Left (Driver's side) trailer tail light doesn't work.

[jason.yergler]

My left trailer tail light doesn't work as a marker light or turn signal. Hazards and brake lights work. It doesn't work only when the left light is the only one functioning. I'm assuming the most likely issue is a fuse, but I couldn't figure out which fuse is the faulty one. Is there a dedicated fuse for those functions. I guess the next most likely issue is in the 7 pin connector or a faulty ground. Is this a common problem? Is it a fuse or am I going to have to follow that wire?

I haven't used a multimeter or test light to see if power is coming from the 7 pin, but it is the same with both of my trailers, including the new one I bought today, so I know the fault is in the truck. Thanks.

 

[04fxdwgi]

Without being able to actually see it, just spitballing here on things I have seen before.

Pull the wiring harness connector for 7 pin apart and check for corrosion. Also check the harness / 7 pin ground on the truck.

Pick up one of those 7 pin testers and see what is going on with trailer(s) isolated.

 

[crash68]

My left trailer tail light doesn't work as a marker light or turn signal. Hazards and brake lights work.

Not sure how the brake/hazard function can work but not the turn signal as those three functions are all the same bulb/wire. For the turn signal to not work but the brake light does, that would indicate most likely problem with the BCM. Each of the 7-pin trailer light circuits have their own fuse.
Attached is the wiring diagram for the Trailer Tow 7-pin

 

[BossHogg]

What crash68 said. All trailer circuits are controlled by the BCM through a fuse and relay if my memory is correct. It has been a while since I looked at a schematic, I no longer have.

If the right marker light works but the left marker light does not, the issue is on the trailer. It could be a wire, bulb, connection issue, or a combination of these.

If the left trailer light works as a turn signal light and a hazard light but not a brake light, this suggests all is well with the trailer, the 7-pin connection, the relay, and the circuit fuse; it is all the same physical circuit. The BCM commands the trailer lighting circuits, and since the BCM can command the brake light and the hazard light, it suggests an issue with the BCM's internals not being able to command a turn signal event. Do you get the DIC turn signal indicator? If you do, this simply means the BCM can detect and send a CAN message for a turn signal. This is a bizarre problem.

Check your owner's manual; there is a comprehensive listing of all fuses, amp ratings, and circuits. The underside of the under-hood fuse block cover will have the relay information.

 

[jason.yergler]

Thanks, guys. I'll double check to see if I'm wrong about the turn signal function. I know the brake and hazards work. I know the issue isn't the trailer, as I've connected three different trailers in the last week and they're all having the same issue.

I tried to find the fuse, but I couldn't find which one controls the left trailer marker light. I'll get into the seven pin and see if I can find any corrosion. I'll update this weekend when I have a chance to get into it.

 

[04fxdwgi]

Not sure how the brake/hazard function can work but not the turn signal as those three functions are all the same bulb/wire. For the turn signal to not work but the brake light does, that would indicate most likely problem with the BCM. Each of the 7-pin trailer light circuits have their own fuse.
Attached is the wiring diagram for the Trailer Tow 7-pin

Unless it's a ground issue and there are stray currents back feeding thru lamps that are not energized.

Very common problem.

 

[crash68]

I tried to find the fuse, but I couldn't find which one controls the left trailer marker light.

7-pin fuse #s along with fuse type and amp rating:
F53 ATR 20 amp Left Trailer lights
F81 ATR 20 amp Right Trailer lights
F29 FMM 20 amp Trailer Park lights
F28 FMM 20 amp Trailer Back up lights
F30 FMM 30 amp Direct from battery
F11 J Case 30 amp Brake Lever
F87 ATR 10 amp ITBM module

The Trailer-Tow wiring diagram I attached above has the fuse numbers listed also.
If you don't have power on the bumper side of the 7-pin, pull the plug out of the back and check there.