Integrated trailer brake controller not working

[RNAV]

I hooked up my '21 SLT Duramax to my horse trailer and the trailer brakes aren't working.  Sanity check me here: I took my multimeter to pins 1 (ground) and 2 (brake control) on the truck's 7-pin connector.  I had my wife depress the brakes, set the gain to 10, and depress the manual lever.  Regardless of gain setting, manual lever position, or brakes on/off, the voltage was constantly flipping between 0.9V and 1.2V.  Am I correct in understanding that the brake controller is not working?

[redwngr]

I likely does a check to verify it is connected to a trailer with brakes before it sends out 'brake' voltage/current.  Checking pinouts with a meter and no trailer connected wouldn't satisfy that type of check. 

 

 

Are you getting a 'trailer connected' message after it's plugged in? 

Are you able to set up a trailer profile for it in the trailering app?

 

 

Edited February 1, 2021 by redwngr

[redwngr]

There might be hints here: 

 

https://gm-techlink.com/?p=12721

 

Trailering App Trailer Connection and Lighting Diagnosis Update

February 27, 2020

The new trailering app (RPO U1D) (Fig. 23) available on the 2019-2020 Silverado 1500 and Sierra 1500 and the 2020 Silverado HD and Sierra HD uses the K68 Trailer Lighting Control Module (TLCM), or Trailer Interface Module, to constantly monitor for trailer connection status, trailer lighting faults, and trailer theft deterrent purposes through the lighting circuits of the trailer.

 

Fig. 23

 

Trailer Connection Status

When a trailer is connected, the Trailer Lighting Control Module senses the trailer connection using the lighting circuits of the trailer and alerts the driver by requesting a trailer profile setup through the trailering app on the infotainment screen (P17 Info Display Module). (Fig. 24) The Trailer Detection Alert setting must be enabled for the alert to display when a trailer is connected.

 

Fig. 24

 

With a trailer connected and the ignition off, the Trailer Lighting Control Module will periodically pulse the lighting circuits of the trailer to verify it is still connected. Depending on the configuration of the trailer lights, the trailer lights may periodically flash as part of the trailer connection detection or theft deterrent functions. These flashes may be more visible in dark ambient light environments and correspond to when the Trailer Lighting Control Module pulses the lighting circuits to ensure the trailer is still connected. (Fig. 25)

TIP: The flashing or flickering lights is a normal condition and this operation should be explained to the customer when a vehicle is brought to the dealership for a flickering/flashing lights condition. GM is receiving CCND warranty claims for this condition and it should not be charged through warranty.

 

Fig. 25

 

Depending on the settings, a Trailer Connected or No Trailer Connected status may be displayed by the trailering app on the infotainment screen. (Fig. 26)

TIP: Trailer disconnection detection by the Trailer Lighting Control Module requires that a trailer light circuit must be activated after the trailer is disconnected. If a trailer is disconnected while no trailer lights are active, the trailer will continue to be reported as connected by the trailering app. Always have the vehicle lights on when disconnecting the trailer to ensure the system properly detects the disconnection.

Other causes that may result in a trailer not being detected include poor trailer wiring or a poor connection at the trailer connector. It may be necessary to update the trailer wiring, trailer connector, or trailer lights.

 

Fig. 26

 

Trailer Brake System

The available Trailer Brake System with an Integrated Trailer Brake Control (Fig. 27) and the trailering app both report trailer connection information. However, the Trailer Brake System and related DIC messages are independent from the Trailer Lighting Control Module trailer connection detection system that feeds the trailering app on the infotainment screen.

 

Fig. 27

 

If the truck is equipped with a Trailer Brake System, a Trailer Brakes Connected message will display on the Driver Information Center (DIC) when a trailer with electric trailer brakes is connected. In addition, this system will display Check Trailer Wiring on the DIC every time a trailer with electric brakes is disconnected.

 

Trailer Disconnected Message

On some 2019 Silverado 1500 and Sierra 1500 models only, a “Trailer Disconnected, Check Connection” message may display on the DIC without a trailer being connected. (Fig. 28) If the Trailer Lighting Control Module on 2019 models detects enough of a load on any of the trailer tail lamps or stop/turn signal circuits, it may determine that a trailer has been connected. However, any moisture or corrosion in the trailer receptacle or Trailer Lighting Control Module connector, or anything left plugged into the trailer receptacle, such as trailer adapters with built-in test LEDs, may cause the module to think that a trailer is connected.

