Troubleshooting Trailer Brakes That Lock Up with Trailer Disconnected from Tow Vehicle

Updated 04/20/2016 | Published 04/18/2016

Products Featured in This Question

Pollak Replacement 7-Pole, RV-Style Trailer Connector Socket - Vehicle End

(745)

$15.62

10 Gauge Wire - Blue Primary - Per Foot

(108)

$1.61

Hook Probe Circuit Tester

(66)

$14.40

Question:

My trailer brakes lock up when I put the truck in Reverse even when electrical is unplugged. Never had a problem. Its 12 years old. Also Ive checked all wiring from truck. Pretty sure theres a short somewhere in trailer wiring.

asked by: Terri R

Helpful Expert Reply:

If your trailer's electric drum brakes are locking up even when the trailer is disconnected from the tow vehicle, then it sounds like the battery on the trailer is somehow finding a connection to the brake circuit. This could be from something as simple as a case of corrosion in the trailer connector to damaged/shorted wiring along the trailer frame.

Please refer to the linked article on trailer wiring for images of the 7-way RV-style plugs typically found on trailers, such as # H20046 from Hopkins. Check the contacts (both inside and outside, if possible) for any signs of corrosive build-up on the contacts. Bad corrosion can cause two circuits to become bridged together. Also trace the brake circuit wiring all the way from the trailer connector to each of the brakes. Check especially for wear marks, cracks or other clear damage to insulation, especially at points where the wire has to make sharp bends and where it comes into contact with the trailer frame. If you find bad brake circuit wiring, you can use # 10-1-1 to replace it.

You can also test the vehicle socket using a circuit tester like # PTW2993. Have a helper activate each function on the truck while you apply the grounded tester to the circuits on the 7-way. This will tell you whether the problem is exclusive to the trailer or if there is an issue in the vehicle wiring too.

If your truck's connector does not test okay, then please let me know the year/make/model/version of the vehicle and I will be glad to help you with a replacement wiring harness.

expert reply by: Adam R