Troubleshooting Brakes that Only Lock Up When Powered Straight From a Battery  

I RandI all 4 drums on the horse trailer and repacked all the bearings. I had to adjust the shoes out so that they would grab the drums correctly. The issue i am having is once it is plugged into a truck 3 different ones the brake controller does not engauge the brakes when you step on the brake pedal or when you manually engauge them. If you take a battery to the trailer plug and send power through the trailer plug to the brakes it locks them up no issues and you cannot drag the trailer easily when hooked to a vehicle. The break away switch works too and you cannot move the trailer easily without straining the truck. 1 ford factory BC 1 chevy hd with a husky quest and 1 chevy hd dually with a P2 BC. All 3 do the same thing. I did check both truck plugs and found 10.7 vdc applied manually through the BC. In my tester it shows 10-12 vdc applied manually. I replaced the trailer end plug and it made a slight difference. Is it possible i have 3 bad brake controls? Its not a ground issue because the trailer works fine on stand alone. Its only when hooked to a truck does it not work. Going to try one more truck tomorrow that has a basic brake control manual dials no led display. 2 trucks i know for a fact have weak brake controls. One unsure of, and the forth is a positive it works due to bejng a teansport hauler for a RV business owner. Any ideas? I think its in the battery pack and distribution block the trailer plug is wired into. It uses lugs and terminals which are clean. But i think its in multipul eye lits where i am getting a weak connection.

asked by: Mike F