Hey FTE, ive got a 1973 f250 with a 360/c6. I have had the truck up and running for a year or 2 now (on/off due to mechanical/transmission issues). I recently put an EFI system on the truck and it was running great and i drove the hell out of it for the couple weeks i had her back on the road.
Recently, however i started having electrical issues with the truck that started when i went to take her for a spin and the truck randomly lost ALL power and left me on the side of the road.
Cycling the key would make the dome light flash for only a second and would stay on if the key would be held between the ON and RUN positions. i figured that the ignition cylinder was faulty and swapped it out but im having even more issues now. The Cab gets 0 power unless i maneuver the key just right and in the instances where i am getting power into the cab, if i pull out the headlight switch (newer switch maybe 2 years old) or turn on the turn signals i lose all power all over again. I have cleaned off the engine ground (had discovered it to be oily as hell) as well as started trying to trace wiring but im at a loss on what the issue is. Ive removed power from the EFI system to help isolate the issue and it remains the same. Jumping the solenoid is hit or miss weather the starter cranks but when it does the engine will fire however it doesn't stay running, also jumping the solenoid has lead to the efi fuel pump priming in some instances however the fuel pump is not wired in with the solenoid so im more confused. The only certainty i have is that there's something up between the solenoid and something under my dash. I had to call it a day as the sun started coming down but i did notice that the ground location the blower motor uses has quite a bit of surface rust, and i plan on cleaning that up as soon as i get out there next. could such strange issues be caused by small auxiliary ground like that? im hoping so...

 

Possibly the ignition switch (not the cylinder) or the light switch.

 

Sounds like you have a hot wire grounding out somewhere. The headlight switch has a internal circuit breaker. If you turn the headlight on and then they shut off shortly afterwards, sure sounds like a intermittent dead ground. And EFI computers do NOT like that. Have you checked all your fuzes?

Any chance you have a mouse issue chewing on some wiring up under the dash or in the engine compartment? Maybe a wire or 2 got on a exhaust manifold or exhaust pipe? Even though you have a new ignition switch, new parts are not always 100% good parts. Have you double triple checked all your elec connectors and even added some dielectric grease?

Adding another ground strap can not hurt anything. Do you have any sort of trailer plug wiring in the back end of the truck? If so, how does it look?

 

I'll have to go through my hot wires under the dash then see if anything has exposed wiring. From what i remember most of my fuses looked fine, but ill double check that as well. Not certain about mouse exposure but its possible. Whats interesting is sometimes when i wiggle or reposition the wiring harness from the ignition itll cut or restore power but i couldnt find any exposed wiring then. i hope its not in the big loom-ed harness.. Only one way to find out!

 

I was thinking the same thing switch not the cylinder.

Now you check the engine ground, oily as hell, but what about any of the other connections from / to the battery, remember it needs a complete full circle to work right.
Ground from battery to motor, motor to frame, 10 ga wire from motor to firewall to ground cab.
Pos battery to solenoid, solenoid to starter.

Now as someone said with the head lights a "dead short" is it not a dead short or a fuse if lucky or melted wire if un-lucky would happen.
My bet and I dont bet, is a bad battery connection. it is just making contact to make the light come on but put any more juice to that bad connection and it stops working.
Tanks all the battery connections apart and clean with find sand paper and see what hapens.
Dave ----

 

Yes dead short is not the best term to use when hunting a elec issue. Pay no mind to the red box ID'ing the radio plug, it was a reference for another FTE member I saved and updated my diagram by mistake.

 

Update: dug into the wiring and noticed all cab accessories would trip the power (lights and blower fan). all of which have the same blue/red tracer wire running to them all. i pulled the temp controls out and unplugged the fan motor switch. i found that the housing for the fan motor switch was grounding out to the metal frame that holds it. i cleaned up the connections and wrapped the switch/housing in electrical tape and now she runs again! hopefully it was just the rusted/corroded terminals! i also cleaned up the blower motor ground on the cowl for good measure.