If the truck started with a jump across the solenoid with key on. it means the coil is getting power from the ignition switch. It means the ignition switch is getting power from the battery. No fusible link problem. It does not have a factory fusible link. One could have been added but don't think it is that. Solenoid not being triggered by ignition switch?
The wiring is, as you know: big cable from battery to starter. Smaller wire from battery to horn relay. Horn relay has a two screw terminal used for power distribution. Relay distributes, basically, direct power to ignition switch, headlight switch, fuse panel, alternator and voltage regulator. All RED wires. Some of the distribution goes thru fuses, connectors and terminal points. Check that distribution and all fuses.
IMPORTANT: Check to see that there is hot at the ignition switch battery terminal. If power there....Turn ignition to start. Ignition switch solenoid terminal (purple wire) should go hot. If it does not, it's a bad ignition sw. If it does, follow it thru the 6 pin connector and check it's continuity along the way to the solenoid "S".
Still trying to figure out why turning on lights kills motor. Large amp draw from lights is stealing power to coil? Ignition switch has weak ignition "ON" contact?
I assume no one has messed with ignition wiring or any wiring? Started one day and not the next. I assume static battery voltage is ≈12.6V and charging voltage at battery is ≈14V at high idle.
A dash volt meter is powered by switched power, so if key off, it would read zero voltage at it's battery terminal. If that's what
"the big terminal" means.
If you mean your multimeter voltmeter, I don't know what "the big terminal" means.
