I just bought this truck and it has a weird electrical short. It is a 1991 F150 with a 302ci 5.0L 4x4, all original. The truck will not start until you turn the headlights on, the key ignition doesn't work but if I jump the solenoid it starts fine (only if the headlights are on and the key is in the ignition and on). If I get the truck running by jumping the solenoid, all power is lost when I turn the headlights off. Whether the key is in the ignition or not, turning the headlights on engages the ignition, the fuel pump hums for a second, all dash lights appear and the radio powers on. The truck has an aftermarket radio but the previous owner did not mess with the original wiring. For diagnostic purposes I disconnected the two ends from the aftermarket radio harness to factor that out.

I replaced the headlight switch in case it was an internal problem but no luck. All bulbs are fine and secure in their sockets. I checked and sanded a few grounds just above each headlight, and will continue to check any grounds I can find. The headlight switch gets real hot, obviously because of too much current from the short. The rheostat would always get hot but this is to the point that it is almost melting the plastic connector.

As you can imagine this is a frustrating problem as any short would be and I can't really drive it for fear of a fire from the overheating wires.

I know basic diagnostics and am decent with a multi-meter but not really sure where to check.

Any help would be GREATLY appreciated!

 

That's pretty f*cking weird.

 

Indeed!

 

Agreed it sounds as if someone has hacked the main wiring harness. Best bet get a diagram and find out if those wires cross someplace and check it. I do not know of any relays or fuses that would cause this. It is either a short or someone spliced it together. Talk about a cobbled up mess if they do it on purpose.

 

That sounds like a good idea, most of the diagrams dont seems to show where there wires are routed so ill have to find one that does and check that. Thanks

 

Sounds like someone hacked into the light switch wiring, Maybe to make it harder to steel or something, or I would think you have some melted wires somewhere.. Try tracing the wires coming off the light switch you just may find it.. Maybe in the fuse panel as well.. Just a thought..

Ant...