If the brakes are locking, it can be caused by an electrical or mechanical malfunction. If the problem goes away when the plug is disconnected, the problem is usually electrical.

If the problem is electrical, the locked brakes affect is caused by B+ voltage being applied to the braking system at the wrong time. This can be caused by a bad controller, the wrong wiring or adapter, or errant wiring within the plug connection or the trailer itself.

If the wiring were broken, the brakes would cease to work, or only work intermittently. The brakes would not remain locked. If the braking problem is only common to one trailer or during the usage of a particular adapter, then the truck's controller may at first, be eliminated as being the problem source.

The only other source of power to the brake magnets, can be by a short in the wiring. This can usually be found in the plug or socket, an adaptor that is behaving badly, or the trailer itself, that has had changes or maintenance made to its original wiring. This will usually occur at the front near the plug area.