Guess the cause of my truck-trailer brake malfunction

[Tony Zona]

Here's a puzzler, and I'd like your thoughts. It's automotive, but what the heck?

My truck was unable to move my fifth-wheel camper the other day because the brakes were applied without my foot being on the brake pedal.

The camper dealer identified the problem, but not the cause. The truck's factory tow package applies 12 volts to the brake line thereby applying the trailer's electrical brakes. When the key is on, the brakes get 12 volts or so.

My mechanic went over everything he knew of and didn't find the cause, and he is good.

The truck will be going into the Toyota dealer later this week.

What do you think is the cause?

[Chris Padilla]

Seems to me the brakes are getting voltage applied without the brakes being applied indicating something up in the wiring.

Are there any corrosion issues in the hook-ups? I'd put a meter on the plug where the trailer's wiring connects to your truck. If that checks out, then time to check the trailer's wiring.

[Jerome Stanek]

It could be the running lights are powering the braking circuit. I would test each lug at the trailer plug on your truck and see if there is any power there

[Adam Herman]

The 12 volt supply for charging should be pin 4, assuming you are using the modern 7 blade connector. brakes are pin 2. if your truck has a trailer brake controller (should be a gain up/down and a switch to apply the brakes in the cab) this will be wired to pin 2 of the connector. I am unaware as to toyota and if they have a factory controller like my GMC. Did you test the battery and the saftey switch on the trailer? The plug should have 0 V on pin 2 when you are not stepping on the brakes, and ramp up voltage as you press the brake or activate the switch on the controller.

[Dan Hintz]

It could be the running lights are powering the braking circuit. I would test each lug at the trailer plug on your truck and see if there is any power there

This was going to be my guess... if power is supplied to the brakes from key-on and beyond, it's likely connected to the wrong wire.

[Tony Zona]

Chris, yep. I think something is definitely up with the wiring. We did find a critter nest in the main fuse block container, but no chewed wires.

Adam, battery and safety switch checked out.

Jerome, the only lug that has power when it should not is the brake lug.

Dan, this truck and camper have been functioning well for five years or so, including four or five trips up and down the east coast.

And, by the way, we tested with brake controller in and out of the circuitry. The camper dealer pronounced the controller alive and functioning well.

[Tom M King]

There is a short somewhere between the controller and the plug on the truck if that lug on the truck plug is always hot. I'm not familiar with Toyota trucks. Is the brake controller factory installed, or is there a controller installed by someone else? A lot of trucks with tow packages come with a plug for a controller already wired into the trucks wiring, but the controller itself is not part of the package. All controller manufacturers sell adaptors so their controller can plug into the plug waiting for it in the truck.

If it's not factory installed, which controller is it?

[Jerome Stanek]

Your brake controller is most likely the problem then check the sensitivity adjustment.

[Tony Zona]

Tom,
The controller is a Tekonsha Voyager. A pigtail came with the Tundra, and it plugs into a factory tow package receptacle under the dash. And then it is connected into the controller.

The camper people installed the controller, and the fifth wheel hitch.

Jerome, the controller was checked and eliminated.

[Tom M King]

If the controller was checked, how did they check it? If they tested at the plug for the trailer, then the problem is in the wire going to the brakes on the trailer getting power from somewhere else.

[Mark Blatter]

I had the same issue a few years ago. A friend built a new trailer wiring plug for me, changing the positions on two of the wires. It was as other said, I think the running lights and brake wires were being switched in the plug. Never had an issue with any trailer being pulled by the same vehicle since.

[Bill Orbine]

I'm gonna take a gut hunch and say the controller has poor grounding.

[Tom M King]

Grounding is the first thing I check when something on a trailer doesn't work, but not high on the list when something works when it's not supposed to. Trailer wiring can be very aggravating, especially since most manufacturers use copper coated steel wire. The trailers that I haven't had wiring issues with are the ones I changed years ago to tinned copper marine wire, and all soldered connections.

[Adam Herman]

good idea Bill, Grounding is an issue on trucks and trailers that can cause many odd symptoms.

[Kev Williams]

I have a little (bad) experience with this, having caught a set of electric trailer brakes on fire once...

I'll skip the long story and go straight to the cause and effect, which took many days to figure out...

The brakes activate when the pedal is pressed, and the brake light circuit is used as the 12v source, yes?

So what did I find? After searching everything to do with the wiring, I finally got around to checking the tail light bulbs, and what did I find? The left tail light on the trailer, the brake light filament had broken away from one of the posts and was dangling, and it eventually got stuck to the TAILLIGHT filament... and when it got dark and I turned the headlights on, the taillight filament energized the brake light circuit... since the right brakes weren't working due to a loose wheel, the left brake was the only one working, and since we weren't decelerating the brakes were minimal-- I thought the headwind was the source of all my drag. But within a few miles I noticed smoke on the left wheel and when I pulled over the wheel was in flames!

That's how mine happened, but it took turning on the taillights to do it.

Not sure if a light bulb filament is your culprit, but maybe worth a look. I'm more inclined to think you have something going on in with your plug, or where the brake controller is initially wired up. I've seen auxiliary spades and whatnot jammed into the fusebox come loose, if/when that happens it could make contact with a different hot spot in the fuse box...