I want to install a momentary switch so that when the trailer parking lights are on, I can flash/interrupt them like the big rigs do. I bought a marine grade momentary switch (normally closed) rated at 10 amps. Can I simply intercept the parking light wire at the factory headlight switch and run that through the momentary switch. I realize that it will flash/interrupt both the truck and trailer lights, and only when set to "parking" or "headlights on" and not "auto", but I'm ok with that. It's a '14 f250. I've seen posts with guys do this setup to earlier model year trucks with the brown wire at the switch, but I know the 11 and newer are wired differently through relays to the BCM. One doc I found showed that the leads are (-) not (+). I think it's the yel/org wire I'm after. Can anyone confirm?

 

Here's a diagram. You are correct, the headlight switch is wired to the steering wheel control module, and then that module gets on the network and communicates to the body module to turn the lights on. There is a parking light relay inside the body control module.

http://www.revbase.com/BBBMotor/Wd/D...Pdf?id=1010695

Here's another diagram.

http://www.revbase.com/BBBMotor/Wd/D...Pdf?id=1010696

I think what I would do is find fuse f30 in the body control module. I am not sure where the body control module is, it's pretty confusing. Ford has or had what they called a "smart junction box" which was a fuse box with a circuit board made into it. Now they say they have a "body control module" and it has fuses in it. See if you can figure out where f30 is located.

Once you find f30, turn the parking lights on, and then go in and pull f30 fuse. Go back out and see which lights are still on and which are out. If you are happy with which lights are out, I would find some sort of male connector that will plug into where the f30 fuse goes. I would get two, one for each side of the fuse. Also before you do any wiring, fire the parking lights up again and with f30 fuse out, probe each side and find out which side is hot with 12v.

Once you figure out which side is hot, put your connector in the fuse socket on that side, and then run out and install a inline 15 amp fuse to restore the fuse we are not going to use now in the fuse box.

Then run out of the fuse to your switch. Run back from the switch and plug that wire into the other f30 fuse socket.

Won't this work? And you will not be modifying or cutting any of the original wiring.

 

Dave, thanks so much for the reply. What you said would work, I think. But you got me thinking now... if I'm running wire anyway, maybe I should try to grab the brown wire (12v +) coming out of the battery distribution block on the way to the towing 7-way plug. That would allow me to get exactly what I want, which is to interrupt/flash only the trailer running lights (for whatever reason they're on...auto lights, parking lights on, headlamps on) with a push of the momentary button. See page 2 of this diagram:
https://www.revbase.com/BBBMotor/Wd/...Pdf?id=1010704
What do you think? Am I reading that right? Now finding that harness between the batter junction box and the rear 7-way may not be easy.

 

It would be easiest to simply interrupt at BCM fuse F26 as it will only affect the trailer running lamps.

Dave, you need to find the trailer adapter schematics instead of the vehicle drawings. In the EVTM, it's section 95.

 

Noob question, but what is the best method for doing this, ideally without cutting wires? A fuse tap wouldn't work, would it?

 

I have never tried this before. A fuse tap would work if you used two fuse taps and the fuse was blown out. The fuse tap requires the fuse to be in place.

What I have done before on other vehicles was to solder a wire directly onto the fuse. This was the little bit larger colored fuses, they had a little spot on each side where the metal was exposed. I would tin this spot, it would partially melt the area also, and then solder a piece of wire to this spot. But if you did this, again the fuse would have to be blown out so it would not connect the circuit, you want to connect the circuit with your added switch.

Whatever you do, however you do it, make sure you still have a fuse somewhere in the circuit. You certainly do not want to blow out any of those internal relays and have to replace the whole fuse box or body module.

 

F26 (which protects the trailer running lights) in the fuse box under the hood is a 30 amp square female (j-case style) fuse, best i can tell. In that case, I wonder if this would work. It's an extension that I could cut into for the switch, but it also relocates and preserves the original fuse.

Breakout Leads/Boxes

My momentary switch is only a 10-amp switch. Is that a concern?

Also, could I take advantage of the factory up fitter switch wiring that goes through the firewall? Is it thick enough? Switch 1 I believe is rated at 25 or 30a, so presumably it would be.

Finally, should I just use factory up fitter switch #1 in place of my momentary switch? Prob with that, though, is that it's normally open. So I'd have to leave it "on" all the time in order for trailer lights to work, then I'd flip it down to kill trailer lights.

 

I am not sure what the actual circuit load is, but it's usually no where near the fuse rating for the circuit. The fuse is rated for the wire size they use. If it's only for the trailer lamps, I doubt your trailer lights draw over 5 amps, especially if they are LED.

To use your upfitter switches, you would have to modify more wiring. They are pre-wired by the factory to the fuse box with a fuse for each one. You would have to cut the upfitter loose and add your wiring to it.

 

Cole Hersee has a switch that does exactly what you want.