Alto said: "I don't understand about powering the coil then removing power- that happens every time I turn the key to run and off again."

That is what the TFI does to the coil as the engine is running. If one side of the coil is at ground, when you apply power to the other side, primary (12v) side, the coil energizes and creates a magnetic field. When you take away power, as the distributor does, the magnetic field collapses. As that happens, a tiny amperage hi-voltage spark is generated in the secondary windings. Each and every time. Key On/Off/On/Off would generate 2 sparks. You could actually start the engine if things were exactly right. If a piston was at TDC, ready to fire, compressed fuel/air in place, and you cycled the key, a spark could light that mix and start the engine. could.
The resistor embedded in the LED is to protect the LED from overdrawing too many amps. You tried to pull even more amps using it as a conductor. That was not what it was designed to do. It was only supposed to be a glowing indicator of the presence or lack of voltage. I would not say they were inferior as I think they weren't designed to do what you wanted. Maybe.
tom

 

Thank you for that clear explanation, Tom.

After a good night of rest, I redid the diagnostic. Earlier tests were flawed, because I misinterpreted Abe's picture and thought my TFI was upside down, and had reversed the number order.
I didn't need to do the PIP test with an LED, as I was getting test light flashing on the #5 circuit wire (as numbered by Abe in the diagnostic) on the TFI while cranking. That's supposed to be the pulse sent to the coil. From the diagnostic:

"The TEST LIGHT blinked on and off as the engine was cranking. This means the ignition module is triggering the Ignition Coil and therefore the Ignition Control Module is good."

I still don't have spark, so I probed the positive side of the coil input with a test light and grounded the other end. I expected to see flashing while cranking, but I didn't. Aside from dimming when first starting to crank, the test light remained steady.

I probed the neg side of the coil input for continuity to ground and found none. The coil is known-good.

Am I correct in assuming the TFI switches the ground input to the coil? I jumpered from the coil negative wire at the TFI (#5 in last test) to the coil negative input and still got no spark. I was trying to eliminate the possibility of that wire being bad in the harness.

Alto

 

I probed the coil neg input wire with a test light and verified ground switching from the ICM. Fooling aroung with the coil input connector, the coil grounding wire fell out minus its spade terminal grippy end. I redid the end with a female spade connector and VROOOOOM.

All this drama, and it was only a bad connection.

Tom, I want to tell you how appreciative I am for your time, patience, and clear explanations. Although this turned out to be a lesson for me to check the simple things first, your input has helped me understand the ignition circuit on this truck mo' better!

I sir, am going to go have a stiff drink in your honor.

Alto