Troubleshooting Trailer Wiring Using a 2021 GMC Sierra Truck
Question:
I have a 2021 GMC sierra. My trailer had a mouse problem with the wires, so I need to re-wire. My pickup is putting out no power to the 7 blade except the auxiliary by volt testing. I checked all fuses, which were fine. I took it to the dealership and they told me that volt meters no longer work on trucks as they only provide power to the 7 blade if there is a draw. They used a special machine to verify it is working properly. Does this sound plausible? If so, how do I wire my trailer if I cant test each blade separately? I guess I grab a 12v battery to wire and hope it works when I hook it up to the 7 blade at the end? Thank you.
asked by: Jason
Expert Reply:
The Dealership is correct that the 2021 GMC Sierra is not going to send power back to the 7-way unless it senses a load. Since your trailer wiring is chewed up, the 7-way is most likely not sensing a load. The fix for this would be to use an older vehicle with a 7-way that will not have this same issues, or you can use the 12v battery like you were talking about to power your trailers wires. Then use a circuit tester like the Hook Probe Circuit Tester # PTW2993 to see what wires are receiving power and help find where the power is not making it to.
I have also included a help article that goes over wiring a 7-way trailer plug.
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Dave
3/17/2024
How do I wire in tailgate light bar turn stop reverse that uses the 4 flat for power? The light bar is all led and will not light on my 21 gmc 1500. Took it to a 16 Jeep wrangler 4 flat and it lights up. I did wire the inline harness for an extra 4 flat connector but tried using existing 4 pin before installing to make sure it is oriented correctly. Left side left and right side right. Activated the trailer wiring check on the center display no lights.