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ray_mcavoy · 12-29-2013, 08:29 PM

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.

12-29-2013, 08:29 PM	#3
ray_mcavoy Registered User Join Date: Nov 2009 Location: Sherman, ME Posts: 2,358	Re: Back End Fuse Problems! 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.