Trailer Brakes Stopped Working But Trailer Lights All Still Operate

Published 11/06/2014

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LCD Digital Circuit Tester

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$28.06

Question:

i have a break box on my 2005 avalanch and now they have quit working all lights on trailer work but no breaks

asked by: Gerald B

Helpful Expert Reply:

The first step I suggest for troubleshooting your truck's trailer brake controller is to disconnect the truck and trailer and clean the connectors. If that does not correct the problem, test using a circuit tester like # PTW2992. See the linked video for an illustration of using the tester. Apply the grounded tester to the pin on your 7-Way located at the 5-o-clock position - please refer to the linked photo for guidance. Have a helper press the manual override button on your brake controller while you test. You should detect a signal on the 7-way's pin for the brake circuit.

If you do then you know the issue is likely to be on the trailer side; if not, then you will want to test the vehicle further upstream at the back of the controller. If no signal appears on the 7-way, then apply your tester to the blue wire at the back of the controller and again press the manual override button. If no signal appears on the blue wire at the back of the controller the unit may be damaged. If you have a signal at the back of the controller but not at the 7-way, then you have a short somewhere in the truck's wiring.

If you do detect a brake signal on the truck's 7-way you can move on to test the trailer brakes directly by applying 12V power to the brake circuit pin on the trailer plug. You can use your truck's battery or a spare battery to apply power directly to the trailer's brake circuit. Be sure to connect the battery negative terminal to the trailer's ground and apply the positive terminal to the pin for the brake circuit. Your trailer's brakes should engage. If not, then you will need to troubleshoot the trailer wiring and brakes from the plug all the way to the brake assemblies themselves.

Start with the connector itself. Look for signs of even slight greenish or whitish discoloration on the contacts. Corrosion does not have to be severe to cause a problem. Check the brake circuit wire all the way from the plug to the brakes, inspecting the insulation for any signs of wear or cracks that could be causing a short. Repair or replace any damaged wire. If no signs of a short are found, then you can check the brake magnets themselves. If any of the copper wire in the magnets is visible, then they need to be replaced. Please refer to the linked article for more information about testing the brake magnets.

expert reply by: Adam R