Trailer Brakes Only Work when Interior Light Switch is On on Horse Trailer

Question:

On my horse trailer I am trying to connect it to my 2009 GMC sierra 2500. The running lights work and the left turn and right turn signals work. The problem I have is that the trailer brakes do not show any output until I flip the internal light switch of the trailer on. The internal light blinks one time and then the truck shows trailer connected and trailer brakes will work as long as that light switch is on. If I turn that light switch off and on a couple times I get check trailer wiring. I cant for the life of me figure out what I am doing wrong. The internal trailer lights wont work either. I have the 7 way connector wired up according to the diagrams. The trailer has a red wire which I assume is the + a black wire which I assume is the ground a yellow wire which appears to be the left turn a green wire which appears to be the right turn a blue wire which I assume is the electric brakes a brown wire which appear to be the running lights.

asked by: Richie

Expert Reply:

With trailer wiring, you can't make any assumptions. The wires have to be tested to determine the functions. An easy way to do this would be to use a separate 12 volt battery and apply power to each wire on the trailer and then see what turns on. Then wire up the trailer connector accordingly.

There may be breakaway system that is wired in with the 12 volt circuit from the trailer connector so when the switch is off it basically closes off that circuit. That would explain why the brake controller does not sense the trailer magnets when the switch is off.

If there is a breakaway system involved then there is a break away battery as well and if the interior lights are trying to run off of it they may not work depending on the charge level and voltage rating of the battery.

You will also need to make sure that the truck and trailer side connectors are all clean and free of corrosion inside and out. And make sure that the grounds for the trailer connector and trailer lights are attached to clean and corrosion free bare metal surfaces.

expert reply by: Michael H