Trailer Brakes Do Not Work when Using Brake Controller But Do With a Car Battery

Question:

I have a 97 Chevy 3/4 ton Diesel that I pull my 33foot Fifth wheel with tandem axle brakes. There is a 7 pin connector that is used between the two. When I pull there are no known brakes working on the trailer. The pads have been adjusted and I have disconnected each magnet and checked to make sure they activate and they all draw about 3.5 Amps when hooked up to a 12V car battery. I then connected a car battery at the trailer connector that plugs into the truck and the brakes work good too. I thought it was a bad controller but Im getting around 10V to 11V at the truck connector when the brake pedal is applied. When the trailer is hooked up the lights work and the brakes are the only things that do not work. I am mostly confused now and Im not sure what to think? The only thing I can think of is that the controller is not supplying enough current to provide for all four brakes while the battery is. Any help would be appreciated as now Im at my end of logical ideas!

asked by: Mark

Expert Reply:

With a total of 4 brake assemblies, the brake controller power wire will need to be connected to at least a 20 amp circuit breaker and then to the battery. If it is on a lower capacity breaker or fused power source then the draw of the magnets could be too great and cause the brakes not to work. When connected directly to the battery the brakes can draw as much amperage as the battery is rated for.

If there is any sort of short, corrosion, a bare wire touching metal somewhere, that is going to cause an overload and should cut off power to the brakes. With the trailer plugged in and testing the brake circuit behind the connector on the trailer side does the power make it to that point? How far back does it go? I would test to see if there is a break in a wire from the connector to the brake magnets while also looking for nicks in the wiring.

Make sure that the 7-Ways are grounded to corrosion free bare metal surfaces and do the same for the brake magnets. A weak or bad ground can cause this problem as well.

expert reply by: Michael H