Troubleshooting Low Brake Output Circuit Voltage on Trailer Wiring

Question:

I have a 2012 F250 with the factory brake controller and a 2008 Fuzion 393. The brakes were horrible. Come to find out the factory used 16 gauge brake wire for 6 brakes. I installed 10 gauge wire to each brake. I checked the voltage at each brake Im getting 9.6 volts. At the junction box Im getting 10.7 volts. These voltages were with the slide manually squeezed. I then check voltage at the plug with the trailer disconnected and I got 13.7 volts on the truck side so I began to wonder if the 7 way on the trailer side was bad, disconnected the brake wire from the junction box. With the manual slide squeezed the terminal at the end of the trailer plug read 13.7 volts. Is it normal that the truck detects the magnets and s the voltage from 13.7 to 10.7? All these voltages were with the gain on 10.0 which is max. I have also inspected all the brakes and readjusted the shoes and thats what led me to change out the wires.

asked by: Derrick

Helpful Expert Reply:

The more resistance a circuit has the more the voltage will drop. The longer the wires are on a trailer the more resistance is put on the circuit. What I would recommend you do is check the conditions of all of the connections of the brake output circuit from the trailer connector all the way back to the assemblies themselves. If there is unnecessarily long wires anywhere make them shorter.

If the trailer connector has 13.7 volts and then from there to the junction box it drops down to 10.7 volts then it sounds like there is some resistance there. Check the connections and wiring looking for anything that might need attention such as corrosion or shorts. You also want to make sure the junction box has 10 gauge wire run to it as well.

The next place to check would be the brake magnets. I attached an FAQ article on troubleshooting bad magnets for you to check out as well.

expert reply by: Jameson C