Hello all! I am new to your site but have been following for some time.

I have a 68' F100 that I am having some issues with. When I turn on my left turn signal, the front indicator lights up but does not blink and the rear does nothing. The right works perfectly normal. I have replaced the left rear socket with a new one and of course bulbs and still nothing.
The truck is original and no wiring has been altered.
The issue came on gradually. I would notice when sitting at a stop light, if I turned on my left signal the indicator on the dash would light up solid and flash about every 15-20seconds.

Any thoughts would be greatly appreciated and thank you in advance!

 

Have you check the flasher?

 

I have not as it still works for the right side? If it was bad or going bad wouldn't it effect both left and right or am I mistaken?

Thanks!

 

Bad grounding at frame, firewall, loose or corroded cables etc

 

I was afraid of that...
Best place to find wiring diagram?

Thanks!

 

https://fordification.com/tech/schematics_h.htm

 

Eh, don't really need a wiring diagram. Anything that connects to ground whether at frame, block, firewall, etc etc needs to be clean and tight. Disconnect whatever it is, and grind down to bright shiny bare metal. If battery and start cables are old replace them. Tighten securely and coat with grease or vaseline to keep corrosion away. Turn signals, brake lights, headlights &c have their own ground path through the body or fenders to frame, all the way back to the battery. Same thing applies. Block connectors through firewall, SAE connectors all turn green with corrosion on the pins and receptacles. A brass bristle brush, drill bit, and kerosene, WD40, lighter fluid, contact cleaner will clean these up. Best of all it doesn't cost anything except a little time. Put your battery on an external charger overnight and clean the cables up while you're at it, it's gonna get frosty soon enough.

 

I would start at the non-working light and work my way back to the turn sig switch checking for power along the way.


I also got to ask does the brake light work on that side? If not I would point more to the turn sig switch. (see below why)


I don't know the way your trucks wiring is run.
My 81 F100 has a rear light harness that plugs in to the rear wiring harness that run along the frame. If you have a plug there check for power there.
If not move forward, my 81 also has a plug at the bottom of the firewall/frame rail/engine bay area. This plug ties the rear frame harness into the main harness.
Look for power there.


If no power then check at the turn sig switch.
(below)The brake lights go thru the turn sig switch. When you turn left or right a wire goes to the front and another to the rear. Now if you hit the brakes the switch does not let the brake light come on on the side the switch is on.
So if both the brake & turn do not work for the same light it would be the switch.


Sometimes the contacts get dirty and sometimes can be fixed with electric spray cleaner and work the switch. If not replace the switch.


That is my story and I am sticking to it.
Dave ----