Here is my senario: I have a 74 F250, all front lights and side markers work fine, rear lights and turn signals don't work. The only exception on the rear is the right side lights up when the headlights are on, but turn signals still don't flash.

I recently had the box off and I rebuilt the rear wiring harness because the old one was brittle and coming apart. However the rear lights and turn signals did work before I rebuilt it. I spliced the ends of the stock plug connectors onto a little bit larger guage wire and soldered all joints. It was only 4 wires in the harness and seemed an easy splice. I'm assuming I did something wrong however, but can't figure out what.

Did using a slightly larger guage wire have anything to do with the rear not working? Any ideas?

 

Larger wire would not make a difference. What I would look for first is grounding problems, especially if you just painted the frame.

The running lights circuit will be easiest to trouble shoot first. It was originally probably a brown wire, and it ran to both taillights and also the license plate lights. You should have 12 volts on this wire when the headlight switch is pulled into either position. Make sure if you are using a testlight or voltmeter, that you put the negative of the testlight or voltmeter on the bed sheetmetal. If you get nothing, move the negative of the tester to a clean place on the frame. If you get voltage with it on the frame, you know the bed is not grounded properly.

 

Thanks for the grounding suggestion. When I had the box off I did paint the frame with POR-15, so not having a good ground from the frame to the box would make sense. Will try your test and let you know what happens.

 

I attached a ground wire from the bed to the frame, which made the L-side running light work, but didn't help the blinkers. Here is where it stands: all front lights, turn signals, side markers work fine...side markers at rear work on both L&R sides, running lights work on R-side tailight only...blinkers do not work on L&R rear...nothing works at L tailight.

Would a bad turn signal switch cause just the rear signals to not work? I'm not sure where to go next. Any other suggestions?

 

I would keep going on the running lights in the rear. Try to find out why the left rear running light will not work. You may have a grounding problem up in the light itself for the left rear.

Why I am recommending you keep going on the running light problem, is it's a simple circuit with one common wire that runs everything. You know you have power back there, because the right light works. There is a chance that if you get the left running light working, maybe part of the blinkers/brake circuit will start working.

The rear turn/brake lights are a little bit more complicated. You have a left brake/turn wire and a right brake/turn wire. As you can see from my description, these two wires and their filaments in the bulbs do double duty as the brake and the turnsignals.

Once you get the running lights fixed, see what happens when you hit the brake pedal with the turnsignal in the center position. We can go from there depending on the results. Yes, it could be the turnsignal switch that is causing the turn/brake lights not to work, but I am suspicous you have a ground problem in the dead left light assembly.

 

I have connected a ground wire from the box to the frame, sanding to bare metal at each spot, but no change in lights. The L-rear lights are still completely dead, turn signals don't work on either rear side, but running lights work on the side markers and the R-rear. I also connected a ground from the frame to the ground spot behind the L-rear light box, but no change with that either.

Not sure what else to check to help determine where the problem lies.

 

Get a testlight or voltmeter, and take the left rear light apart and probe the wires for voltage. You are losing it somewhere. If the light socket has no voltage at all on the brown running light circuit, you may have to skin the insulation back on the wiring and work your way down to the splice where the license plate and the right rear light tie in.

 

Using a testlight, I tested the wires coming from the TS switch and had power for both rear lights. When I tested where the rear wiring harness plugs into the front in the engine bay near the firewall, I got a flashing light for the R-rear, but nothing for the L-rear.

Then I tested all of the sockets in the tailights. I got some more to work as I wiggled the lights in their sockets. I tried to clean off some of the rust in the sockets and bulbs, then put some dielectric grease on both. After doing that, I got the running lights to work on both L&R rear, and the R-rear turn signal to work. However, L turn signal still doesn't work, nor do brake lights.

Since the truck sat for the winter while I had the engine out of it for some work, do you think the sockets got corroded and are the cause of most of my problems? The more I look at the wiring harness that runs from L to R rear lights, the more I think it is too corroded and worn out to work properly. I'm thinking of pulling one from a truck in the junk yard, but wonder if it would make much difference. I'm also thinking of just re-creating a new one and buying new sockets at Advance Auto.

Any additional thoughts on what I should try next?

