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.

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.

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

@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

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

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 3

Sounds like you've followed every troubleshooting tip I could offer you Omar. I do think you need to reach out to GM about your wiring issue especially knowing there's a recall.

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Answer 4

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…

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Answer 5

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 6

For a brake controller I would recommend the Prodigy P3 Brake Controller part #90195 for basically every towing situation but especially for an electric over hydraulic setup like what you have. This is our best selling brake controller because of how…

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Answer 7

What we have found is the integrated trailer brake controllers on trucks can be very temperamental when used with electric over hydraulic brakes. I do recommend using a HydraStar Electric Over Hydraulic Actuator Adapter Module #HBA-CAM with your…

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Answer 8

If you're speaking of the OEM brake controller, then the ITBC on the 2021 GMC Sierra is compatible with electric over hydraulic braking systems such as the Dexter brake actuator used on your Keystone Montana Legacy. If your truck has the OEM…

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Answer 9

The integrated brake controller of your 2022 GMC Sierra 3500 has a hydraulic mode which would allow it to work with electric over hydraulic actuators without needing an adapter. With that said if for some reason the controller wasn't recognizing…

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Answer 10

As you can see on the attached photo, the wiring diagram for the actuator shows there can be a 25-40 amp circuit breaker in-line with the black wiring which goes from the battery's positive terminal to power the actuator. This is actually only…

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Answer 11

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 12

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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Troubleshooting Trailer Wiring Using a 2021 GMC Sierra Truck

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