Troubleshooting Voltage Loss on Trailer Brake Circuit Between Truck 7-Way & Trailer Brakes


with truck running, brake controller on manual, I am getting 13.9 volts at the plug. Checking voltage at the brakes, only getting 9.9 volts. Should the voltage at the brakes be higher than it is?


Helpful Expert Reply:

If your 7-way's brake controller circuit shows 13.9 volts when you apply your controller manual override, but the voltage reading at the brakes is only 9.9 volts then you do have some issue with your wiring. A minor voltage drop is normal but losing 4 volts tends to suggest you have one or more wiring issues.

You can check the condition of the contacts in your trailer's male 7-way plug to see if there is corrosion or a bent pin. Trace the wiring all the way to the brake assemblies, looking for worn or cracked insulation, especially at any points where the brake circuit wire has to make sharp bends and where it could come into contact with any part of the trailer. Replace or repair any wiring with damage.

Also check the trailer's main ground wire to ensure it is solidly secured to a clean bare metal surface free of paint, primer, rust, grease, etc. A weak or marginal ground could also limit the power reaching the brakes.

helpful expert reply by:
Adam R

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