Back End Fuse Problems!

[79Betty]

On my 77 C30 i'm having some fuse issues. It's the 20A taillights fuse. I have used the process of elimination and determined such:

Undid front half wiring to headlamps and parking lights still blows.
Undid back half wiring same results.
Undid turn signal switch same results.
Undid floor dimmer switch same thing.
Undid the cluster housing plug same thing...
This is annoying as hell! Seems to me my short is in the cab wiring somewhere? I have a new headlight switch... Help! Any suggestions? I finally got me driveshaft made so after this i'll be ready to rock!

[mrolds88]

You pulled the harness out of the front (engine compartment side) of the fusebox? If so, I guess pulling the fusebox out and go from there. If it is blowing without the brakes applied or turn signals on or the hazard lights on, thats another thing to check. Did you unhook the wiring the connector to the upper column? If everything else checks out, follow the brown wire after it comes through the firewall into the back of the fusebox.

[ray_mcavoy]

The factory 77 wiring diagram shows the 20A tail light fuse powering an 18ga orange wire that runs directly from the fuse box to the headlight switch. At the headlight switch, it shares a terminal with a second 18ga orange wire that runs to an under-dash courtesy light.

If the fuse is blowing with the headlight switch off, I would suspect a short somewhere in that orange wire. Either in the fuse box to switch branch. Or in the switch to courtesy light branch.

When you turn the headlight switch on, it feeds power from the orange wire to the brown wire that leads to the front & rear light harnesses. It also feeds power (through the rheostat part of the switch) to the the "panel lights" fuse and then on to the instrument panel lights. And finally, if your truck has cab lights, the headlight switch feeds power from the orange wire to them.

[79Betty]

I tinkered some more with it. Think the wire going to the light in the ac controls were grounded straight to black assembly... Only thing i can think of. The cargo light and door jamb are all disabled (unpluged) and there is no under dash lights. I know it's not outside by it blowing the fuse and the rear assembly at te firewall undone. Did the same for front and blew it. I undid the cluster, steering column, and all that with same results... Could this be it? After i get it i'ma wire in the cab lights and cargo light.