1994 E350 7.3 Uhaul Van

Issue:REAR Running Lights are not working.

I had this issue "fixed" last week.

Some useful info: (before it was "fixed")
The headlight/running light switch/**** has always given me problems. But I thought it was just the inside cab dimmer light that was a problem, so I ignored it. It wouldn't always light up. Had to play with it to find the sweet spot. No big deal I thought.

Then one night the corner lights, including rear running lights stayed on. I had to disconnect the batteries since I couldn't get them to turn off. The problem went away on its own so I figured id leave it alone.

I Got pulled over buy a police officer due to the REAR running lights being off. The next day i took it to get fixed.

The shop said it was the switch/****. They replaced it and showed me the old one. It was definitely damaged. After they replaced the switch, everything was fixed. The dimmer is perfect now. But the running lights stopped working a week later. The lights work, they are LED lights that Uhaul installed so its not the actual lights. It's not getting power to the lights.

Last night I did some research and found the fuse box diagram on the engine side of the truck, not the fuse box under the driver side dash. The running lights are controlled by the engine side fuse box.

I matched the diagram and behold, there is a 10amp fuse missing on the engine side fuse box. Which is weird because if its missing, how is it possible that the running lights were working at all?

Anyways, i go get a 10amp fuse, plug it and it caused the yellow front lights to stay on. I couldnt find a way to shut it off so I removed the 10amp fuse.
That 10amp fuse also didnt get the rear running lights going.

Any suggestions? Very confused here.

Below is the fuse that was missing, and remains missing since it activated the running lights and didn't allow me to turn them off.

 

LED lights can do funky things on older vehicles. My experience has been the typical finding the right flasher that'll work and also issues with the LED taillights and backup lights randomly being lit dimly while everything is off. One solution to this is to isntall a resistor in the LED circuit, as many issues are from the LED bulbs burning only a fraction of the pixies as incandescent.

Either way, when they won't light check voltage at the taillight socket. If none, jiggle the harness along the drivers frame rail and see if you can find a short or break. Check the plug at the back left under the bed, could be loose of corroded. Check for wiring abortions from PO wiring trailers. That is in my experience the #1 cause of intermittent tail/brake/turn lights. Everyone loves to use those junky splices that pierce the insulation and then the factory wiring corrodes. Either do a proper crimp or solder joint and seal it or if unadultered just use one of those plugs that goes between the harness plugs at the rear if you only need a 4 flat.

 

Wait. Look at the trailer tail light(running light) relay. It's located under the hood, driver's side fender well, near the cab.
It's a black/gray box hooked up to a 4/5 wire pigtail.

It has battery power on one side, lights on the other, and tail lights(from the main switch) to drive it.
If it's corroded, power could be "leaking" across it.

 

Marcrob, I do remember my mechanics telling me that it was corroded and showing me. They "fixed" it but probably did it wrong. I think that's the problem. I'm going to take a look. But then, what do i do? I don't remember it having any labels to tell. Me which pin went where. Is there a diagram somewhere? What would I search for? 7.3 idi trailer light relay? On my phone at the moment, on a bus. But I'll take a look when I get home.

 

You need a multimeter.
You can then take all the wires loose and find:
1. ground - want to keep this one separate.
2. Power(heavier gauge, will be powered at all times. Probably from your fuse)
3. Tail lights - only powered with the switch on, unpowered with it off.
4. Output - heavier gauge, brown I think, and if you connect it to B+ with a jumper, the extra running lights should turn on.

At this point, you can then wire up /any/ relay to it.
Ground and tail lights go to the two "coil" wires on it - either way works.
Power goes to the Normally Open contact
Output goes to the Common contact.
(Note - these could be either way as well, but if you do it this way the Normally Closed contact won't have power with the relay /off/)