I have a 74 ford f100 ranger with a c6 automatic transmission. I bough the truck from a buddy who was in the process of rebuilding the engine before it got sidelined 15 years ago. I’m having an issue with the front headlights, turn signals and side marking lights working. The rear side marking lights work but the drivers side brake light glows very dim and the passenger side doesn’t work. The turn signal on the drivers side rear works but the passenger side doesn’t. If I pull the hazard switch my front turn signals go out and my rear drivers side turns on. I’m so lost I’ve been up and down the truck looking for broken wires I’ve found some spots that I’ve repaired but still nothing.

 

Not sure where exactly to look yet. The symptoms are seemingly all over the place. But a good place to start, whenever something is wonky with wiring function, would be checking, fixing, or simply adding new grounding connections.
Starting off, make sure the battery negative is connected securely to the engine block. If the cable, or either of it’s ends look old or compromised, just jump in and replace them.
Make sure hat there is an additional ground wire of at least 10 gauge connecting the battery to the body.
There should also be one from the back of the engine to the firewall.

These aren’t primarily for the lights only, but they pretty much do help everything electrical.
Then make sure the front radiator core support is still well grounded to the fenders, because that’s likely where headlights, front turn signals, and maybe even side marker lights get their ground connection.
When these trucks were new, the body panels had good contact with each other. But fifty years of real world living with rust, paint, and even the light corrosion that builds up on metal may have led to at least some of the problems you’ve been experiencing.

For the rear, make sure that you have a separate grounding strap from the frame to the pickup bed. There’s not much grounded to the bed, but that might enhance the tail light connection in case there is a fault there.
And as an aside to that, you might as well add a small grounding strap say, for example, from the engine block where the battery cable meets, down to the frame in the front.
The frame isn’t used for much grounding either, but adding this might help, even things out overall.
Happy grounds lead to happy owners!

And of course, don’t forget to start with the basics. Pull each lightbulb out and make sure that both filaments work on the turn signals and running lights. Look inside and make sure the sockets are clean, straight, and free of rust. If they are, add some di-electric grease to the bulb and socket housings to keep moisture and future rust at bay.
If they’re not rusty, they’ve been leading a charmed life!

 

Thank you, that’s gonna be my next step of adding a whole bunch of grounds. My buddy was saying fords are notorious for grounding issues.

 

Agreed with 1Ton. I would clean each ground near the lights and check all the bulb sockets for corrosion, clean or replace those as needed and look at the bulbs, if there are any signs of soot/burning/smoke in the bulb replace it whether it is still working or not. Also clean the primary engine and battery ground straps.

After checking all that, if still not fixed, I would unplug the t/s switch at the bottom of the steering column, grab my ohm meter and a wiring diagram and test the functionality of the switch to make sure it functions as intended and can pass electricity when it should and doesn't when it shouldn't. Since I am under the dash, I would unplug the headlight switch and do the same and be sure to clean any corrosion. Bonus points if you use some dielectric grease on all the bulb sockets and electrical connectors when you put things back together.

If everything tests good there, time to test the wires themselves, which this can be a fun chore since testing the taillights is going to require some big jumpers to reach from dash wire origination point > multimeter > wire end point. Grabbing some wire probes that pierce the insulation and they plug directly into the leads from your multimeter can be helpful.

Good luck, electrical is tedious when things aren't working as they're supposed to.

 

Thank you. So all bulbs on the truck are new. I put dielectric grease in all connectors before installing and cleaned any rust or corrosion on connectors before installing bulbs. The headlight switch is brand new and I’m still having issues. One thing I found out yesterday is I don’t have running lights on the rear unless I pull the hazard switch. When I pull the hazard switch I lose turn signals on the front but get running lights on the rear. Makes me think either the turn signal switch is bad or something is wired wrong from the previous owner

 

Could be bad t/s or wiring. Since you have already swapped the headlight switch that is presumably good, but I would still test it, the t/s switch, and though I doubt the brake light switch would have any impact on this you can test that too. For the t/s switch you will need your wiring diagram, and determine input vs output wires for the wires going into the switch. Grab your ohm meter, when you turn a specific function, say left turn, the in and out wire for left turn should show 0 to almost 0 ohm. Disengage the turn signal and that resistance should go back to infinite resistance. Test left, right and hazard functions. Do the same for brake light switch, button pushed in, infinite resistance, button released 0. Do the same thing for headlight switch. While testing the switches move the lever/rod around to simulate vibrations that those switches experience bouncing down the road and just from the engine running. It may test okay in a static state, but when there are vibrations that could change. Might not be a bad idea to plug some jumpers in where the flashers are connected for hazards and t/s to take them out of the loop during testing.

If all the switches have good test results, move on to testing each of the wires and make sure all of them show 0 or close to zero resistance, if there's anything higher there is likely a break in the wire. Having a helper to move around wires at the connectors can help determine if it is an intermittent issue.

All this testing only requires time and you get to learn a lot about how power moves through the system. Much better than just tossing parts at it crossing your fingers, and you would probably be surprised how often a part gets replaced and simply in the process of replacing it you end up cleaning a connection that was the entire issue to begin with, but since it started working after the t/s switch was swapped it is assumed the switch fixed it, when in fact it was two or three bullet terminals in the t/s switch connector that got a better connection simply due to unplugging it, scraping corrosion off in that process then plugging in a new terminal and making a secure connection.

 

One more thing to add, after you have confirmed all wires are good and there are no breaks which may cause short to ground, you can then test grounds and sockets by finding a good body/frame ground with your multimeter, then probing the positive lead going into any given bulb socket. There will be some resistance in this case since the load will be included, but what you're looking for here will be if you see really high or infinite resistance, that load is not getting a good ground. Before you start, test the resistance of the bulb itself across its terminals, then plug in and test resistance again. If it is close to the bulbs resistance, you're good, if it is higher than that, you either have an issue with the socket or the ground. To test the socket, test resistance of the ground with the positive input(s), if close to zero, good, if higher, replace socket. If socket tests good, the problem is on the ground side. Should be a short wire with a ring terminal screwed to body nearby, remove screw, wire brush screw and ring terminal, scuff up contact area the ring terminal makes contact with, and put it all back together.

I know this sounds like a lot and this is the primary reason people don't like dealing with electrical stuff, but I rather enjoy the troubleshooting side of this even though it is tedious and once you've done it a few times it becomes much easier to wrap your head around what you're doing. I am also that guy that enjoys sitting for a few hours soldering 20 some odd wires, twice, to add extensions to the factory wiring harness to fit taller handlebars on a full-dress Harley so perhaps I am just odd...

 

Glad to know I’m not alone! :-)
And speaking of buggered wires… Trace along the driver side frame rail all the wires that go back to the tail section.
Look for anywhere they might have come close to the exhaust pipes of a custom exhaust, or been pinched by some sort of accessory such as a custom trailer hitch, etc.

 

Plastic bulb sockets need double checking, had to replace all 4 on my 78. Some replacements are not color coded correctly.

 

I did not read every line in every post but I kept seeing check grounds at the lights.
If the main grounds at the battery are not good the rest will be bad too.

Battery> motor> frame.
Motor (intake bolt)> fire wall (cab ground) with a 10 gauge wire.
I would also run a 10 gauge wire from frame to the bed so it is grounded.
That is how the factory did it for the 80's trucks and I have not had any light issues in 5+ years the truck has been on the road.
Dave ----

 

Thank you all very much. I went through and added a whole bunch of new grounds. I bought a test light and was able to figure out the bulb sockets were bad on the back. I replaced both tail light sockets and I got all lights working.