So... I've read several posts about this and then watched several videos on the same, but none appear to have a solution for the problem. I thought I'd post it again to the group in hopes that one of you have both experienced the issue and have a 'definitive solution'. So, here goes...

My '90 Ford F250's running lights do not come on when the headlight switch is moved to either the first nor the second position. Instead, running lights ONLY come on when I've pulled the switch past first position (doing so VERY SLOWLY) to a "mid-position"; the same can be said of the dash-light.

I thought it might be my truck camper, so I unplugged it: No joy.
I thought I might be the new(er) harness to the truck camper, so I unplugged it: No joy.
I thought it might be the headlight switch, but I'm now on my second new switch: No joy.
I thought it might be the Molex, but the switches exhibit the same behavior on either the old OR the new Molex: Again, No joy.
I thought it may be an errant cross-wire, ground, etc... so I replaced the portion of the harness to the Molex and then made sure the appropriate ground: Again... No joy.
I thought it may be because of a power issue to the switch - ya, a long shot, nevertheless... I took JPR's advice and put in a relay on the switch: No joy.

Current situation:
Headlights - work (both high and low)
Running lights - don't work in either switch position but "do work" on a "mid-position"; same with dash-lights.

My current setup is:
New switch
New Molex
New relay
Definitive ground
No camper connection nor OTS harness.

Honestly...? I'm at a bit of a 'wtf moment' and am not sure where to even begin troubleshooting given what's already been tried. I'm hoping one of you "nearly OBS Ford omnipotent" types out there (you know , the ones we used to call 'Master Tech' and other cert-related things) has seen this before, knows the solution OR can at the very least point me in a direction that may lead to my resolving this problem.

Thx in advance,

-Thomas

 

I was thinking it sounded like you have a wiring error somewhere and then I saw you picture. What is up with all the splices and what is the purple/blue thing?

 

Purple box is a temporary power tie-in for relay post #30. The splices are part of my troubleshooting as I thought I may had crossed relays #86/#87 (but I hadn’t).

The problem both predates and is post the relay insertion. That’s not the issue.

 

I plugged in my camper top because it’s easier to see its running lights. Not the best video, but one can see/hear the problem:

Switch position #1, nothing, nada, zilch

Switch position #2, headlights (as designed) but no running lights.

Switch position #1.25-ish, running lights.

Note: High-beam floor switch and lights work as designed.

<scratching my head…>

 

We need a wiring diagram for the lighting circuits.
Seems to me the wiring for the running lights is messed up somewhere. Are there any wiring splices at either corner behind the lights? It would seem the running lights only work when the other lights are open circuit, that suggests that the running lights are seeing 2 hots or 2 grounds when the switches are in the normal positions. What you could do it pull all the bulbs out of the sockets and reinsert them 1 at a time to see if the problem can be isolated to one of them. If you find that then you have to trace those wires to see if there was a repair where the wires were accidentally reversed.

 

Test the main lamp switch using a meter and a test lamp. You may have a new one, but that does not mean it is a good one.

 

I see a problem with your switch wiring.
You need to make sure the correct wires are connected to the correct terminals.

Take a better shot of the wires and the molex plug., may be I'm colour blind, but I think you have swapped wires.

 

Problem solved.

After more research I happened upon another Forum wherein a 1992 F250 Diesel owner had the identical issue; literally identical. Following that rabbit hole of a discussion to its conclusion I found an entry by a 90’s Ford service guru who stated (in response to the longer thread):

“…So it sounds like when you pulled the **** a little past the running lights detent, the moving contacts were touching both the headlights and running lights contacts and sending headlight power to the running lights. Interesting. I've never seen that before, but I've never tried it either.”

This got me thinking: If that could occur, by definition, the ground is solid and there’s something not correct on the power-side of the circuit, otherwise the running lights would never have lit in position “1.25-ish”. A bit more investigation led me back to the basics of troubleshooting. Check the obvious first: The fuse.

However, fuse #5 looked great. Nevertheless, I switched it out for a new one just to be sure. No joy.

Then I got to thinking that perhaps I’m looking at the wrong fuse? Checking the diagram, I found that #4 said ‘exterior lights’ which sounded to me more like the light above the rear window. I pulled and checked it anyway. Looked good. No burn, looked connected, so I put it back in. No joy.

Much gnashing of teeth...

“Screw it!” I thought, and simply swapped in all new fuses across the board - just to be sure, right? And… Voilà!! Running lights came on at the first position (detent) and STAYED on when moving on the second position with the headlights. Everything lit up as intended. All other lights work as well. I then plugged in my truck camper and it lit up like a Christmas tree.

Happy with the outcome but unsatisfied with an understanding of the “solution” I decided to continuity test the fuses. Wouldn’t you know it, the fuse pulled from position #4 (aka “exterior lights”) looked perfectly fine but didn’t show continuity. It was a dead fuse. That was the problem all along: I was looking to the wrong fuse AND the fuse in question wasn’t any good. (Start rant and extended string of expletives here…)

Net-Net: All the wiring is correct and connections are solid. Everything is put back together and we’re running on all new fuses (with a bristling box of spares in the glove box). I’m heading out to a BSA Camp Friday with my son and the confidence of running lights on Big Blue and my Alaskan Camper.

If you're Brick is experiencing a similar issue, check fuse #4. It may very well be the culprit...

 

Glad you found the root cause and reported back with your solution.

 

It looks like where the BK/PK wire is on the schematic, you have a Brown/White.
Maybe its just the picture.

 

Yessir, that's just my rather inept photography skills...