Headlight issue 1969 Fj40 (1 Viewer)

[FARMAN33]

I had a 100% functional electrical F40 three days ago. Today I replaced my front indicators for the round earlier style (preference only). Afterwards, signals worked and thought all was good. WRONG. Headlights now don't work. Checked fuse and it was blown. Replaced fuse switched the headlights on and nothing. Fuse blew again.

Thus begins the hunt. Took new indicators out of the loop in case something didn't want to play nice with headlights. No change (another fuse killed). Brightened up all the contacts in fuse block and squoze the holders so it gripped the fuse tighter. Same result. If I run 12V to the dimmer, I get headlights but no tail lights of instrumentation lights.

This tells me that there is a dead short somewhere between fuse block and dimmer (everything down from dimmer working with 12V applied) Fuse is fine until switch is pulled.

Does this sound correct? Any clues with the tail lights and instrumentation out? Missing something obvious?

 

[Coolerman]

It could be the head light switch itself, but my bet is its in the Park/Tail circuit.

To trouble shoot this:

Unplug the head light switch. Jumper across the Red and Red/White wires in the connector. That will turn the headlights on. If the fuse blows then it's a wire shorted somewhere in the headlight circuit.
Next jumper from the Red to the Green wire to activate the tail lights. Again if the fuse blows it's somewhere in the park/tail light circuit.
Jumper Red to Red/Black to activate the meter lights. If the fuse blows now you know its in the meter light circuit.

If no fuse blows after testing those three circuits, then time to take the switch apart to see where the short is occurring.

 

[FARMAN33]

@Coolerman Just the guy I wanted to see this!

So I got tired of blowing fuses and rigged a trouble light across the tail light fuse circuit. When I jump Red to Red/White, no trouble light. Red to Red/Black, same. Red to Green, trouble light. With the jumpers however, NOTHING lights up. I popped a fuse in the top headlight circuit just to verify. Same. Jumped all with no lights but only Red to Green blew the fuse.

Honestly thought it could only be a short in the headlight circuit seeing it only blows the top fuse. I see that the fuse block is bundled in threes though. I will see if anything jumps out at me in back of the 40. Thanks again. Will update!

 

[FARMAN33]

So! @Coolerman was right. Tail light circuit was the culprit. Under the dash (of course). Everything is running fine but the dimmer switch is also bad. After that is replaced I'm back to 100%!

 

[Coolerman]

Great! Really nice looking truck BTW. Love the pic of you upside down troubleshooting!

I should have mentioned about using a bulb across the fuse contacts. Doing that will only allow about 1.2 amps to flow, even if there is a dead short. Not enough to melt wires, but enough to verify that circuit is pulling current. Ideally you would pull the bulbs out of the circuit being tested so that they don't try to light giving a false reading.

You stated that you had a tail light fuse but it's actually a brake light fuse. The tail (Park) lights are on the same fuse as the headlights.