,
you must be sick of ECU/Ignition conversionquestions so I found out most of the info I was looking for from yourprevious tech Q’s as well as links from your website. But I can’t findanywhere that tells me the correct value of ballast resistor to use for theOrange ECU upgrade from the stock black ECU. I learnt that the 5-pin blackECU needs a dual ballast (5 ohm and 0.5 ohm) and that the new Orange 4 pinECU only requires a single ballast =- But what value?? Parts suppliers say a1 ohm, but they supplied me with a 1.25. I have a collection here rangingfrom (as measured on my ohmeter) 0.6 to 1.6 ohms. What is the correct valueto use and how critical is it to the operation? And another thing I can’twork out is why they don’t use pin 3 on the ECU any more? I guess they musthave an internal resistor to drop the voltage for the unit? I am also doingthe upgrade on my other vehicle — an Australian ’73 Valiant Charger 340.Thanks for any help for this simple question, and also thanks for aninformative and sharp publication.


The dual ballast’s 5 ohm side wasm’t needed on 4-pin ECUs, which began inthe late ’70’s, since the ECU’s internal circuitry was completelymodernized, rendering it unnecessary and superfluous.

The ballast’s value is pretty much an inverse curve: as the resistancedecreases, the spark out goes up (“hotter” spark), RPM range also goes up,and emissions and low-RPM misfire go down. Sounds like a no-brainer, right?Well, there’s no free lunch, mate! As the resistance goes down, current draw(amperage) goes up, and so does power dissipation, which is what makes allthose good things happen. But more dissipation = higher operating temps =reduced reliability. That’s why the “Gold” box has an extra heat radiator onthe switching transistor.

Now that the engineering lesson is over, a direct answer: around 1.0 ohmseems to be a good compromise. Anything less, for long-term street use,either toss spare parts in the glove box, or add an extra heat radiator(it’s a TO-3 power transistor, see sources such as or Radio Shack, etc.)



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