Hey guys, once again i have another problem and would love the expertise of you fellow FTE members. I have a 79 f-150 and ever since i've had this truck (over 3 years) the dome light in the cab and the hazards never worked. I know the cab light clicks on when the doors open and I got tired of searching in the dark at night trying to find something. So i dug into it, found a fuse panel diagram and found which one it was. Before i figured it was just a blown fuse but instead it's perfectly fine. I then unscrew the panel and come to find out that the wire leaving the fuse is snipped for some reason. Now my question is, does anyone have a single clue to where this wire might connect? I have my haynes book but i'm not exactly sure how to follow since it's somewhat confusing. Anyways, that's my classic truck issue for the day and thanks again in advance. You guys rock.

 

Have you checked that the headlight switch is not rotated all the way counter clockwise and locked on the dome light on?

Out of the fuse box, up the A piller and to the dome light. Along the way some where the door jam switches and headlight switch come into play. The PO might of has a problem with them and just cut the wire to shut off the dome light.

Dome light 101: For the dome light wire, hot-at-all-times power comes goes to the headlight switch as a GREEN with YELLOW stripe wire. Dome light power comes back out as a BLACK with BLUE stripe wire. The same color scheme is used at the door jamb switches. BLACK with BLUE stripe runs out to the actual dome light. BLUE with a RED stripe is instrument panel illumination. It comes out of the headlight switch to the fuse box, leaves the fuse box again as the same color and is distributed throughout the dash.

There is no ground connection to the dome light switches or the headlight switches. These switches switch power, not ground.
1) Make sure you have 12 volts on both sides of the courtesy lamp fuse. This circuit is hot at all times, so it doesn't make a difference what position the key is in. If you have power on only one side, replace the fuse. If you don't have power on either side, then there is other work to do first.

2) Pull the dome light switches out and remove them. Make sure the terminal for the GREEN with YELLOW stripe wire in the connector has power.

3) Check the dome light switches for continuity. With the plunger all the way out (switch CLOSED), the switch should be shorted. With the plunger pushed all the way in (switch OPEN), the switch should be open. If not, replace the switch.

4) With power to the switch proven to be good, and the switches proven to be good, the dome light should light up. If the dome light does not light up, remove the cover and check for power at the dome light (not the side that's permanently grounded to sheetmetal) with the door OPEN. If you have power here, yet the bulb does not light up, replace the bulb.

This will prove out the door-side; if you get to the point that the doors turn the light on but the headlight switch will not, then we can cross that bridge.

 

The strangest thing happened. So was checking the fuses with a voltmeter and i put the positive wire on one side with the ground, shows nothing. But then i put just the positive on the other side, not even a ground on, and the dome light lights up. The doors are open but im mind blown.

 

So here is the cut wire on the fuse panel. But there are 2 green/yellow wires coming off the same spot? But when i give the fuse power, the dome light comes to life. So is it like a circuit with the power to one side cut off? So doesn't the fuse panel get power in from one wire then distributes to different wires which then pass through the fuses? Kinda lost.

 

So now i know for a fact that both door switches and the headlight switch are good. I gave them power with it all on and it all works. Just gotta find out how to give it that constant power..

 

A fuse is just a bridge... the lil wire in the glass fuse is formulated to "smoke check" if there is a runaway current. Hence the fuse rating of 2A, 5A, or whatever.

The second contact you applied the voltmeter to somehow completed the circuit Therefore, the circuit is bad at the first contact.

Disconnect the battery and clean the contacts on both sides. A very small piece of emery cloth wrapped around a straight slot screwdriver usually does the trick. Verify in fact that the fuse is good by Ohming it out... it should read zero, right?

 

Its new, just put it in. Im out here now and its showing zero yes.

 

Oh i think i might know what it is. Maybe the connection from the fuse holder to the fuse is rough and by putting the needle in the right place, that completed the circuit. Even with the voltmeter off it works, so thats gotta be it. Let me test it.

Correction, thats exactly what you said HIO. My bad. Lol.

 

HOORAYYYY!!! The dome light finally works after almost 3 years!! Thanks alot guys! You rock!

 

Congrats, you did all the work, we just arm chair GB'd it.

 

Cool beans. Onward!