Hi guys, I have searched this forum to no end to try to resolve my electrical problem. I have a 1997 F- superduty, with a rear running light and left turn signal problem. The rear running lights do not work at all, however the fronts work fine, not a single glitch there. I have traced the wire bundle from the cab back and have not found any ground outs, or spots where rodents have ate through.I have pulled the headlight switch for the burnout problem, fine there too.
Now for the left turn signal, up front, once again, works fine, out back with LED tailights, it only lights up 2 leds out of 14, very dimly at that. The turn signal switch is finicky on the left hand turn, but it works fine up front. Right turn works, stop works fine on both lights, but no marker lights. I have googled for a wiring diagram , and have yet to find one for this truck. So what is the problem, bad turn signal switch? I am baffled by all of the lights working on the cab forward, and very few working on the rear end. This truck is the shops flatbed, and cannot be driven without marker lights and especially a left turn out. Any help is appreciated.
Thanks,
Dirk

 

Get your self a test light.

Brown is Running lights
Black is backup
green and orange are left and right.(cant remember which)

As for your LED tail lights, sounds like you have diodes burned out

 

The leds are brand new, and work fine for stop. I have checked this whole truck with a test light. The test light gives a faint light on the left turn, also , just seems like it is not getting full juice? A test light doesn't help much up front a with a maze of wires.

 

Unplug the tail light harness from the main harness, do you have full lighting at the main harness at that point?

How many times has the tail light harness on it been hacked into for this trailer and that trailer etc etc, previous owner/s didn't buy T-harness to do it?

That sort of thing happens to trucks all the time, did you get under there and look it over real close? Might be buried under layers of black tape and dirt, might be hard to spot right off.

Guys cut into tail light harness tap in lighting but use nothing but black tape to protect it, weather and road salt takes it toll tend cause issues such as you describe.

Only way you're gonna find it is trace it down as Brad suggest using that color code no way around it, test forward till you find juice then back track until you find the problem.

If the front lights work its not a fuse.

 

Where does the tail harness unplug? Underneath the cab?

 

Main harness ends at last frame cross member, just forward of the bumper just inboard of driver side frame rail.

The tail light harness it tees off, goes to each tail light (feeds license plate lights too) just after the connector at that point.

Unplug it test for turn/stop/tail verify you have it there at that point.... or not, based on what you find decide what need check next.