09 F-250 Super duty. 5.4 V8 Auto 4x4.
No Brake Lights or Shifter unlock. Power to NEW Switch. TILL I apply switch then it Looses Power.
Tested all Fuses and Relays (GOOD) Had 11.9 V to input yesterday now 2.6 Today ?
If I jump 12v to switch input I have Lights.
What do you guys think ? I need help as this is My Dads only Truck.
Thank you up Front !

 

Are you blowing fuse 12 or 13? I’m guessing fuse 13 as that’s the input to the SJB to control the brake lights.

Not much attached to the brake switch circuit, start with the easy stuff and unplug the SJB and PCM (be gentle with the PCM plug and do lots of wiggling so it doesn’t break) and see if the fuse blows with those unplugged.

 

No Blown fuses or Bad Relays. Thats the Kicker. it must be in a connecter or harness ?

 

Check fuse 13 in the under hood fuse box, pull it out and look for corrosion, put the fuse back in and measure voltage on both sides(if you have a incandescent test lamp, that would be better to test with). From the under hood fuse box, the Grey/Red wire runs to C145, that’s below the steering column when you remove the trim piece on the dash. From there it goes to C237 which is on the right side of the steering column (there’s maybe 6-8” between these plugs) and then it goes to the brake switch.
The wire flow is from battery fuse box to engine bay harness, then to dash harness, then to cab harness and then brake switch. If there is going to be a wire issue it’s either going to be from the fuse box to firewall section of harness which is right above the drivers side wheel well, or from the brake switch to cab harness. This is assuming no one has molested under the dash at all. (If an aftermarket remote start was installed at one time, really check the wires around the brake switch as they tie into the brake switch)

Start with testing both sides of the fuse, if that’s good then disconnect C145 (large plug) and see if you have power on pin 46 (female side of plug), if that’s good then plug C145 back in and unplug C237 and see if you have power on pin 34 (male side of plug), if that’s good then your problem is in the cab harness to brake switch. Use a test light or small automotive bulb to test these to eliminate poor connections that will look fine on a multimeter.