My 2012 F350 cab and chassis doesn’t have any rear blinkers or rear brake lights. Front blinkers and the center brake light on top of the cab DO work, but I have no voltage to the back. I thought it might be the Body Control Module, but I also don’t have any voltage to the left or right blinker/brake prongs in the trailer plug. The Ford service guy said he didn’t think the trailer brake lights went through the BCM, but it didn’t sound like he knew for certain. Also don’t get any voltage to the electric brake prong when the brake pedal is applied, but I do get voltage when I use the built-in trailer brake controller. Couldn’t find any blown fuses. It had incandescent bulbs in it. Some were burned out. No LEDs. Anyone have any ideas?
Check the plug on the harness for the rear lights, usually back by the left rear tail light. There may be more connectors up by the cab. Do the tail lights work?
The tail lights DO work. Reverse lights also work. I don’t have any voltage at the rear harness plugs for brakes/blinkers, either side. It looks like all the wires end up near the fuse box on the left side of the engine compartment. There looks to be about 50 wires packed in an area that’s too small to work in. I see 2 large plugs, but I don’t know what’s what.
LN. You said you checked all fuses. Does that include the fuses in that box inside the engine compartment on the left side?
I’m pretty sure all the circuits run through the computer now, if it didn’t blow a fuse the CPU may have shut down that circuit if it thought there was a problem or if it sensed an overload. You may need to go to someone with the ability to get into the cpu and check it and “turn it back on”. I had a friend that had the same thing happen but with the reverse lights, no voltage going to the wires, had it read and realized that the computer had shut down that circuit, it just needed to be turned back on and he was back in business.
I checked the fuses that the book showed to be associated with the brakes, blinkers or trailer lights.
I’ll try this.
Maybe try disconnecting the battery for an hour or so, sometimes it will reset the computers and clear codes.
I tried that. Let it set over night with the battery disconnected. Didn’t make a difference.
Are the rear turn signals and brake lamps LED, by chance?
I'm looking at the wiring diagram trying to wrap my head around this. Does your 3rd brake lamp work, or does it even have one since it's a cab and chassis?
First, I would check fuse. #21 (10amp) Then, I would check for power at the brake lamp switch. Here is the pin out. If you have power at the switch, try jumping it to see if your stop lamps work.
Jump the stop lamp switch like this;
12v power for the brake lamp switch is supplied from the BCM. So, chances are if you don't see power at the switch, it could be a bad BCM. Also, if you don't see power at the switch, you may try carefully disconnecting/reconnecting the harness at the BCM to check for a bad connection. But, it's sort of a pain to access. I fought with a similar lighting system issue on a Super Duty like yours, in Dutch Harbor of all places, over the summer. Holy cow. It turned out to be a bad BCM. I don't have a Ford IDS, nor did anyone on the Island. I had to pay a fella with IDS to fly to Dutch to program the new one for me. But, I digress. Back to your rig. But, you say your turn signals don't work. Do your front signals work and not the rear? If that's the case, chances are you will find a bad ground. Both the brake lamps and rear turn signals charge the same ground. Hopefully it's that easy. BTW, The turn signal circuit actually has diagnostic trouble codes that you can check with a scan tool.
Do you happen to have a wiring schematic for this vehicle? If so, could you send me a private message? I’ll have you email it to me if possible.
PM sent to you, LN Pipeline.
As it is a cab and chassis check the fuses for the trailer lighting connector. The brakes and blinkers in the connector have separate fuses than the truck tail lights and blinkers and both are separate from the truck cab and standard tail/ brake lights. Many flatbeds have been wired connecting into the trailer lighting coupler circuits.