I have a 68 F100 Flareside 8' Longbed that I'm working on. I have already taken the bed apart and reworked it and is now in primer and have relocated my fuel tank from the cab to behind the rear end. Before I start taking the cab apart I wanted to make sure everything works. The lights, heater controls, fan, switches, radio and gauges. Now here is what I knew that worked, Head lights low/high beams, Dome light, Amp and gas gauges and thats it. After working and going over the electrical diagrams for 2 days, I throught I had everything fixed. All the fuses are good, the heater controls now work including the fan. I removed the radio. I replaced five bulbs, new tail lights for the back, a new light switch and replaced both flasher units. All the wiring checked good for continuity and checked all the switches. They were good too. Everything checked good. Now here it comes. I started the truck. No problems. This truck has always started. I pulled the light switch on and all I got was the instrument lights, which I didn't have before, the head lights which I had before and the front parking lights that I didn't have before. That's it. No tail lights, no heater control panel light. No brake warning light. When I tried the turn signals, the indicators lights worked for right and left and the parking lights flashed according to the side indicated and both tail lights flashed at the same time regardless of the direction. The following still does not work. Rear tail lights, emergency flashers, Brake light switch, and the back-up lamp switch. Like I said earlier. all the wiring, switches, fuses, bulbs, all check good. I completely removed the rear lights wiring and rechecked it and the results was the same, good for continuity including the new tail light assemblies. I don't know what to do now, Recheck everything? Have I missed something? I need all the help that I can get on this one guys. Thanks in advance.

 

My first guess is that you replaced a good part with new junk. Did you keep the old switches and flashers? Try putting them back in one-by-one to see what happens.

 

Before I replaced the light switch, the only things I had was the Head Lights low/high beams, Dome Lamp, Gas and Amp gauges and that's it. The Flashers that I replaced, the case on the Turn Signal flasher was broken off and the Emergency flasher was dented in against the insides. I had no chose but to replace them. When I replaced the switch, I got the Instrument Panel Lights and Front Parking Lamps, Turn Signal Lamps. The front Turn Signals work on both sides and the back Turn Signals both flash at the same time. And that's all I got. No Rear Lights, no Brake Warning Light, no Back Up Lights and no Stop Lights. Like I said before the fuses, bulbs, wiring, Back Up Lamp switch, Stop Lamp switch, Rear Tail Light assemblies all check good. The Heater fan switch was unplugged. But works now. I just don't know what it could be.

 

I don't have the time to dig into the schematics right now, but it sounds like you've got something crossed up in the rear circuit. Your turn signal circuits are somehow feeding what should be the hazard circuit on the rear branch. And the other circuits are dead because they're hooked to the wrong place. My guess is that you've got a connector where the pins have been plugged into the wrong places, or you've got a mismatch of harness sections.

The only solution is to test back through the harness to see where the voltage is correct, and trace how it should be on the schematic.

 

Thanks xstrange. I have been thinking the pass couples of days about this problem. Going as far as drawing out each circuit separately in order to get a better view of how they should be and connected to. You and I both are thinking along the same lines. There is a mismatch somewhere. I bought this truck from the son of the original owner at an estate sale. It even came with a good title that was in his mother and father's name. I know where they bought it, how much they paid for it, date built, date sold and it had 16.3 miles on it when they drove it off the lot. It was a rare find. I'm off Monday and Tuesday and I will start checking everything out according to how both of us are thinking and post the results. Thanks for your help.