You better not run that motor. You better disconnect the battery. You better check the wiring more carefully. What are you using to "check the wiring"? I would NOT use the AMMETER ever again. You have a volt meter. Do you have the foot start? or a solenoid? Do you have a wiring diagram for your truck? If you answer these questions, I will explain what to check. You obviously are NOT turning off the coil. That's how a motor stops. So just wait till we get this sorted. Make your own wiring diagram from following each wire. Get the rat's nest sorted and bundled in meaning full branch harness. Get the Ignition switch dropped down and hanging so we can test. Ammeters are a PITA and absolutely not necessary. Too many unprotected/unfused/previous owner wiring/no fusible links..... on these old trucks. That must be addressed. If someone is working on their wiring, they need to understand wiring. Otherwise they will cause issues. Our trucks basic wiring is very simple. It's not house wiring or appliance wiring. It's very easy to understand.
If you have time, please answer ALL the question above. Glad to help, late this evening. You have already received some great advice and explanation.
