I have been chasing an electrical gremlin for a month or so now. Truck will start just fine for about a week. Then it won't. I turn the key and nothing....no solenoid, no click ...zilch. Funny thing is though, no headlights, no lighter, cab is completely dead. If I let it sit it will start eventually. I will get some clicks, the brake warning light will come on dimly then fade out. I believe it is the ignition switch going out. Does everything in the cab run through the ignition switch? Its not the battery. Battery tests out fine. I have power going into the cab, tested the green/black wire and get 12.6 volts. That is the wire that powers the cab. I had to fuse it because the PO removed the fusible link. If all power in the cab goes through the switch, that will be my next purchase.

 

Unfortunately, no it's not going to be that simple i don't think. You're on the right track, but the headlights are not powered by the ignition switch. Nor are the brake lights and probably one or two other things. Maybe double-check the brake lights just to be sure?

Sounds like a dying battery cable to me. But usually before it dies you get that last heavy click of the starter relay and then nothing. Usually not fully dead just turning the key.
But it's still a place to start. What cables are you using? Not the starter cable (though that's important too) because it won't kill power to anything other than the starter. If it's a cable, it has to be either the positive or negative from the battery.
I've only ever seen the positive cable do it (happened to me to) but I think other members here have experienced ground cables going bad too.

You can still measure voltage even when the cable is failing and nothing works. The meter does not take enough current to show the break. But when something heavier is demanded of it, such as the heater or whatever, you don't get enough flow to power anything up.
So that's the basics. The headlights were my biggest clue in that direction. So a bit more detecting on your part is in order it sounds like.

Next time it dies, connect a jumper (maybe even a jumper cable) between the battery positive and the battery side of the starter relay. If still no power, move the jumpers over to the negative side of the battery and then a good ground on the engine.
If anything changes, you're getting closer.

paul
Good luck.

 

Headlights, front park lights, side marker lights, emergency flashers, dome light, brake lights, cig lighter, glove box light all direct power from battery, regardless of key position. I believe the brake warning light on then off, is like the seat belt and just a self test of the circuit, once the key is in the run/start position.

I'd check all the grounds, battery cables. Ditch the ones from the cheap local parts house where the wire is molded in the end, or buy quality ones. Take the one large elec connector off the back of the ign switch clean and insp.

Double ck the battery voltage.

 

I was trying to pull from memory where the cab feed ran I've had the dash apart so many times doing other wiring projects. It is the only wire feeding power to the cab and if memory served it seemed to only run to the ignition switch. I do not recall any splices except one, about a foot from the switch, and both of those go to the switch.It didn't make sense to me that all the power ran through the switch, just thought I would ask, because someones memory is always better than mine.....I will try a new cable first, looks like it needs replaced anyway. Thank you.

 

Not sure what year truck you are talking about but the main feed doesnt only run to the ignition switch. The main feed probably goes to the fuse panel, ignition switch, and headlight switch. From there, other switches are fed. I have never seen wiring diagrams for the dents, only the bumps. Earlier dents probably arent that much different and maybe not the later either. I found some diagrams that I havent looked through. https://www.fordification.net/tech/wiring.htm

 

All my elec diagrams start on post #10. Here is a link to the start of the thread.
https://www.ford-trucks.com/forums/1...then-some.html