no power to right trailer brake light

Pure Diesel

Randsburg Drifter
Apr 22, 2008
685
3
18
So. Cal.
Went out this weekend and was doing my double tow. All lights, blinkers, brake lights were working for the truck, 5th wheel and side x side trailer. Go to come home and I have no brake lights(right side) for the 5th wheel and side x side trailer. The truck has brake lights. Its gotta be a fuse. I check all the related fuses and their all good. Visually and with continuity meter. I supply all the power(while towing) via the 5th hook up connector in the bed. I check it and the bumper connector and there's no power(only right side). I know the trailer and 5th are working by checking it with another vehicle. I pull the fuse for the right side(driver's fuse panel), apply the brake pedal and check with a continuity light. There is no power to either side of the fuse terminals. I double check the left side and get power out of one side. I'm at a loss here. Why do I not have power to the fuse block(drivers side fuse panel)? Need some help.
Thanks
 

MAXX IT OUT

<<<IT WORKS
Mar 1, 2013
1,648
0
36
Des Moines, Iowa
What fuse Block? Under hold or in the side of the dash? Whenever I have this problem its one of the fuses under the hood, since there are fuses for the truck and the trailer plug.
 

Pure Diesel

Randsburg Drifter
Apr 22, 2008
685
3
18
So. Cal.
What fuse Block(drivers side dash)? Under hold or in the side of the dash? Whenever I have this problem its one of the fuses under the hood, since there are fuses for the truck and the trailer plug.(As I said, I checked all fuses engine bay fuse block and drivers side dash).
There is no power to the fuse for the right trailer brake light. Drivers side dash.
 

ironmax

chock full of goodies
Jul 6, 2010
455
4
18
Col. Ohio
Try looking under the fuse block for a burnt wire, Or critter damage.

Sent from my SM-J260A using Tapatalk
 

LBZ

Super Moderator
Staff member
Jul 1, 2007
9,524
2
38
41
B.C.
Or green shit. Had that happen in my LBZ when it was only a year old. Fuse box was full of corrosion underneath.
 

KyleC4

Tech
Dec 30, 2016
312
3
18
Bay Area
Built into the driver side fuse block is a Turn signal flasher module that supplies power to the left and right side trailer brake lights. If you have power to left only It’s possible that the flasher module went out. Just remove the knee bolster panel and there should be access to it. Looks like a large relay. Put turn signal on and feel for the relay clicking. That’ll be the flasher module as GM calls it. Terminal J (if they are printed on relay) is what feeds right side. So I’d say remove relay and test with your meter for continuity from relay terminals at fuse block to right trailer light fuse leg. That way you know fuse block is good and then quick check for corrosion, spread terminals, etc. Hopefully this helps.
 

dndj

Boost!
Apr 13, 2019
134
1
18
Oregon
Kyle I think you're right. Probably a bad/corroded terminal on one of the internal relays inside the flasher module. One clarification however - terminal J is the right turn fuse for the truck turn signal, terminal C is trailer right side turn/brake circuit that leads to the "RT TRLR ST/TURN" fuse and trailer lamps.

Pure Diesel, the left and right trailer fuses are downstream from the relay, so if it's bad, you wouldn't see power on either side of the fuse as you reported.

If memory serves, there's 3 relays inside - one left, one right, and the flasher. When neither right nor left turn signal is on, the relays sit in normally closed position. If you activate the turn signal, you should hear an audible click as the relay switches. Mine went bad a few years back, what would happen is I would intermittently get no turn signal on my trailer. Investigation revealed the relay wasn't reliably closing, so I bent the little metallic tab on the relay a little and I've been running it ever since without any issues.
 

Pure Diesel

Randsburg Drifter
Apr 22, 2008
685
3
18
So. Cal.
Thanks for the extra help on what to look for. I didn't get to look at it yesterday as it was 104 when I got home and wasn't in the mood. Tonight was our anniversary and well that explains the rest. I'm looking at it tomorrow as it will be 30 degrees cooler.
 

KyleC4

Tech
Dec 30, 2016
312
3
18
Bay Area
Good catch on my error. I just rechecked the schematics and in one it shows terminal C as right trailer and the other shows terminal D as right trailer. Different docs in service information. But definitely not J lol. And totally agree with you on waiting til it gets a little cooler to go diving under the dash. Congrats on anniversary
 

Pure Diesel

Randsburg Drifter
Apr 22, 2008
685
3
18
So. Cal.
Ok something happened and I got these three codes. All I did was unplug the relay, pull the plastic cover off, look at it and then plug it back in. What happened?
I erase the codes and they come back. I'm looking at fuses now.
Thanks ahead of time.


Sent from my LM-G820 using Tapatalk
 

dndj

Boost!
Apr 13, 2019
134
1
18
Oregon
Take a look at the VEH STOP fuse in the left side dash fuse panel.

The 2 relays on the left side of your flasher module are for left and right brake/trailer brake/turn. The single one on the right creates the on/off signal for the turn signals. I will post a schematic I made for this if I get a chance later this evening. If you have a really thin file, you could try cleaning up the normally closed contacts on the two relays on the left to see if it solves your trailer brake light issue.
 

Pure Diesel

Randsburg Drifter
Apr 22, 2008
685
3
18
So. Cal.
I went out and bought a new relay. Unfortunately, I showed the manager that it was a used one but it looked new. The tabs had scrape marks on them from being pushed in. They said take it and they will order me a new one. Plugged it in still not working. The service 4wd light is on in the display, the parking brake light is on and the ABS light is on. Also, the truck is in limp mode. Won't go over 30.

Sent from my LM-G820 using Tapatalk
 

dndj

Boost!
Apr 13, 2019
134
1
18
Oregon
Doesn't sound good. Something must have been knocked loose or there is a fuse blown or relay knocked out of place. U1064 indicates the BCM is not talking over the class 2 serial bus.
 

Pure Diesel

Randsburg Drifter
Apr 22, 2008
685
3
18
So. Cal.
Ok good news. I have fixed it. The original problem was the flasher/relay. Put the so called new one in and was looking at the connector just above it. Well I some how knocked this connector loose in trying to get out the flasher. Fat hands in little places don't work well. Pushed it in and heard it click. Started the truck and all the lights went away. Cleared all codes and then hooked it up to my trailer. We got brake lights on the right.
Thanks for the help.


Sent from my LM-G820 using Tapatalk
 
  • Like
Reactions: darkness

dndj

Boost!
Apr 13, 2019
134
1
18
Oregon
Super glad you got it sorted out, and thanks for posting up the resolution. It really helps others who may be reading this thread years later.

Here's a partial schematic of the flasher module internal wiring. I traced things on the circuit board because I was curious about the logic. I also modified mine internally to work with LED lamps on my flatbed without hyperflash.
 

Attachments

Users Who Are Viewing This Thread (Users: 0, Guests: 1)