Guess the cause of my truck-trailer brake malfunction

[Ken Combs]

I gotta ask: has any work been done one either the truck or trailer since the last time it worked correctly?

As stated by others, the only way brakes can be applied is to have power on the brake apply pin. First suspect to me is always review previous work. Only then would I suspect a failure in the wiring. If it is like my Tundra, the tail lights are not on when the key is on, only the front DRL. So, that power source can likely be dismissed. But, if yours, like mine, has the factory tow package there is a battery charge pin for the breakaway battery, or other trailer batteries, it will be hot when key is on. A mis-wiring there would do it.

I also suspect that a breakaway switch failure would put 12v on the source wire as well as the brake apply circuit. I'm not sure how the tech tested that function. But if I were doing it, I'd put a diode in between the breakaway switch and truck power lead and test for voltage on both sides. that would determine if the truck is definitely the culprit.

curious problem! Please let us know the outcome.

[Tom M King]

I keep these in the traveling electrical tool kit in my truck. They save a lot of time fumbling with a multi-meter and two leads at the same time.
http://www.ebay.com/itm/231910498308...%3AMEBIDX%3AIT

[Jim Koepke]

It could be the trailer brakes are wired to the brake lights. If the break lights are in the same circuit as the running lights and there is a poor ground at the tail lights. This would then send some voltage to the brake circuit.

I had cars with a problem like this many years ago.

jtk

[Tony Zona]

Ken, the only work has been oil changes and routine service.

I don't know how the breakaway switch was tested.

It goes in to Toyota tomorrow. It's beyond me and my team of crack consultants.

[Tony Zona]

The toyota tech was stumped yesterday.

The truck goes in Monday for examination by an electrical system heavy hitter.

[Jerome Stanek]

I would start by pulling fuses and see when the power to the brake goes off.

[Ken Combs]

Any more info from the specialist?

[Tony Zona]

This took a whole lot of time, and just finished yesterday.

Toyota replaced the hitch plug pigtail, about an 18-inch long wire bundle with a new four-up header and a spade plug for the fifth wheel. And they replaced the brake controller.

Ultimately I found both of those to be good, but they did not work correctly on the truck. Nobody knows why. It had worked correctly for five years. It's kind of maddening, but I can tow the camper now, with brakes.