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.

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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

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Answer 1

Yes we do have the inner member available for the Dexter DX 7.5L A-60 #DX89FR that you mentioned with the #DX99FR.

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Answer 2

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…

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Answer 3

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…

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Answer 4

Hey Paul, try connecting to a trailer and seeing if the lights work like they should. A lot of modern trucks require a trailer connection to be sensed before the trailer wiring circuits are completely active.

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Answer 5

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.

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Answer 6

I attached links to this page that have all of the dimensions for the Fulton F2 Trailer Winch #FW32000101 that you referenced. When looking at it from the front the winch will have an overall width of 14.77", which includes the extension from…

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Answer 7

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…

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Answer 8

The Axle Riser Kit - Single Square Torsion Axle part #LC382741 will lift both sides of the trailer frame up 3-1/2" to add clearance between the frame and the axle. To install the axle riser kit you want to raise the trailer one side at a time by…

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Answer 9

We do have a lift kit for your Lippert 7K torsion axle but the Dexter Torflex Lift Kits like part #K71-725-02 are designed to fit specific Dexter torsion axles. For example, the above mentioned kit will only fit a #12 Torflex axle that has a 5,500 -…

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Answer 10

Based on the pictures you sent in you are in need of the MB Sturgis Propane Hose - 3/8" Female Flare x 3/8" Female Flare Swivel - 15' Item #MBS57FR. If you need just a 12' hose then you'll want the MB Sturgis Propane Hose -…

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Answer 11

It is not recommended to have a hose above 20 feet as then you will start to notice pressure droppage. For that reason I recommend the MB Sturgis Propane Hose #MBS57FR which measures in at 15' long. This hose has 3/8" female flare fittings…

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Answer 12

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…

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Troubleshooting Trailer Wiring Using a 2021 GMC Sierra Truck

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