Troubleshooting Trailer Wiring Using a 2021 GMC Sierra Truck
Updated 02/22/2025 | Published 11/10/2021
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.
Matt
2/21/2025
i also have a 2021 gmc serria that the interior lights of my enclosed trailer are not working on. They worked when I bought the trailer but do not work now. I cannot find a blown fuse to save my life. Hooked the trailer up to a 2018 Silverado and lights work as they should.
Andrew N.
2/22/2025
Helpful Links
Product Page this Question was Asked From
Featured Help Information
Miscellaneous Media
Continue Researching
- Search Results: dometic penguin 2
- Shop: Trailer Wiring
- Search Results: lb20
- Search Results: bk4~150
- Search Results: 8k bearing kit
- Search Results: 10k jack
- Search Results: #42 spindle
- Shop: Dexter DX7.5L A-60 Brake Actuator w Electric Lockout - Bolt On - Disc - 2" Ball - Zinc - 7.5K
- Search Results: 2" coupler
- Shop: Gen-Y Hitch Anti-Rattle Reducer Sleeve - 3" to 2.5" Trailer Hitch
- Shop: Erickson Ratchet Tie-Down Strap - Chain Leads and Hooks - 2" x 27' - 3,333 lbs
- Shop: etrailer Lock Set - 2" / 2-1/2" Hitch Receiver Lock and 2-5/16" Surround Coupler Lock
- Shop: Replacement Shroud for Dometic Penguin II RV Air Conditioner - Polar White
- Q&A: Availability of Inner Member of the Dexter DX 7.5L A-60 # DX89FR
- Video: Let’s Check Out the Dexter DX7.5L A-60 Brake Actuator w Electric Lockout
- Shop: Dexter Bearing Kit for 7.2K - 8K Axles - Inner and Outer Bearings 02475/25580
- Q&A: Troubleshooting 7-Way of 2022 Chevy Silverado that Shows No Voltage at Connector
- Shop: Replacement Inner Member for Dexter DX7.5L Actuator - Tandem Axle w/ Disc - 2" Ball - 7,500 lbs
- Q&A: Troubleshooting Trailer Wiring Not Working on a 2023 Chevy Silverado 3500
- Q&A: Troubleshooting LED Trailer Lights Causing Error Code On 2021 GMC Sierra Denali 2500
- Article: Trailer Wiring Diagrams
- Video: Curt 7-Way to 4-Way/5-Way Trailer Adapter Review
- Video: Review of Curt Wiring - Wiring Adapters - C35FR
- Article: Best RV Roof Vents
- Q&A: Troubleshooting Check Trailer Wiring Message on 2021 GMC Denali Sierra 2500 After Driving
- Article: Wiring Trailer Lights with a 5-Way Plug (It's Easier Than You Think)
- Article: Wiring Trailer Lights with a 4-Way Plug (It's Easier Than You Think)
- Q&A: How To Install Lippert Axle Riser Kit For Single Torsion Axles
- Q&A: Lift Kit Needed for Lippert Torsion Axle
- Article: Wiring Trailer Lights with a 7-Way Plug (It's Easier Than You Think)
on Automotive Tools
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.