Need to find a trailer brake relay for my 2015 GMC 3500HD. Getting the constant service trailer brakes when towing form my hitch. Replaced the dash switch, next up is the relay. Anyone know part number or where to grab a new one?
When I do a search it shows part # 20904439 but if I search on a GM parts site with my vin, I cannot find a trailer brake relay and on the other sites it says that part number does not work on my 2015 gmc sierra 3500 HD denali. I figured as inexpensive as it was I would just replace it as I have already checked the trailer, the wiring connector from the trailer to the truck, and replaced the trailer brake controller in the dash.
Have you tried other trailers, or this trailer on another truck?
I find it hard to believe that this is being caused by a relay. If a relay fails it will typically not close at all, which would lead to the truck not recognizing the trailer brakes as being connected (i.e., you'd never get the "trailer connected" message). I believe the "service trailer brake system" message is related to the resistance values across the trailer brakes not falling within the system's expected values, and the only way to check that directly is with an ohmmeter.
Rather than do a bunch of exploratory part replacement I would pick up one of these
and see how your truck behaves with it. It's not cheap but the amount of time it will save you when diagnosing trailer wiring issues is more than worth the cost. Trust me.
In either case, the next thing I'd look at is the trailer brakes themselves. Take the brake drums off and inspect the internal parts and wiring to ensure there are no short circuits.
Good luck and let us know the outcome.