Trailer wiring not working

[handstr]

Had the great idea to wire some rigid reverse lights into my trailer harness, but found out it doesn’t work. Plugged a tester into and the 12v works but nothing else. Brake, turn, reverse all won’t work. Any ideas ? I check the fuses under the hood and all were fine.

 

[Doubeleive]

Had the great idea to wire some rigid reverse lights into my trailer harness, but found out it doesn’t work. Plugged a tester into and the 12v works but nothing else. Brake, turn, reverse all won’t work. Any ideas ? I check the fuses under the hood and all were fine.

not sure if this is still the case with the new trucks but on previous models you have to connect the trailer wire power up by the underhood fusebox 1 of the wires would be tucked on the side between the fender and fuse box (wrapped with tape) and other would be tapped up to the main wire harness on the other side in the engine bay area.
they would connect to corresponding post's on the forward side of the fusebox
1 is for power and the other 1 for brake, I wired some reverse lights to my 2012 thru the tow harness and they wouldn't work until I connected those wires

 

[Fless]

not sure if this is still the case with the new trucks but on previous models you have to connect the trailer wire power up by the underhood fusebox 1 of the wires would be tucked on the side between the fender and fuse box (wrapped with tape) and other would be tapped up to the main wire harness on the other side in the engine bay area.
they would connect to corresponding post's on the forward side of the fusebox
1 is for power and the other 1 for brake, I wired some reverse lights to my 2012 thru the tow harness and they wouldn't work until I connected those wires

^^This for 12v auxiliary power, and brake controller circuit. And fuses might need to be added for those.

This is a 2021+ model year? Need more detail on what your truck is.

If these newer models are like the older models, there are individual fuses for the trailer lights, separate from the truck lights. There may be left and right trailer turn signals (brakes) and running lights. Be sure you're looking at those in the fusebox. SWAG maybe they leave those fuses out of those from the factory?

 

[handstr]

It’s a 22 suburban with max trailer. I would expect everything to be hooked up and ready to go. I checked the obvious fuses under the hood. Example trailer left turn but not even that one is working. I bought a 7pin plug in that tells you what lights are working and only the 12v is lit up.

 

[gooffeyguy]

It’s a 22 suburban with max trailer. I would expect everything to be hooked up and ready to go. I checked the obvious fuses under the hood. Example trailer left turn but not even that one is working. I bought a 7pin plug in that tells you what lights are working and only the 12v is lit up.

Was the vehicle started when checking the lights?

I know my brake and turn signal lights won't come on unless the engine is running

 

[handstr]

Yes vehicle was started. And the ground works bc I put a small led prob on the 12v and ground and it lit up.

 

[R32driver]

If it was mine I would have it back at the dealer to see what the deal is. Brand new rig with max trailer package you should have no reason to be troubleshooting wiring. I towed with ours a while back and no issues, all my trailer lights worked like normal

 

[Doubeleive]

It’s a 22 suburban with max trailer. I would expect everything to be hooked up and ready to go. I checked the obvious fuses under the hood. Example trailer left turn but not even that one is working. I bought a 7pin plug in that tells you what lights are working and only the 12v is lit up.

so you have a oem trailer brake controller on the left side of the dash? or no?

 

[Doubeleive]

Was the vehicle started when checking the lights?

I know my brake and turn signal lights won't come on unless the engine is running

should only have to have key-on

 

[handstr]

so you have a oem trailer brake controller on the left side of the dash? or no?

Yes

 

[Doubeleive]

on these backup lights what are you using a for a ground?, have you checked the pins with a meter? I know you said you had some kind of tester is that thing for gm or is it some kind of universal tester

 

[handstr]

I was using the reverse wire and the ground wire from the 7pin connector. A curt plug tester. I used it on a few other vehicles and it worked. Definitely feel like I missing something. Does something else need to be to be connected for the system to work?

 

[Fless]

If it was mine I would have it back at the dealer to see what the deal is. Brand new rig with max trailer package you should have no reason to be troubleshooting wiring. I towed with ours a while back and no issues, all my trailer lights worked like normal

^^^ This. I know the curiosity factor is strong, but why mess with it at home?

 

[Doubeleive]

I was using the reverse wire and the ground wire from the 7pin connector. A curt plug tester. I used it on a few other vehicles and it worked. Definitely feel like I missing something. Does something else need to be to be connected for the system to work?

from what I have seen so far it should be working already, you could try just using a meter and ground one probe to the frame and test the output from the reverse pin, same for the brake & blinkers they should all work if not then take it in and have the dealer check it out

 

[three_stacks]

@handstr What was the fix? Having the same exact issue (no trailer lights, 7pin tester showing only one light for 12v) and taking my '22 Yukon Denali to the dealer on Monday. Hoping I can skip the trip or at the very least steer them in the right direction.

 

[Fless]

The previous gen had individual fuses for the trailer left brake, right brake, and running lights. I can't imagine they changed that configuration, so my question would be does yours have those and how are the fuses? Voltage on both sides? Maybe what controls the trailer lights is now a module.

 

[three_stacks]

Same situation here, there are individual fuses for LH trailer brake light, RH trailer brake light, reverse, etc. There is no power to either side of the fuse. Whatever is supposed to be sending 12v into those fuses is either disconnected or not working. A bit odd for a 2022 Denali with 1k miles...

 

[Fless]

Have you checked the running lights fuse? It should be live when the truck lights are on.

It could be that the brake light circuit doesn't get activated until the turn signal or brake is on. Not sure if the fuse is before or after the activation, but a wiring diagram would tell.

 

[Fless]

After reading a bit about earlier models, I would suggest that you connect a trailer if you haven't already. Apparently there is a trailer lighting control module that senses whether a trailer is connected electrically. The module may need a bigger load on it than the test plug.

 

[three_stacks]

This was good advice. The first thing that I had plugged in was my boat trailer and after seeing that the lights were not working, grabbed the 7pin tester to see that only one of the lights was on. This was with headlamps on, hazards blinking, etc. I ran home to grab my other car and had no issues with the trailer lights which led me to believe there was an issue. After reading your posts last night, I decided to fiddle with it some more and borrowed my neighbor's trailer as a test. Sure enough, they lit up just fine behind the Yukon. It seems there is something going on between my boat trailer and the Yukon; perhaps the load from the LEDs is not sufficient to engage the module. Oddly not an issue with with my other car's 7pin plug.

For those that stumbled upon this post in the future and are experiencing the same issues, it seems that a 7pin test plug will not draw enough for the T1s to engage the module. Suggest plugging in a trailer with incandescent bulbs to test functionality.