2016 F-150 XLT, 2.7, 4x4, all the options.

Grab your popcorn and prepare to get a good laugh out of my stupidity. About 5 months ago I put 35x12.50 and a 2in leveling kit on the truck. Rubbed a lot in reverse still so I took a pie cut out of the rear crash bars and welded them back together (controversial but better than removing them all together). What I wasn’t aware of, was the massive BCM wiring harness hiding behind the driver side rear crash bar. You guessed it, I cut some of the wires. I repaired them as properly as possible and reinstalled the conduit and taped everything up nice. No issues with the truck, everything worked.

2 days ago I’m going down the road and the truck flashed the message “Trailer connected” then “Trailer rear left lamp malfunction, check trailer” then “Trailer disconnected”. It did this about every 15 minutes or so, while driving. Yesterday, I’m driving and suddenly had hyper flash. Had my buddy get out to see which bulb was out, and they’re all out. No rear turn signals, reverse lights, tail lights, or brake lights. When the headlights are in auto, the turn signal will flash the license plate lights. And the brake lights will also illuminate the plate lights. I obviously checked the wiring I cut first, everything was solid, no corrosion, actually re-did a few connections just to make sure. The wires are coming out of C316 and C275, I used alldata and verified all the wires for anything in those circuits were solid. I cleaned the ground on the back frame rail. Forscan shows some weird stuff, I’ll post below. I tried resetting BCM and TRM did the self test to see what codes it had.

B1247:15-48 - Left Rear Turn Indicator Circuit Short to Battery Or Open

B1445:15-48 - Rear Park Lamps Output Circuit Short to Battery Or Open

B1330:02-08 - DC/AC Inverter Module General Signal Failure (AC outlets work fine)

B1D08:12-28 - Left Trailer Direction Indicator Circuit Short to Battery

Tried unplugging the trailer harness to see if that would make a difference, none made. Unplugged the battery for 15 minutes as well and made no difference. I have no clue what is going on, fuses are all good too. I wanted to replace that harness coming from C316/C275 but had a hell of a time finding it, and the dealer wasn’t helpful either. I’m not sure if I got unlucky and fried the BCM or if there’s a simple fix for this somewhere. What I don’t understand is how this suddenly came up, I would’ve thought I’d have issues months ago when this first happened. Any suggestions are appreciated, I feel like a bit of an idiot over this whole thing.

 

I would unplug all the connectors at the back of the truck by the spare tire. They have a tendancy to get water inside and this can cause all sorts of issues. C405 often leaks water through the lager terminal plugs on the outsides.

 

I’ll definitely try that. If things already aren’t weird enough, I went to continue diagnosing it a few minutes ago and everything works fine. All the BCM codes still exist, and when self testing it shows three more codes.

Headlight On Input Circuit Short to Ground

Headlight Off Input Circuit Short to Ground

Auto Headlamp Circuit Short to Ground

Ill check out those connections and see if there’s any change.

 

Turn your Headlamp Switch to "Off" and test. I think you will find that some of those DTCs go away. If not I would start checking for water intrusion into the BCM. Do you have a Vista Roof?

 

Yeah they stick around even with the headlamps in the off position, and yes I have the panoramic sunroof.

 

The Vista Roof is favorite leakage point for water getting into the BCM or the BCM connectors. Have you checked all the C2280 connectors? Is the ground good? It is just beside the BCM

 

This may sound weird, but I took the truck through an automatic car wash to see if I could find water getting anywhere. BCM is dry and that ground is clean & secure. I did pull those 4 connectors apart and found that C405 was a little wet. I blew it out with compressed air. As I mentioned, the lights randomly started working again yesterday. Is there a proven way to keep water out of those 4 connectors in case that’s what’s causing my issue?

 

Dielectric grease.

 

What the hell did lectric grease do to you you want it to die mang?

 

Duh, I should’ve known that. Lol. Thanks for your advice and knowledge this has been an annoyingly painful process.