Spark when key is turned to the off position?

67Dart273 said
Sorry this is not true. At least not as you explained it. One side of the resistor supplies the COIL and the other side supplies the ECU. THAT is the half that was deleted. The coil side has never changed so far as the circuit. IN FACT if the ECU goes bad, you can pull the ECU connector, drop in an old points distributor, hook the dist. to the coil NEG and if the ECU was the only problem, it will run

When you START the car with a 4 terminal ballast, "things are a trick." There is no "ignition run" power during start, the coil PLUS is powered through the ballast bypass (brown, IGN2) circuit. So the power BACKFEEDS through the resistor to power the ECU during crank. VERY POOR DESIGN in my opinion

Here is the circuit. The "ignition run" at lower right is DEAD during cranking. Only power to the circuit comes from "Ignition 2" pink in the diagram

Current flow from coil is from ECU ground, through box switching transistor, to coil NEG and through coil, out coil + and back to battery through the pink "ign 2"

Box circuit is from ECU ground, out pin 1, through resistor from right to left lower section, out resistor and back to battery on pink

Other section is s from ECU ground, out pin 3 (blue) to top section of resistor, through top section from left to right, through bottom section from right to left, and back to battery on pink.

This is "properly described" as neg to pos electron flow

View attachment 1715152060

Thanks for the detailed explanation. I stand corrected and have just learned something. I'll have to read more on this.

67Dart273 said
MORE: The vast majority of ECU boxes you run into are ACTUALLY more modern 4 pin boxes, which only require a TWO pin resistor, but "can" run with a 4 pin installed. The "other half" is simply not used

This simplified circuit omits the "ign2" bypass and shows the "run" as "existing wire" off to top right. THIS CIRCUIT shows that the coil resistor is same as points.........feeds from points (transistor switch) up through coil, up through resistor, and back to the battery. "Just like" the 4 pin, in start, the resistor has to feed the box to power it during cranking. The "existing" wire is dead. ONLY POWER is IGN2 "bypass" being fed to coil+ during "start"

While it's true that most ECU boxes have probably been replaced with the more modern 4 pin unit, we don't know that is true with the OP's car.

The '73 cars originally had a 5 pin ECU with a dual ballast resistor. I think we can agree on that... and agree that we don't know what ECU is in that car right now. If it is a 5 pin ECU, I think we could also agree that if the side of the ballast resistor used in the "start" circuit is bad, the car won't start.

Regardless of my incorrect explanation, I think that given the description by the OP of "no spark" while starting and "spark" as the key is released, it kinda points to an issue with the "start" circuit. (at least that's the way I read the first post)

So... While that could be a lot of issues, given that the car is a '73 with quite possibly a 5 pin ECU, the backyard mechanic in me will point at the ballast resistor as being the easiest and quickest thing to check. With the little tidbit about the ballast not being bolted down, I would be even more suspicious that there was physical damage to the ceramic and that has caused the ballast resistor to fail.

We can debate it ad nauseum, but bottom line it that it's a simple check to replace the ballast resistor with the spare that most Mopar guys carry in their glovebox as a sacrifice to the gods of spark. Given that the OP hasn't gotten out his DMM and diagnosed the ignition circuits before posting and we're giving advice over the internet.. well... if it runs, great... If not... it's something else.