I'm going to start over as my input. I have a GMC 305.
1. The coil was originally mounted in a hard to reach place and many have been moved like yours has.
2. You have a resistance wire. You don't have a ballast resistor unless a PO added.
3.
DO NOT use that diagram. It is some guys redraw of the original. He has two tries at it after finding mistakes. Use the original wiring diagram.
4. If your truck was starting and running fine,
AND NO ONE screwed with anything you haven't told us about, and it now shuts off after spring loaded switch returns from "start" position to "on" position,
it is the ignition switch. I would say a broken off wire but you said you have 12V at coil with ignition switch ON. You should have TWO wires to the coil positive. Do you have two wires on the positive and one wire on the negative?? The coil is marked - and + if you clean it off!!
"I tried fixing the ignition switch thinking that might be a good place to start but that was a no go." What does that mean exactly?
See if there is voltage at the coil positive terminal with key OFF and then see if there is voltage with key ON. Report results of this test. Then we can proceed. Get a ice pick lighted voltage
tester. Forget the multimeter.
Clean up the wires and the coil!!!!! So you and we can see what's going on and see the colors.
ALSO: to be clear, you are saying that the motor FIRES, starts to run, but when you let go of the key it stops. Right?
FIY: Never use a sketch or a redraw of a schematic or wiring diagram on the internet for our old trucks. Find the original from a manual.
Your problem will be very easy to fix. We need to proceed slowly. Don't do any wiring or disconnecting. Just test the coil pos again, key "On" and Off". "On" does not means "start" position.
You are checking ohms and fuse panels and breaking into harnesses and don't know about electrics. Lets back off a little. Getting too confused possibly.
What did this mean? "Are you suggesting that I try testing the harness on the ignition? I've already replaced this." What did you
replace?
