Hi gang...My 86 F-150 4.9L 300 Six....Was running fine, UNTIL A raining snowy freeze overnight...ALL electrical Dead. [ Has new battery] I banged around under the dash... I got her running fine BUT IF YOU TURN ON LIGHTS, OR HEATER, OR WIPER, OR TAP BRAKE, EVEN A TURN SIGNAL. SHE WILL SHUT OFF AS IF YOU TURNED OF THE IGNITION? THEN, EVERYTHING WILL BE DEAD FOR A SPELL. I AM THE ORIGIONAL OWNER OF THIS TRUCK AND KNOW ALL ITS HISTORY AND REPAIRS.
After this episode So Far I:
I REPLACED the ignition switch, battery, and new starter soleniod, I cleaned and re-mouted the main frame/ starter grounding cable comming off the solenoid. When I do get her running I have14.2 V across battery.[13.v engine off]. Alternator checks out by these readings.

What could it be that would kill ignition,lights, acces, ect?

I pulled the fuse box cover, and I only have I empty curcuit at the bottom of box that is hot to the test light, all other fuses/ or empty curcits are dead when this black out happens? Could it be wet? Also, add on Amp gage is always hot at both terminals as it should be, even when die out happens.
Could the computer cause this Killing of all electic power.[tho it will run like a dream if THE RARE TIME it starts/ electricals come back? ANY HELP APPRECIATED FOR SURE!

 

Turn on the headlights with the engine off. If they are dim or won't burn, keep them on, and then measure the voltage at the large terminal on the solenoid that goes to the battery +. Make sure you have around 12 volts there. If you do not, then I would suspect a loose connection or a bad cable going to the battery.

If that checks out with 12 volts, then turn the headlights off, disconnect the battery, and then start at the large battery + solenoid connection, and look for a smaller wire coming off this big terminal. This smaller wire feeds the whole truck's elec system. There should be a fusible link in this wire(colored rubber thing in the wire). Follow this wire down through the harness. You might have to unwrap the harness. You should come to a splice. Check all this wiring and the splice out. You should come to a splice where you will find more wires and more fusible links. One of them should be a black/orange wire(I think). This is the wire that feeds the headlights. So your problem should be at that splice, or upstream to the battery.

Once you get the wiring unwrapped, if you want to do some more voltage tests, make sure the wiring is not shorted out where you unwrapped it, hook the battery back up, and then turn the headlights back on, and do more voltage checks.

When you are troubleshooting this problem with your meter, before you take any readings, always turn the headlights on first to put a load on the system. If you don't, you will get false good readings.

 

Thank you for your help...I had it started today[after a thaw/dry weather] I had all electric- ran for 30 miniutes with just heater on MED...when engine reached full temp I but heater on HIGH [stalled at that point before, but was fine today?] but as I added small electrical loads like wiper/ and or tap brake...she kills right out and all curciuts dead. then will restart within 15mins to 1 hr. later?????
Could a diode in alternator fail off and on with this problem? Though alternator does charge bat to 14.2 v with engine on.

When I turn on the head lights with all off...they do NOT burn at all. I have yet to do your voltage checks and follow your instructions on the Sol. hot wires/links...I just got bomb with 8" of snow and truck is buried again. I will keep this problem posted....Thanks again

 

ATTN: FOUND PROBLEM...ELECTRICAL CONNECTION PLUG [located on to of right wheel fender cover near soleniod] OF THICK YELLOW, THIN YELLOW/GREEN AND RED WIRES ROTTED OUT AND WAS SMOKING WHEN LIGHTS ON...SPLICED ALL WIRES AND EVERYTHING IS A-OK....THANKS FOR READING AND I HOPE IT HELPS OUT OTHERS.