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View Full Version : Guess the cause of my truck-trailer brake malfunction



Tony Zona
05-17-2016, 4:53 PM
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
05-17-2016, 5:17 PM
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
05-17-2016, 5:28 PM
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
05-17-2016, 6:17 PM
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
05-17-2016, 6:55 PM
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
05-17-2016, 7:00 PM
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
05-17-2016, 7:19 PM
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
05-17-2016, 7:32 PM
Your brake controller is most likely the problem then check the sensitivity adjustment.

Tony Zona
05-17-2016, 7:35 PM
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
05-17-2016, 8:00 PM
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
05-17-2016, 8:07 PM
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
05-17-2016, 8:26 PM
I'm gonna take a gut hunch and say the controller has poor grounding.

Tom M King
05-17-2016, 9:21 PM
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
05-18-2016, 12:28 PM
good idea Bill, Grounding is an issue on trucks and trailers that can cause many odd symptoms.

Kev Williams
05-18-2016, 4:05 PM
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...

Ken Combs
05-18-2016, 6:09 PM
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
05-18-2016, 7:13 PM
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?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Jim Koepke
05-18-2016, 8:43 PM
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
05-18-2016, 9:44 PM
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
05-20-2016, 1:44 PM
The toyota tech was stumped yesterday.

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

Jerome Stanek
05-20-2016, 4:15 PM
I would start by pulling fuses and see when the power to the brake goes off.

Ken Combs
05-27-2016, 9:52 PM
Any more info from the specialist?

Tony Zona
05-27-2016, 10:03 PM
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.