Troubleshooting Trailer Brakes That Engage When Silverado Running Lights Are Turned On

Updated 07/29/2015 | Published 07/28/2015

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Adapter 7-Pole to 4-Pole and 5-Pole

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Hook Probe Circuit Tester

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Question:

I have a 2004 chevy Silverado I hook up my bobcat trailer and when I turn my tail tights on my trailer brales come on. When I hit the button on my trailer brake system it turns my tail lights on but if I unplug my Trailer brake system and turn my tail lights on my trailer brakes do not come on. Is this due to a faulty trailer brake system? The one I am using I had bought less than a year ago brand new.

asked by: Lucas

Expert Reply:

There are several potential causes for your issue.

You could have a corrosion problem in either the vehicle trailer socket or the trailer plug. Because of the many circuits in a 7-way connector it is possible for corrosion, dirt or moisture to bridge two adjacent circuits together. When this happens one function leaks over to the wrong pin. If for example a running light circuit became bridged to the trailer brake circuit the brakes would engage whenever the truck's lights were on. Inspect both the outside and inside of both the truck and trailer connectors for signs of even slight discoloration and clean as needed.

Another possibility is that either the truck or the trailer connector is mis-wired. The linked article shows the usual wiring configuration for all types of trailer connectors. Wire colors alone are not a reliable way to confirm what function is carried so testing with a circuit tester like # PTW2993 is the way to go.

Yet another possibility is that the brake controller's red signal input wire is connected to the wrong wire on the vehicle. The controller's red wire must be connected to the one wire at the brake light switch that carries power ONLY when the brake pedal is pressed and at no other time. You can use the tester to confirm the correct wire at the brake pedal.

expert reply by: Adam R