Service Trailer Brake Message With DTC C1114 And/Or C1112 - keywords connector module TBCM tow 7 way wheel wire

This PI was superseded to provide additional fix information and remove the Engineer contact info. Please discard PIT4595A.

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Some owners may comment of a Service Trailer Brake System message displayed on the DIC. When checking for DTC's, C1112 or C1114 will be set in the Trailer Brake Control Module (TBCM). This concern may occur with or without a trailer connected to the vehicle.

GM is receiving a number of returned Trailer Brake Control Modules (TBCM) and Trailer Brake Relays that are completely functional. The TBCM monitors the voltage of the electric brake wire (dark blue circuit 47) that runs from the trailer brake relay to the 7-pin trailer connector in the bumper. If the voltage on this wire is shorted lower or higher than the designed output voltage of the relay the TBCM will set a DTC.

Please investigate all wiring between the trailer brake relay and the 7-pin trailer connector (including any 5th wheel jumpers/connectors) for damaged wiring/connectors (nicks, cuts, corrosion, pin damage, etc.) and poor quality or non-sealed splices. Even a small amount of water intrusion can wick its way to both the trailer brake relay and the 7-pin trailer connector in the bumper, potentially causing intermittent DTCs.

If a Current DTC is present, please follow the trailer brake service procedures found in SI for the specific DTC.

If only history DTCs are present, follow the diagnostic procedure found in this PI to help with the identification of an intermittent short condition.

Please follow the appropriate diagnostic path PRIOR to attempting to ordering a Trailer Brake Control Module or a Trailer Brake Relay from the Product Quality Center (PQC). Please provide as complete of an answer as possible to the steps below (for a history DTC) or to the steps listed in the trailer brake service procedures (for an Current DTC).

IMMEDIATE DIAGNOSIS

1. Check for blown fuses related to the trailer brake system.
2. Disconnect trailer harness (if trailer is with vehicle).
3. Examine vehicle for unsealed splices or damage in vehicle trailer wiring (including 5th wheel wiring harnesses).
4. Examine G401 ground for proper grounding. Clean as required.
5. With the ignition on, use Tech 2 to monitor TBCM Relay Feedback Signal. Monitor signal for 10-15 seconds. Determine the minimum voltage value that occurs. (This voltage varies once per second and always settles down to the minimum.)
6. The minimum voltage should be between 3.4 - 3.8 V.

•	If the voltage is higher or lower - go to Short Investigation section (seen below)

•	If Service Trailer Brake Code set - go to Short Investigation section (seen below)

7. Repeat step 5 with brake applied for 10 seconds.
8. The minimum voltage should be between 3.4 - 3.8 V.

•	If the voltage is higher or lower - go to Short Investigation section (seen below)

•	If Service Trailer Brake Code set - go to Short Investigation section (seen below)

9. Repeat step 5 with vehicle in reverse and with brake applied for 10 seconds.
10. The minimum voltage should be between 3.4 - 3.8 V.

•	If the voltage is higher or lower - go to Short Investigation section

•	If Service Trailer Brake Code set - go to Short Investigation section

11. If damage to wiring harness or splices found in step 3, spray with water and repeat steps 5 - 10.
12. If no issues found in previous steps, follow service procedure for C1112/C1114.

SHORT INVESTIGATION

Investigate wiring for shorts between trailer brake (electric brake) wire (blue) and any/all of the following trailer wires: Right Turn/Brake, Left Turn/Brake, Running Lights, Back Up, Battery, Ground. Please note that even heat shrunk splices are not necessarily water tight.

PRODUCT QUALITY CENTER (PQC)

If diagnosis leads to TBCM or Trailer Brake Relay replacement, review latest version of PIT4634 prior to contacting PQC.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.