Should Blue Wire From Brake Controller Have Constant Power

Updated 04/07/2025 | Published 07/27/2018

Products Featured in This Question

Tekonsha Voyager Trailer Brake Controller - 1 to 4 Axles - Proportional

(730)

$84.94

Tekonsha Prodigy P2 Trailer Brake Controller - 1 to 4 Axles - Proportional

(2710)

$134.95

Hook Probe Circuit Tester

(58)

$12.11

Question:

I am hoping you can help. Should the blue wire from the back of the controller have constant power?

asked by: Daniel

Expert Reply:

The blue wire coming from the back of your Tekonsha Voyager Brake Controller # 39510 is for brake output to the 7-Way connector. This wire should only have power with the brake pedal depressed or the manual override activated on the brake controller. If you are getting constant power on this blue wire this may be an indication that the controller is bad but there is an easy way to test this.

You will want to first sever the blue wire several inches from the back of the controller. Then use a circuit tester like # PTW2993 to test the end of the blue wire coming out of the controller. This should only have power when you apply the manual override. If you are still getting power there at idle, then there is an internal issue on the controller and it will need to be replaced.

If a replacement is in order, I highly recommend the Tekonsha Prodigy P2 # 90885. This is a great proportional controller that features continuous diagnostics to alert you to any potential issues and comes with a limited lifetime warranty.

If it tests normal (only showing power with the manual override) then go ahead and reconnect the blue wire. You will then want to move back to the 7-Way connector on your vehicle. Inspect the connector for any dirt or corrosion that might be built-up around and behind the pins. This can cause a short that might be bleeding over into the brake output pin. Clean the connector out as needed.

Then test the brake output pin on the 7-Way (5 o'clock position) to make sure it is only getting power with the manual override on the brake controller activated. If it shows power at any other time, then the issue is somewhere between the brake controller and the 7-Way. If this is the case, you will just need to inspect the wiring along the vehicle for any damaged or exposed sections, fixing/replacing as needed.

expert reply by: Chris R

Nick M.

7/15/2020

The test you describe above... is it implied that the trailer is plugged into the back of the tow vehicle? Is it true that the blue wire would test differently with the trailer plugged in or not? Thanks,

Chris R.

7/20/2020

If you slide the manual override on the brake controller, it should send back full power on that blue brake output wire regardless of whether or not a trailer is attached. So you can perform this test without hooking up the trailer.

Dave D.

4/6/2025

@ChrisR What about power on the trailer side? I have 12v on the blue pin at the connector(trailer side)

Timothy

4/6/2025

@ChrisR I am having a similar issue trying to figure out what post go where on my new seven pin junction box it seems they mislabeled some of them. The Blue post that they say I should put the brake controller wire on is 12 V hot when the engine is running I don’t think this is correct. There is another post that doesn’t have power. I wonder if that should go there.

Kate F.

4/7/2025

@Timothy Hey Timothy, You're right to question the labeling on that junction box. The blue wire is supposed to be for the brake controller output, which means it should only have power when you're pressing the brake pedal or using the manual override on your controller. It definitely shouldn’t be hot just because the engine is running. If that post is constantly live, then it’s most likely mislabeled. On a standard 7-way RV plug, the blue wire is for the trailer brakes, the black wire is your 12V power (used for charging a trailer battery or running 12V accessories), and the white wire is ground. There are also wires for turn signals, tail lights, and reverse, but for this situation, focus on those main three. You can use a multimeter to check which post is which, or something like the # PTW2993. The black (12V) should be hot with the ignition on, and the blue (brake controller) should only show voltage when you hit the brakes or use the manual slider. It’s not uncommon for junction boxes to be labeled incorrectly, so the best bet is to go by function, not the printed labels. Once you figure out what each wire does, you can relabel the box to avoid confusion later. Let me know how things work out.

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