Will 12V Wire at Vehicle End Trailer Connector Be Hot Even When Truck and All Lights Are Off

Updated 06/01/2020 | Published 01/24/2016

Question:

Does the black 12v wire stay hot, even when the truck isnt running with no lights on?

asked by: Brian

Expert Reply:

The 12V wire at the vehicle-end trailer connector being hot even when the truck is off can vary depending on the application. For all aftermarket hardwiring installations of a 7-Way wiring harness, the 12V wire will be hot at all times, even when the truck is off and no lights are on in the trailer. This is because the 12V power wire is ran from the vehicle battery, then typically to a circuit breaker, then to the trailer connector.

In OEM applications, it will vary depending on the make of the truck. For Chevrolet and Dodge trucks, the 12V wire will always be hot, even when the truck is off.

Ford trucks can vary depending on the year and make. Some Ford trucks have a relay that requires the ignition to be on for the 12V wire to have power. Other Fords will have a hot 12V wire all of the time. I believe Toyota trucks are also this way.

expert reply by: Victoria B

Scott N.

5/28/2020

I have a 2011 gmc 2500. When a trailer is plugged into truck with power off I still have power to passenger side rear trailer light. I put the light tester plug on truck it showes power to passenger side. All other lights work when truck is on but I have no turn signal on the trailer but the light is on. I know the issue is on the truck side because all lights shut off on trailer when truck is shut off

Chris R.

6/1/2020

I would give the trailer connector on your Sierra a good visual inspection. Look for any corrosion or debris that may be built up inside the pins, cleaning out as needed. I'm thinking some corrosion might be causing a short on that trailer turn signal circuit that's bleeding over to the truck's tail light circuit. Let me know what you find and we can go from there.