Troubleshooting Trailer Wiring Using a 2021 GMC Sierra Truck

Question

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

0

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.

expert reply by: Scott S

0

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.

Reply

Kevin C.

3/19/2024

@Dave Have you checked the 4-pole connector on the truck to make sure that the signals are actually reaching the light bar? If the 4-pole connector on the truck wasn't working and the light bar worked fine on another vehicle it's highly likely that you have a blown fuse or some other issue with your 4-pole connector. Looking at an online fuse box diagram it shows fuse port 41 as your trailer running lights so I'd test with a multimeter to see power is passing through the fuse or if it's blown. You can check the other trailer-related fuses as well: Trailer reverse lamp - 54 Trailer back-up lamp - 55 Trailer stop/turn lamp left - 73 Trailer stop/turn lamp right - 81 I'm not quite sure what you mean when you're talking about wiring in an inline harness. Are you not using the factory 4-pole connector and instead have a separate 4-pole wire harness spliced in that you're using? If so I'd doublecheck your wiring to make sure you're spliced into the correct wires. For reverse you would need a 5-pole wire harness to get the reverse light function. If you have a 7-way you can use an adapter like part # A10-7275VP to plug in the light.

Reply

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.

Reply

Andrew N.

2/22/2025

@Matt Since the lights work when hooked up to the 2018, but not the 2021, that leads me to believe the 2021 has a wiring issue somewhere. I'd take a circuit tester to the 7 way on the back of the 2021 and see if you get 12V on the top 1 oclock pin which is what gives the trailer 12v, this is the auxiliary 12v power pin. Assuming you don't have power there, you'll need to follow the wiring to check for a break or short somewhere and check your fuses in the truck. If you DO have 12v there, you'll want to make sure you have a clean good connection on the 7 way pins of your truck, and check for any that are bent.

Helpful Links

faq-wiring-7-way.aspx

Reply

Answer 1

The only trailer tire sensors that will work with the factory TPMS on your 2021 GMC Sierra 3500HD Denali is the GM kit 85110397 that is sold through GM. These sensors will need to replace the valve stems on your trailer wheels. I recommend staying…

view full answer...

Answer 2

B and W recommends a 10 inch lube plate with their Companion hitches like the part #BWRVK3715 that you referenced. For that we have the Reese Lube plate part #RP83002 which would work really well.

view full answer...

Answer 3

Yes, the puck system used on the 2016-2019 Chevy/GMC trucks is different than the pucks used on the 2020 and newer trucks. So you would need a 5th wheel like the B&W Companion #BWRVK3710 for your 2020 GMC Sierra 3500 with an 8' Bed. If you…

view full answer...

Answer 4

Seeing as your 2021 GMC Sierra 2500 Denali recognizes the trailer but then shows the error message once you start driving leads me to believe the issue is with your trailer. I would start by checking the 7-way connector and adding Dielectric Grease…

view full answer...

Answer 5

There are a few troubleshooting steps to help you figure out why you have no power. I would start by making sure that there is not any corrosion or debris blocking your connection points. If they are clean or still no power after cleaning, I would…

view full answer...

Answer 6

I checked with Curt and you are correct, the flashing red light indicates a brake circuit overload. Since the light is flashing that most likely means you have a short somewhere on the brake output circuit drawing too much power. To correct this…

view full answer...

Answer 7

The newer Chevrolet and GM pickup trucks have a known power deficiency issue with the Furrion Vision S Wireless RV Observation Camera System w/ Side Marker Light Cameras #FCE48TASL-05. There is likely nothing wrong that you did, the problem is the…

view full answer...

Answer 8

The issue you are having is the circuit you chose to power the running lights circuit of your trailer 7-way. If that circuit sees voltage when the key is turned on and your trailer is connected then all of the trailer lights will illuminate when the…

view full answer...

Answer 9

When replacing shackle straps they should be the same length as the ones that were previously on your tandem axle trailer (if possible); replacing them with longer straps will result in additional play in your trailer suspension which could lead to…

view full answer...

Answer 10

To slightly lower your trailer that has had an "axle flip" kit installed you would want to use slightly shorter shackle straps, not longer ones. Shorter shackles will let the axle rest closer to the trailer frame, which lowers the trailer…

view full answer...

Answer 11

Trailer manufacturers don't usually keep a record of parts like these that they install at the factory, so I cannot be sure as to the exact length of the shackle strap that would have come originally on your trailer. With this in mind, a 3-1/8…

view full answer...

Answer 12

To correct the issue the couple is having powering LED trailer lights with their 2020 GMC Sierra Denali 2500 I recommend going with the Trailer Connector Adapter w/ Lamp-Out Sensor Bypass #20142 due to it being a simple plug and play adapter that…

view full answer...

Troubleshooting Trailer Wiring Using a 2021 GMC Sierra Truck

Product Page this Question was Asked From

Featured Help Information

Miscellaneous Media

Continue Researching