This bulletin applies to 2024-2025 GMC Canyon, Hummer EV pickup, Hummer EV SUV, Sierra EV, Sierra 1500/2500/3500 and Yukon vehicles. Some customers may comment when they connect a trailer that has a Hydrastar Electric Over Hydraulic brake system, that intermittently a Service Trailer Brake message will appear. They may notice that this issue happens more often during lighter trailer brake applications. When checking, DTC C114 will have set.

The cause of the concern is with the Hydrastar Electric Over Hydraulic trailer brake actuator generating a voltage spike on the blue trailer brake command circuit 47, which can interfere with the vehicle’s trailer brake diagnostics.

To verify that the truck’s trailer brake system is working properly, use the Trailer Brake Simulator Tool P/N EL-52641 to test the vehicle’s trailer brake system.

The issue with the trailer may or may not be the voltage spike from the Hydrastar brake actuator, but there is an issue within the trailer and it should be inspected by a trailer repair facility or by Hydrastar. If the trailer shop has determined that this issue is caused by the voltage spike from the actuator, Hydrastar has released a trailer mounted module to correct this concern, Hydrastar P/N 381-7074 (GM Controller adapter module). This module will be installed to the trailer, connected to the blue trailer brake command wire, between the 7-pin trailer connector and the Hydrastar actuator. This module will prevent voltage spikes from the actuator from reaching the vehicle’s trailer brake control module.