Troubleshooting Trailer That Locks Up Brakes When Plugged in to Tow Vehicle

Question:

2016 ford f250. When the trailer is plugged in the brakes lock up. checked the wiring at the plug and its wired like my other 17 trucks. check the controller the blue wire has power when brake pedal is pressed and no power when its released, yet brakes are locked. please help me its driving me crazy

asked by: Ron

Expert Reply:

Thank you for clearly describing the trailer brake issue you're having.

Since you checked your brake controller and found that the blue brake output wire has power only when you press the pedal this tends to suggest the problem with the brakes locking up as soon as you plug in the trailer is due to corrosion of the vehicle socket or trailer plug.

What can happen inside a trailer socket or plug is that water (or another contaminant) can find its way into the interior of the connectors where it can cause corrosion. This corrosion can cause two adjacent circuits to become bridged together. This could allow, for instance, the right stop/turn signals or those from another circuit to bleed over to the brake circuit. Please refer to the linked photo showing a 7-way. You can often unplug the connector from the back of the socket to see the interior and determine if cleaning is appropriate.

You can sometimes clean out this corrosion with a spray-on electrical contact cleaner like Caig De-Oxit or a similar product, but in other cases the corrosion can be bad enough to require replacing the vehicle socket or trailer plug. If the truck has factory 7-way wiring you can use part # HM40975.
If the trailer's plug turns out to be the bad part you can use cable and junction box # HS381-9000 to replace it.

expert reply by: Adam R