There are many, many threads on this site referring to SERVICE TRAILER BRAKE SYSTEM or TRAILER NOT CONNECTED. Lots of possibilities and solutions.
This is general information about the trailer braking system:
Trailer Brake Controls Description and Operation
Trailer Brake Controls Description and Operation
This vehicle may be equipped with a Trailer Brake Control (TBCM) system for electric trailer brakes. The power output to the trailer brakes is based on the amount of brake pressure being applied in the vehicle's brake system. The available power output to the trailer brakes can be adjusted to a wide range of trailering situations.
Important: Connecting a trailer that is not compatible with the ITBC system may result in reduced or complete loss of trailer braking. There may be an increase in stopping distance or trailer instability which could result in personal injury or damage to your vehicle, trailer, or other property. An aftermarket controller may be available for use with trailers with surge, air or electric-over-hydraulic trailer brake systems. To determine the type of brakes on your trailer and the availability of controllers, check with your trailer manufacturer or dealer.
Important: If your vehicle is equipped with an ITBC, the blunt cuts exist, but are not connected further in the harness. If you install an aftermarket trailer brake controller, the ITBC must be disconnected. Do not power both ITBC and aftermarket controllers to control the trailer brakes at the same time.
The vehicle is equipped with the following trailer braking components:
Manual Trailer Brake Apply
Trailer Gain Adjustment
Trailer Brake Control Panel
Trailer Brake DIC Display
Manual Trailer Brake Apply
The Manual Trailer Brake Apply Lever is located on the Trailer Brake Control Panel, and is used to apply the trailer's electric brakes independent of your vehicle's brakes. This lever is used in the Trailer Gain Adjustment Procedure to properly adjust the power output to the trailer brakes. Sliding the lever to the left will apply only the trailer brakes. The power output to the trailer is indicated in the Trailer Brake Display Page in the DIC. If your vehicle's service brakes are applied while using the Manual Trailer Brake Apply Lever, the trailer output power will be the greater of the two.
The trailer and the vehicle's brake lamps will come on when either vehicle braking or manual trailer brakes are applied.
Trailer Gain Adjustment
Trailer Gain should be set for a specific trailering condition, and must be adjusted any time vehicle loading, trailer loading or road surface conditions change.
Setting the Trailer Gain properly is needed for the best trailer stopping performance. A trailer that is over-gained may result in locked trailer brakes. A trailer that is under-gained may result in not enough trailer braking. Both of these conditions may result in poorer stopping and stability of the vehicle and trailer.
After the electrical connection is made to a trailer equipped with electric brakes, the TRAILER CONNECTED message will be momentarily displayed on the DIC. The Trailer Brake Display Page will appear on the DIC showing TRAILER GAIN and OUTPUT, after all vehicle related service messages are acknowledged by the driver. The dashed lines in the TRAILER OUTPUT display signifies a disconnected trailer or TBCM fault condition, and will disappear only when the TBCM fault condition is not present.
Trailer Gain Adjustment
Adjust trailer gain in 0.5 step increments up to 10 gain setting by using the gain adjustment +/- buttons on the trailer brake control panel switch. Pressing and holding a gain button will cause the trailer gain to continuously increment or decrement. To turn the output to the trailer off, set the gain to zero.
Drive the tow vehicle and trailer combination on a level road surface representative of the towing condition, and free of traffic at approximately 32-40 km/h (20-25 mph) and fully apply the manual trailer brake apply lever mechanism located on the trailer brake control panel switch. Adjusting trailer gain at slower speeds may result in an incorrect gain setting.
Adjust the trailer gain to just below the threshold of trailer wheel lock-up. Important trailer wheel lock-up may not occur if towing a heavily loaded trailer. In this case, adjust the trailer gain to the highest allowable setting for the towing condition.
Important: Re-adjust trailer gain any time tow vehicle and trailer loading or road surface conditions change or if you notice trailer wheel lock-up at any time while you are towing.
Trailer Brake Control Panel
The TBCM system has a control panel with the trailer gain and manual apply switches, and is located on the instrument panel to the left of the steering column. See Instrument Panel Overview for more information on location. The control panel and switches allows you to adjust the amount of output, referred to as trailer gain, available to the electric trailer brakes and allows you to manually apply the trailer brakes. The Trailer Brake Control Panel, and switches is used along with the Trailer Brake Display Page on the DIC to adjust and display power output to the trailer brakes.
Driver Information Indicators and Messages
The following indicators are used to inform the driver of several different factors.
TRAILER CONNECTED
This message will be briefly displayed when a trailer with electric brakes is first connected to the vehicle. This message will automatically turn off in about ten seconds. The driver can also acknowledge this message before it automatically turns off
CHECK TRAILER WIRING and TRAILER NOT CONNECTED
This message will be displayed if:
The ITBC system first determines connection to a trailer with electric brakes, and then if the trailer harness becomes disconnected from the vehicle. If a trailer connection is made initially and the vehicle or trailer wiring connections is not correct the TRAILER NOT CONNECTED message will display during key up as if there is no trailer connected. If the TBCM trailer connection is recognized initially and the disconnect occurs while the vehicle is stationary, this message will automatically turn off in about thirty seconds. This message will also turn off if the driver acknowledges this message off or if the trailer harness is re-connected. If the disconnect occurs while the vehicle is moving, the CHECK TRAILER WIRING message will continue until the ignition is turned off. This message will also turn off if the driver acknowledges this message off or if the trailer harness is re-connected or repairs completed.
There is an electrical fault in the wiring to the electric trailer brakes. The CHECK TRAILER WIRING message will continue as long as there is an electrical fault in the trailer wiring. This message will also turn off if the driver acknowledges this message off. To determine if the electrical fault is on the vehicle side or trailer side of the trailer wiring harness connection, do the following:
SERVICE TRAILER BRAKE SYSTEM
This message will be displayed when there is a problem with the TBCM system. If this message persists over multiple ignition cycles there is problem with the TBCM system. Take your vehicle to an authorized GM dealer to have the TBCM system diagnosed and repaired.
TRAILER GAIN and OUTPUT Display
This display menu can be accessed by scrolling through the DIC vehicle Information menu, or any time the trailer gain +/- button is depressed, or the manual trailer brake apply lever is actuated. The trailer gain display is 0 to 10 in 0.5 step increments, and indicates the current user setting of the trailer output gain. The trailer output is 0 to 10 bars in 1 bar in
