Trailer Brakes Come On when tow Vehicle Head Lights are On  

Question:

Expert Reply:

That must make for some interesting night time towing! There are a couple of things that can cause the trailer brakes to activate when the vehicle head lights are on. First is a short between the running light pin and the brake output pin.

This is actually a bit strange if true because the running light pin is in the opposite corner of the brake output pin in a 7-Way connector. To determine if this is the case you can use a circuit tester such as # PTW2993 to test the vehicle side connector. Have someone sit in the truck and turn on the head lights while you test the 7-Way pin (5 o'clock position). If that pin has power when the head lights are on, then there is a short somewhere on the vehicle side. It could be damaged wires to the 7-Way or corrosion inside where the wires attach.

It could also be related to the brake controller red wire and where it is attached to the vehicle. If you use the circuit tester to test the red brake controller wire it should not have power when the head lights are on. If it does then the brake controller red wire is attached to the wrong brake switch wire. So basically what happens is the brake controller thinks that the vehicle brakes are being applied when the head lights are on.

The fix for this is to test the brake switch wires for the one that only has power when the brake pedal is pressed and not when the head lights are on. If you can tell me the year, make, and model of the vehicle I might be able to tell you which wire it is that you need.

If this happens to be a 1999 to 2006 or 2007 Classic Chevy/GMC truck and you are using a plug-in wiring harness then it is possible that the red wire in the connector that plugs into the truck is in the wrong location and needs to be moved.

expert reply by: Michael H

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