02 Suburban trailer wiring issue

[WrenchMonkey]

For those who don't know, I've got a 2002 Burb 2500 4x4 8.1L. I like it.

I've got no trailer brake lights. I do have both left and right turn signals. So everything on the trailer from the plug to the bulbs is good, the issue is in the truck.

The wiring diagrams I've found say the trailer brake light signal comes off the third brake light. That is working.

So the signal is getting lost somewhere between the CHMSL fuse and the 7-way plug under the bumper.

I can't find a wiring diagram to show how the three signal (Left, right, brake) gets converted to two (just left and right). There's a "Trailer Brake Wiring" box (assume a relay) on this diagram of the fuse block under the dash, but what's actually there doesn't match the diagram at ALL (see pics).

Additional clue, or distraction: The truck's hazards don't work either. Replaced the flasher, no help. Assume it's the switch and unrelated, but thought I'd mention it.

Any suggestions? Thanks.

[WrenchMonkey]

Going over the manual, I did find a separate fuse for the hazards, "HAZ LP" in the lower right.

I'll check that one when I get home. MAYbe it uses that signal to "flash" both trailer turns at once? I dunno...

[WrenchMonkey]

Originally Posted by Waffle

After you check all the fuses you can think to check, I would suggest getting your multimeter and start tracing power and ground at either end...

That's my issue. One end is the CHMSL fuse and the third brake light. They're good. The other end is where it comes out under the bumper. There, there's nothing.

I know there's something in between; a relay, diode pack, something. I just don't know what I'm looking for, or where it is in the truck, to check it...

[WrenchMonkey]

Originally Posted by WrenchMonkey

...a separate fuse for the hazards, "HAZ LP" in the lower right. I'll check that one when I get home...

I didn't have much time to mess with it today, had to get the grass cut before the rain. But I did check that fuse, and it looked good. I didn't take time to put a meter on it.

Originally Posted by Waffle

I searched the net for this "mysterious" Center Instrument Panel Utility Block and found tons of people confusing it as being a third fuse panel. It isnt. Its the block with relays...

In my defense, that is the owner's manual calling it a fuse block, and their list refering to the empty sockets as relays.

Originally Posted by Waffle

...it just happens to be the most poorly drawn description of what is actually there...

No kidding, huh? That's pathetic.

Originally Posted by Waffle

...the second fuse block in the left side of the dash...

I did check that one, but I don't have the same fuses there that you do. My "trailer turns" are under the hood, and are good.

Originally Posted by Waffle

...the trailer brake lights are routed through the Hazards system and the "multifunction switch" that the hazard uses.

THAT is what I was looking for. That's what splits the one brake light signal to the left and right legs. None of my wiring diagrams show that at all.

I'm confident that's the issue. That second thread you linked said $340 retail, and he paid $265. RockAuto has the Dorman for $111.

Installation, however, may be a project.

But thanks for your help, and I'll confirm when I've fixed it!

[WrenchMonkey]

Finally got around to installing it, and yep, that was it.

The video was a huge help, but his fingers must be longer than mine. I had to take a few more things loose, that little plug on the lower right was a B!TCH to get back together.

REALLY appreciate your help man, I just couldn't find that portion of the wiring diagram anywhere!

[WrenchMonkey]

The bad spot on my old one: