Another blinker problem.
I've been getting my '78 F250 ready to give to my son, so in the past couple of weeks I've removed or replaced a great many things.

The latest problem is the blinker, brake & headlight issue.
I followed the troubleshooting guide and that helped alot.
After removing all trailer wiring, a couple of bad bulbs, bad front sockets and bad turn signal switch, everything was working fine.

Now, the blinkers (headlights too?) only work when they want to.
Yesterday I removed the dash cluster and when I did that I double checked the headlights to make sure I hadn't turned them on when pulling the ****/shaft out (they were off).
I unplugged the elec plugs that are located in the engine bay that go to the front right & left blinkers and the plug to the rear blinkers. I have power (12v) going thru the turn flasher into the blinker switch. With the signal harness unplugged under the column, I checked the blue to the y-blk, the grn, the w-blue, etc and the switch is good.
I plugged the connector together and when I engage the left or right blinker the flasher makes no sound and the voltage drops to apprx 5v.
The brakes and emerg flasher work fine.
I pulled the blinker fuse and ohmed between ground and the left then right signal wires. When the left or right blinker is selected, the wires short to ground.

With the gauge cluster out, I removed all of the elec tape from the harness and hand checked every blinker wire (all good).
While checking for shorts with the meter again, I noticed that the problem disappered. I plug the front lights back in and everything is working fine.
And Now.....the headlights are on.?.?.?

 

So were are you at now? I was wondering when I was reading your description how the headlights were off. If I remember correctly, you have to pull the headlight switch **** to the on position, and then hit the little button to release the **** and shaft.

If you have a short to ground, you should have seen some smoke or a blown fuse. You have neither, so if you have 5 volts coming out of the flasher, I would guess you have a bad flasher or a bad connection at the flasher or upstream from the flasher. You may have had a bad connection at the fuse, and taking it out and putting it back in fixed it.

 

With the left blinker on I grabbed and wiggled the harness in one location then continued thru the whole harness. I then did the same with the right blinker on. The problem didn't return.

Somewhere during this I thought I felt a warm wire coming from the fuse box but no matter what I did I could not duplicate it again. I then had my son get in the truck and had him do the same test and also start using the brakes, turn the wheel, have the headlights on.....everything that I could think of. No problem found.

The gauge cluster is going back in and the testing will continue.......

 

I found a wire that comes from the light switch and goes over to the radio, probably the radio interior lights 12v line. Anyway, it was rubbing on the radio mounting frame of the dash. That has now been fixed.

The gauges are back in and the truck is on the road with no known elec problems. Ha, I take that back.....I have to find out why the tank selector solinoid isn't getting power. But hey, that's another story for another day.

 

If you would like to wire the radio illumination a little better, you can look in the original radio area and try to find a blue/red wire. This is the illumination wire that feeds all the lights in the dash, and it also fed the original radio illumination. If you use this wire, the light in the radio will dim with the dimmer on the headlight switch with the rest of the guage lights.