Testing to Find Correct Brake Output Circuit at Rear of Tow Vehicle.

Question:

I purchased a Primus electric brake box and I couldnt get brakes on camper to work. Box has power and lights on camper work. Found out that the blue wire on the 7 way located under bumper isnt wired to anything. There is a blue wire under bumper that isnt going to 7 way but is in the cluster of wires that go to harness. Can I splice into that blue wire to get electric brakes working on camper instead of running a wire up to cab to splice into blue wire on back of electric brake box?

asked by: Chris W

Expert Reply:

You found a blue wire in the harness that you believe comes from the brake controller, is that correct? If this is the case, you can have a helper toggle the manual brake switch on the brake controller on and off, while you use a circuit tester like part # PTW2993 or a multimeter to see if the blue wire powers up when the controller is activated. If it does, you have confirmed that wire as being the output circuit from the brake controller.

If you are certain the blue wire you found behind the 7-way is connected to the brake output pin on the 7-way, you can connect it to the other blue wire. If you're not sure, you can use a multimeter and do a continuity test, with one probe on the blue wire and the other on the brake pin on the 7-way (pin in the 5:00 position). If the circuit has continuity, you can go ahead and connect the blue wires.

If you don't see power on the blue wire in the harness while your helper activates the manual switch, you'll need to run your own length of 10-gauge wire (10-1-1) from the blue wire on the brake controller back to the 7-way.

expert reply by: Mike L