Last week, the 79 wouldn't start.

79 F-350 460 C6 4x2

Changed the plugs, wires, rotor, cap, coil and finally the ignition control module. Started many times over the following week and it ran great!

This morning wouldn't start. No spark at the plugs. When cranking, no spark at the coil; however, when you stop cranking, there's a spark. Consistently

Swapped the ignition module and coil with known good. No difference

When checking for spark, I was jumping the S on the solenoid to the battery, because I was by myself

Eventually the battery gave up, and so did I. I put a charger on it, and will attack it again tomorrow. What should I check next?

 

Key at start, 12v at s terminal and run 12v at I terminal(I think) on starter relay,also should have 12v at coil at start and about 7 to 10v at run due to resistor at run mode.

 

Did you change the pickup coil in the distributor??

 

^^^^ Agree. Your description is a classic case of a failed or failing stator, or bad/loose wiring connections.

If the stator is going south, it will not switch the module while cranking....but releasing the key will collapse the coil which will give you a spark.

Check the stator: Resistance check between the orange and purple wires from the distributor. You're looking for 400 - 700 (maybe 1K) ohms. If it's good, check the wiring between distributor and ICM harness...jiggle, cajole, shake, etc, and try again.

If the ohms are above or below the stated range, the stator has crapped the bed. I think it's more likely a loose wiring/connection issue.

 

This is a good read,also has trouble shooting.
1975-79 DISTRIBUTORS & IGNITION

 

It was the pickup coil in the distributor. Swapped-in a known good one (from the 77) and it started right up

Went to O'Reilly for a new one, but the truck didn't like it. It would start, but as soon as I got on the accelerator, it'd die

Put the donor back in, and all was well

Thanks scottscott and filthybeast! Never would have thought of that-need to learn more about that system...

 

In my experience, the first thing to replace is the pickup coil when having ignition problems on these systems. They used to go bad on my Mopar 2.2 turbo years ago too.

 

While I might agree with you to some extent, without some basic troubleshooting/testing, one might be just throwing new parts at the vehicle and getting nowhere fast. Some problems are classic in their symptoms, like this one. Others, not so straightforward.

...and, you're welcome, Papa Bear Yuma. Glad you got her fixed.

 

The problem was, in fact, the pickup coil. Proper and complete testing of all components would have shown it. The big clue was when I quit cranking, it'd fire.

The confusing problem I ran into was with the reluctor. The distributor shaft has just one slot for the roll pin, but the reluctor has two. There is no obvious marking for which slot is correct, and they appear to be the same. But they're not. If you use the wrong slot, the truck will be way out of time and might start, but won't run. Turn the reluctor 180 degrees, to the other slot, and everything is wonderful!

 

Just for the heck of it, run a jumper wire from the ground on the battery and ground it to the base of the distributor, also check the wiring harness that goes the distributor to the module, also where the wires go into the mod, make sure the plastic piece that go into the module are the same color if you replace the module!