Pikkup

'16 SCAB, trlr tow, OEM brake controller.
I get a dash warning every once in a while saying trlr wiring fault, and it is when braking, and it will cause the trlr brks to not work... sometimes the brakes don't work hitting the brake pedal when the warning is lit, but squeezing the controller and brakes apply as they should...
Any help on what the issue might be?
Plug and wiring all look good at rear bumper, trailer is new...
Thanks!

Flamingtaco

I had the same issue, water and dirt in the 7-pin was the cause.

Months later it came back, ended up being damaged wiring on the trailer. Check the wires where they drop from the trailer to the brake, they are exposed there and can easily become damaged.

i2oadi2unnei2

If you don't have one of these, I suggest you get one: https://www.powerprobe.com/ Very useful tool along with an ohms meter. Sorry not much of help and I hate dealing with wiring issues myself

Charles Rollman

I moved up to an F250 and a fifth wheel trailer, but the my brake controller is pretty much the same as on the F150.
I've got a problem with intermittent "Trailer Wiring Fault" popping up on my new rig. I called the Ford dealership and asked what triggers the "Trailer Wiring Fault" error message. The service guy was quite willing to connect my truck to his trailer simulator, but couldn't answer my question. So, I decided to figure it out.

I made a trailer simulator by connecting a 4 ohm/ 50W resistor from my junk box between the brake wire and ground of a spare 7-way connector. An old headlamp would probably work too, and the brightness would be an indicator of the brake output. The resistor/headlamp makes the truck think that trailer brakes and therefore a trailer are connected.

I plugged my simulator into my truck, and started it. When I hit the brakes, the truck recognized that a trailer is connected. When I squeezed the paddles on the brake controller, the bar graph shows braking action that goes up or down as expected.

With the truck still running, I shorted out the resistor in my simulator. When I did this and hit the brakes, the "Trailer Wiring Fault" popped up. So a short to chassis from the brake wire is at least one problem that will trigger this error.
Also, with the short across the resistor, when I move the brake controller paddles, the bar graph pops up but shows zero braking. This suggests that the brake controller bar graph shows the actual measured output voltage to the brakes. A short circuit causes zero voltage and therefore a zero display.

Again with the truck till running, I removed my jumper, and the brake controller and bar graph behaved normally.

Then with the truck still running, I unplugged my simulator and got the "Trailer disconnected" message, but not the "Trailer Wiring Fault" message. This suggests that an intermittent open connection at the 7-way plug won't cause the "Trailer Wiring Fault" message; it will however cause the "Trailer Disconnected" message.

I conclude that "Trailer Wiring Fault" means that there is a short from the brake wire to chassis. While this could occur due to water in the 7-way connector, such a short would have to be very low resistance compared to the brake magnets, which is not a high probability. The most likely cause is a wire with damaged insulation that is contacting the chassis. This kind of fault will likely come and go as the wire moves.

Coolerman

Good research Charles! The wiring fault could also be caused by a shorted brake magnet.
On my recent 2600 mile trip out west I had intermittent Trailer Disconnected messages going out.

Once we reached camp and got settled, I made up a mixture of white vinegar and table salt. (Basic mixture is 2 OZ of vinegar and 1 teaspoon of salt)
I poured this mixture into the 7 way trailer pigtail connector and let it sit for 2 minutes. Did this several times until the fizzing stopped. Rinsed with plain tap water. This cuts through the brass corrosion layer on the terminals. Once it dried, I put silicon grease into the terminals. On the way home not one Trailer Disconnected message popped up!
If your truck side connector is also corroded, you can put the mixture into a spray bottle and spray it inside the connector. Don't forget to rinse it well and let dry with the lid up!

FYI, if you have used any type of grease in your connectors PRIOR to doing this you will not have a lot of luck as the grease prevents the vinegar from reaching the terminals. In that case, FIRST use a heavy degreaser like Simple Green, Purple Power or 409 and a tooth brush to cut through the grease, then do the vinegar trick. Rinse well, let dry, then put grease back in the connector.

Flamingtaco

Did you try, instead of unplugging the simulator, leaving the clearance lights line engaged and disconnecting only the brake line? By removing the simulator, you are doing the equivalent of disconnecting a trailer. If the system monitors other lines on the 7-pin, it could tell there is an open Vs a disconnected trailer.

I know it's monitoring the left and right turn signals. Just like with the brakes, it can send a short, low-voltage pulse down those lines that will reveal connection status, but not light up the lamp.

PerryB

"Trailer disconnected" means the trailer brake circuit is open/no load. "Trailer wiring fault" means the trailer brake circuit is shorted to ground/excessive load. It almost always either a brake magnet causing the problem, or the wire where it drops off the frame and goes into the axle tube.. If you're getting the "Trailer Wiring Fault" message, it's coming from the trailer brake circuit. Not stop/turn or tail/running. Those are simple fuse-protected circuits whereas the trailer brake has an auto reset breaker and will continuously attempt to restore power.

Flamingtaco

It's doing more than short protection, you get this with an open wire:

PerryB

Agreed, but if the message is "Trailer Wiring Fault", it's the brake circuit.

Pikapp

In my case, it was a bad Brake Controller.