 

Fig. 28

 

If this condition is found, remove anything plugged into the trailer receptacle and inspect for any moisture or corrosion in the trailer receptacle at the rear bumper, the available receptacle for when towing a 5th wheel trailer, the chassis harness connector that plugs into the trailer receptacle, and the Trailer Lighting Control Module connector. (Fig. 29) Clean and repair any connection issues.

 

Fig. 29

 

Trailer Lighting

For lighting operation, the Trailer Lighting Control Module receives serial data messages from the Body Control Module (BCM) indicating which lamps have been activated on the vehicle. The Trailer Lighting Control Module responds by applying voltage to the appropriate control circuits for the requested lamps to illuminate the lamps on the attached trailer. The Trailer Lighting Control Module constantly monitors the trailer’s reverse, park, and left and right stop/turn signal lamps.

2019 model year vehicles equipped with a Trailer Lighting Control Module cannot drive as much current on each circuit when compared to the non-Trailer Lighting Control Module trailer lighting system. The Trailer Lighting Control Module drives four trailer circuits using four solid state drivers that are fed from one 30A lighting fuse. If the total current on the four circuits overloads the fuse, it will fail. If any single lighting circuit exceeds the driver threshold, it will deactivate the output for the balance of the key cycle and a reactivation of the lamp load is required. Individual DTCs are activated for each circuit and that load is turned off due to high current. If a trailer draws too much current, it may be helpful to change some or all of the trailer lighting to LEDs.

TIP: For 2020 model year vehicles, the trailer park lamps amperage capability has been increased and can handle electrical loads up to 16 amps. The park lamp circuit has been moved from terminal 5 to terminal 25 and is driven by an internal relay that allows for higher amperage values for the parking lamps. The park lamp circuits are not used for trailer connection detection on 2020 models.

 

Trailer Lamp Test Lights

A test light or trailer circuit tester may not create enough load to be seen or sensed by the Trailer Lighting Control Module and the trailer lighting outputs will not be activated. If a tester draws enough load on one or more of the trailer lighting circuits, the Trailer Lighting Control Module will determine that a trailer is connected and enable the trailering light circuits (when activated on the vehicle). A single LED may or may not draw enough current to be sensed by the Trailer Lighting Control Module.

The available EL-52641 Trailer Presence Simulator Tester tool provides feedback on the vehicle signals sent to the trailer for the reverse lights, battery power, right turn signal and brake light, left turn signal and brake light, and brake controller output.

 

Trailering App Profiles

When a trailer is connected, the driver has the option of selecting a Guest profile or naming the trailer and storing settings for it on the trailering app. The settings can include basic information — profile name, hitch type and trailer type — or more advanced information — Tow/Haul Mode reminder, Trailer Tire Pressure, and maintenance reminders. (Fig. 30)

 

Fig. 30

 

If the trailering app is set up incorrectly for the connected trailer, several DTCs or DIC messages may appear. For example, if a trailer does not have any reverse circuit loads, yet a reverse circuit load is expected in the trailering app, the Trailer Lighting Control Module will set DTC B3890 (Trailer Backup Lamps Circuit) and messages may be displayed on the DIC and infotainment screen. If the trailer does not have reverse lamps or any loads on the reverse circuit, change the trailering app profile settings and clear any DTCs. Do not replace the Trailering Lighting Control Module.

 

 

Edited February 1, 2021 by redwngr

[Fred Longenecker]

I recently towed a very new rental trailer and had no trailer brakes. Unhooking and re-hooking the connector had no effect. After I was finished with it I parked at home. When returning the empty trailer I suddenly had brakes. 
  I had problems with my own camper trailer previously and found that ( through an alert from GM) certain brands of connectors were not compatible with 2019 through 2021 GM trucks. I installed the compatible brand on my trailer and had not further issues.

  It is sad that all 7 pin connectors would not be compatible. Any solutions?

[drafil]

Hi Fred, can you give more details about the compatible brand connector you installed and fixed your issue?

Thank you!

