Voltage Back Feeding to Trailer Breakaway Switch - ???

[unclejoey_vt]

Hi, all -

I just wanted to throw this question out to the group to see if anyone has insights to offer on a strange issue I'm seeing.

The story starts with pulling out of a rough parking lot, which unknown to me pulled the breakaway switch cable out of the switch, tripping the trailer brakes. Truck felt sluggish at first, but then started driving normally after the first couple turns getting back on the road. It wasn't until later, when I found the fried remains of the breakaway switch, that I put two and two together.

I picked up a replacement and went to reinstall it, when I realized GDRV used two black wires to the original switch. I rang out from the 7-pin connector to find which was connected to the electric brake circuit from the truck, connected that to the blue wire on the new switch (per instructions), and connected the other wire to the wire back to the trailer's electric brakes. Makes sense so far, right?

The mystery comes from the fact that the new NAPA switch I'm using has a test LED on it. The LED supposedly lights when the pin is pulled to show if things are working properly. However, when I pull the pin to check, the LED lights up even without the harness connected to the truck. Isn't the supply from the electric brakes supposed to come from the tow vehicle? Any idea why there would be voltage on that from the trailer side?

We're hitting the road again tomorrow, so I'm hoping your replies might give us a little extra peace of mind.

Thanks!

[jkwilson]

The breakaway is fed by the trailer battery. It wouldn’t do much good if the source of power was on the truck in the event of a breakaway.

[unclejoey_vt]

Doh! Got it. The blue wire is the normal signal from the truck's brake controller to the brakes. The other wire is the 12v source from the trailer, which gets dumped to the brakes when the cable is pulled and the breakaway switch closes. Makes total sense.

Thanks for the quick reply!

uj