 

You had flashing for both left and right? Or solid power when you hit the brake pedal? I would concentrate first on leaving the turnsignal switch in the center position, and having someone push on the brake pedal to test the circuits.



This is telling us you may have a problem up front? You may want to double check that again. Use the brake pedal test, since some flashers get flakey if they do not have a bulb load on them.


This is good. You are getting somewhere. But remember the brake light wire/bulb and the turnsignal wire/bulb are the same thing from the turnsignal switch back. So it is significant to me that your right rear turn works, but the right rear brake does not? That is a sign of the turnsignal switch not working. I would double check this again too, since turnsignal switches are not cheap.

Yes I do believe a lot of your problems are caused by corroded sockets. Your truck sat for a year, and look what happened. I would not fool with a junkyard harness myself. It probably sat way longer than a year. I personally like the do it yourself route. You can buy the different color wires in the store, and also buy a cheap soldering iron, and the new sockets. Solder all your connections, insulate or use shrink wrap, and then get some cheap electrical tape and wrap the whole thing, and it will look good and last a long time. Those old rusty sockets are going to be nothing but trouble from here on out.

 

I think I'm going to rebuild the wiring harness this week with new sockets and try the tests again. When I checked the wires at the TS switch connection, they both blinked. Given that, I assumed the switch is ok.

I will leave the TS switch in neutral and work on the brake lights first. When testing for power, should I be checking the brown wires for solid power? I have checked those when running lights are on and have solid power.

So in the rear light clusters, there are two bulbs. The top bulb runs the running lights and flashers? Does the bottom bulb run the brake lights? If so, I have not been able to get those working on either side. Can't remember what color wires those were, but that is probably what I should work on first.

I'm hoping the new wiring harness will correct everything. If not, I'll probably post again next weekend on what is or is not working. Thanks for all of the help so far.

 

This is correct. The brown is the running lights and it should have solid power with the headlight switch on.



One of the bulbs is the back-up lights. It should be in front of a clear spot in the lense(I am not too familar with your year truck).

The other bulb is two bulbs in one. Pull it out and look at it closely. You should be able to see the two filaments inside the bulb. The bulb will have two spots on it on the bottom of the bulb. One goes to one filament, the the other feeds the other filament. The return or ground for both filaments is tied to the brass part of the bulb.

So the top bulbs have three connections. 1. brake/turn 2. running light 3. ground. The brake/turn filament is brighter than the running light, so make sure you get this correct when you are hooking everything up. You can stick the new socket on the battery under the hood and figure out which wire is the brighter filament.

 

UPDATE...I'm getting closer, but the L-rear still doesn't work completely. I just finished rebuilding the wiring harness that runs both tailights, and I got most lights to work, except the L-rear flasher/brake light. The sockets were pretty corroded, as well as the some of the wires under the insulation. The bulb works when running lights are on.

I think I have the problem narrowed down a little more. Testing with a test light at the wiring harness for the turn signal, the left flashers wire has power when the flasher is on (yellow/blk stripe wire). But when I test the same wire on the other side of the firewall in the engine bay, there is no power. I'm losing the power somewhere between those two points.

It seems to be a mass of wires, some of which go into the fuse panel, then thru the firewall to the engine area, but I can't track down what color wire it turns into, or where it goes. Do you have any suggestions what I should be looking for, or what could cause a problem in that area?

 

I do not have a schematic of a 74 truck. I looked at 74 Ford cars, and their left rear brake/turn was lightgreen/orange stripe wire. A 76 truck was lightblue/black stripe, turning into green somewhere after the turnsignal switch.

For and experiment just to make double sure you are on the right track, hook a piece of wire temporary to the good wire at the turnsignal switch, run it out the door, and hook it up to the correct wire at the taillight and see if it works.

 

I finally got the rear brake light problem fixed late yesterday. I went to do as you suggested and hook a wire from the turn signal switch plug to the rear lights, and found the connection within the turn signal plug to be broken. The L-rear blinker wire fell out of the plug as I was trying to unplug it from the rest of the harness.

Anyway, that 12 or 14 wire plug for the TS switch seemed to be the bad spot. The rear wiring harness needed to be rebuilt anyway, so I'm glad I also took the time to do that as well.

Thanks for all of the help!