Compression Ignition Addict
Join Date: Jul 2013
Location: Centerport, PA
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I doubt it's in the bumper area wiring. Otherwise, manually operating the brake wouldn't do anything either. In an after-market controller, there is a wire that taps into the brake lights at the pedal to detect when you've pressed the brakes. Triggering the relay to power the trailer brakes. I'd imagine that the factory controller uses a similar technique, but in a harness some where. To me, it seems like the controller doesn't know you've hit the brakes.
Not sure on your year, but looking at the 2008 F250 wiring diagrams and fuse boxes, here's my best guess...
There's a yellow w/ green stripe wire coming off of stop light relay post 5. This feeds the 3rd brake light and the BOO (brake on/off) relay to the brake controller. If your third brake light works, you know the relay is good. There is a BOO relay feed fuse (#12 - 15A) that could be popped and causing this issue.
2003 Excursion 6.0L 4x4 (w/o rear HVAC)
Insight CTS w/ EGT probe/fuel pressure/coolant pressure/Backup camera
Gearhead SRL tune w/ SCT x4 | Ed's FICM + PHP Eco | River City EGR Delete | Blue spring | DC Power 185
Rotella T6 + Archoil | Optilube XPD | Zerex ELC and DFUser Coolant filter
Hankook Dynapro ATM RF10 | Silverstar ZXE | Excursion high idle mod | Viper Remote Start