[AZRGE]

On 1/31/2021 at 5:11 PM, RNAV said:

I hooked up my '21 SLT Duramax to my horse trailer and the trailer brakes aren't working.  Sanity check me here: I took my multimeter to pins 1 (ground) and 2 (brake control) on the truck's 7-pin connector.  I had my wife depress the brakes, set the gain to 10, and depress the manual lever.  Regardless of gain setting, manual lever position, or brakes on/off, the voltage was constantly flipping between 0.9V and 1.2V.  Am I correct in understanding that the brake controller is not working?

RNVA - did you ever get a fix for this problem? I have the same issue currently with my 2019 GMC 3500HD SRW LB 4x4. Voltage is exactly as you described. Thx - Ralph 

[SoxFan904]

Hello,

 

I currently cannot get my Infotainment system to recognize my trailer at all.  When I plug in my trailer or the above tester (which I bought) into my truck, it shows "Trailer Breaks Connected" and everything tests perfectly.  I have searched the internet and done every troubleshooting step, waiting the time to plug in, while the car is off, taking of the battery, connecting with the car on or off, just doesn't work.

 

I recently upgraded my radio from RPO code IOS to IOU and I am assuming this is causing the issue.  I also have the trailer TPMS sensor installed behind the license plate.

Is there a different module i would need to fix this?  I read somewhere that I would have to change the video processing module, but I can't seem to find where that is on my truck.  I also read that I may need a BCM flash via the OBDII port, but again, can't find anywhere to obtain that.

 

Any suggestions at all would be very helpful

Edited June 26, 2025 by SoxFan904

[newdude]

57 minutes ago, SoxFan904 said:

Hello,

 

I currently cannot get my Infotainment system to recognize my trailer at all.  When I plug in my trailer or the above tester (which I bought) into my truck, it shows "Trailer Breaks Connected" and everything tests perfectly.  I have searched the internet and done every troubleshooting step, waiting the time to plug in, while the car is off, taking of the battery, connecting with the car on or off, just doesn't work.

 

I recently upgraded my radio from RPO code IOS to IOR and I am assuming this is causing the issue.  I also have the trailer TPMS sensor installed behind the license plaate.

Is there a different module i would need to fix this?  I read somewhere that I would have to change the video processing module, but I can't seem to find where that is on my truck.  I also read that I may need a BCM flash via the OBDII port, but again, can't find anywhere to obtain that.

 

Any suggestions at all would be very helpful

 

 

You downgraded your radio?  From IOS to IOR?  

 

Are you trying to get the "Trailering App" to work in the radio?  

 

If you hook up a trailer, do you have lights and brakes?  

Edited June 26, 2025 by newdude

[SoxFan904]

22 minutes ago, newdude said:

 

 

You downgraded your radio?  From IOS to IOR?  

 

Are you trying to get the "Trailering App" to work in the radio?  

 

If you hook up a trailer, do you have lights and brakes?  

Sorry, I mistyped, I upgraded from IOS to IOU.  Yes, I am trying to get the trailering app to work.

 

When I plug in my trailer presence simulator, and my camper, the radio does not acknowledge a trailer is plugged in, but on the HUD i see trailer breaks connected. All the lights work, backup camera, break controller gain, but it says "No trailer connected" on the Infotainment screen.

 

I think this may be due to my vehicle missing the K68 Trailer Light Controller, I think this module sends the signals to the radio.

[newdude]

45 minutes ago, SoxFan904 said:

Sorry, I mistyped, I upgraded from IOS to IOU.  Yes, I am trying to get the trailering app to work.

 

When I plug in my trailer presence simulator, and my camper, the radio does not acknowledge a trailer is plugged in, but on the HUD i see trailer breaks connected. All the lights work, backup camera, break controller gain, but it says "No trailer connected" on the Infotainment screen.

 

I think this may be due to my vehicle missing the K68 Trailer Light Controller, I think this module sends the signals to the radio.

 

 

 

Yea, I don't think you'll be able to get this to work.  At all.

 

You can try wherever you got the IOU upgrade from, but GM won't have any way to enable it.  

 

Without U1D factory installed (U1D is the trailering app), the trailer circuitry for lighting is all different and you won't be able to add the trailer lighting control module to have the trailering app in the radio do its thing.  

[SoxFan904]

Thats what I was afraid of.  This is very discouraging now because I spent all the money to upgrade (to get navigation and wireless carplay) and now i can't even do the light test in the trailering app.  Yeah, my truck did not come with U1D.

 

It looks like the current light controller uses relays, and the K68 hooks into those with a wiring harness.  If only it was that easy.