I have a 1976 F150 Ranger shortbox that I'm having lighting problems with. Blinkers work when headlights and marker lights are off but faint blink on opposite side. When headlights or marker lights are on no blinkers and the dash indicators are always on till you use the blinker switch. I've gone through and put new sockets on tailights and front markers and some of the side markers. All lights come on as marker lights. Brake lights work also. Any suggestions?

 

Not sure, but from the sound of it my first thought would be to clean and fix (if necessary) the light grounding points on the core support. At least I think that's where they are on your truck.
Both for the headlights AND for the turn signal sockets. Follow any ground wires from the lights and clean them where they meet sheet metal.

Also, does your battery have a dedicated body ground? They did from the factory, but after all these years many of them have been removed, with the thinking that the main battery cable to the engine is the only important one. If you don't have one, add one of your own from the negative side of the battery over to the fender skirt/apron, or fender bolt. Also add one from there to the core support too, just for giggles. After all these years, wear and tear in the form of rust and paint can get in the way of electrons moving about the cabin like they should.

Check to make sure the ground strap from the back of the engine to the firewall is still intact as well. If not, here again is a good place to make another one of your own.
For the grounds you can use any old (preferrably Black though) wire you have, or one of those store bought braided tinned copper straps made just for this purpose with eyelets at both ends.
For the battery to body ground I like to use heavy-ish 10ga wire, as well as the one from the engine to firewall. The braided ground straps are the equivalent of something even larger, like 8ga I think. Something like 12ga or even 14ga would be suitable for the core support, but I would still use 10ga if I had it. A nice roll of Black 10ga wire should not cost that much even with post-Covid pricing being what it is. Could be wrong on the gauge sizes, but the point is you would have a hard time ever coming up with overkill for grounds.
I like to say that when it comes to grounds, overkill is just enough. Especially if your truck has seen a lot of wet and/or salty winters in it's lifetime.

Hope I'm not sending you on a wild goose chase looking for a bad ground, but any time you have electrical issues (especially the strange ones) then a good first step is always to check and add grounds.

Good luck.

paul

 

Right on what Paul said, sounds like a ground issue and how to fix it.

Only thing I would add to it is a ground from block to frame and frame to bed out back. Don't trust body / bed bolts to make good grounds.
Dave ----

 

Thanks for those ideas! I will try to do that soon.