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noteven's avatar
noteven
Explorer III
Apr 17, 2021

Ford experts - rear turn signal output to trailer stopped

I have an interesting issue today on my 2012 F-350 chassis cab model.

Quick background - when the up fitter installed the flat bed body, light connections were made to the correct wires for body lighting.
Trailer 7 way connector was wired in to the correct chassis trailer wiring.
All as per Ford body builder electrical manual.
The truck and trailer wiring has functioned properly for 18 months.

Last week I towed a trailer for someone else using a 4 to 7 plug adapter. The info center alarmed a "trailer disconnected" "trailer connected" message of and on for 5 or 6 times - the trailer lights were ok and not disconnected...

Today I connected a new to me enclosed trailer. The trailer lighting functioned properly on the previous owners Nissan truck. It functioned correctly on my Dodge truck last fall.
I connected the Ford to it and there was no right turn signal on the trailer.
The LED right turn signal on the headache rack on the flatbed flashed, the 2 at the rear of the truck bed did not.
Left side OK, tail and brake lights all ok. Front turn signals and mirror signal lights ok.
Disconnect from trailer all truck lights work properly.
Connect truck to a second trailer.
No turn signals work at all at rear. Mirror and front lights work.
Disconnect second test trailer.
Now no turn signals on the truck flatbed work at all. Mirrors and front turns work.
Turn/stop fuses are ok in cab fuse box.

I understand there are relays for the trailer turn circuits in the under hood power center. I have to get back in to the electrical manual to see if relays exist for body lighting.
Park and clearance lights work fine on the flatdeck and cab.

It is like a module of some kind has turned off or possibly failed. There are no error messages on the info center on the instrument panel.

Anyone experience such a step by step failure?

Signed
Confused by Fords sometimes..
Tow Vehicles

9 Replies

  • Chuck___'s avatar
    Chuck___
    Explorer
    May 11, 2021
    If anyone is have problems with your truck display not recognizing that the trailer is connected your left and right turn signal flashing rapidly in the truck, here is the solution. This connector will save your days and hours trying to hunt down the problem.

    Curt
    7-Way RV Blade LED Electrical Adapter (Not a Wiring Extension) #57003
    MSRP $31.75 USD

    Enables vehicles with a 7-way RV blade socket to recognize LED trailer lights 7-Way RV Blade LED Electrical Adapter (Not a Wiring Extension) #57003
    MSRP $31.75 USD

    Enables vehicles with a 7-way RV blade socket to recognize LED trailer lights
    Built-in, integrated resistors for computers that don't recognize LED lights
    7-way connects tail, brake, reverse, turn signals, brakes and auxiliary power
    Simple plug-and-play design eliminates the need for cutting or splicing
    Spring-loaded dust cover keeps connector clean when not in use
  • dodge_guy's avatar
    dodge_guy
    Explorer II
    Apr 24, 2021
    noteven wrote:
    curt12914 wrote:
    dodge guy wrote:
    Fords of that year have separate fuses for trailer taillight, left and right brake/turn lights.


    Correct answer.


    Update - closest answer ^ - but not any of the 3 - 15 amp fuses for each turn signal circuit or the brake circuit...

    Not any of the 3 relays...

    Not any kind of “module”...

    There is also a 25 amp fuse in the under hood power center that fuses all three relays and all 3 fuses in the other fuse box / body module... it was blown.

    Simple enough system once you find the info.

    It’s in the owner manual the previous owner “lost” that I have been too lazy to replace...

    Young tech at the dealership walked me through it in 5 minutes. He recommended a owner manual cause Ford fuse boxes are numbered not labelled.

    Signed
    A bit less confused by Fords


    Glad you got it figured out. Yes, Fords fuse boxes are ridiculous. However Mopar is going the same route, although some are still numbered with the description. Also good to know that there are younger guys willing to go the extra mile for customer service.
  • noteven's avatar
    noteven
    Explorer III
    Apr 24, 2021
    curt12914 wrote:
    dodge guy wrote:
    Fords of that year have separate fuses for trailer taillight, left and right brake/turn lights.


    Correct answer.


    Update - closest answer ^ - but not any of the 3 - 15 amp fuses for each turn signal circuit or the brake circuit...

    Not any of the 3 relays...

    Not any kind of “module”...

    There is also a 25 amp fuse in the under hood power center that fuses all three relays and all 3 fuses in the other fuse box / body module... it was blown.

    Simple enough system once you find the info.

    It’s in the owner manual the previous owner “lost” that I have been too lazy to replace...

    Young tech at the dealership walked me through it in 5 minutes. He recommended a owner manual cause Ford fuse boxes are numbered not labelled.

    Signed
    A bit less confused by Fords
  • noteven's avatar
    noteven
    Explorer III
    Apr 19, 2021
    Update-

    Off to Ford dealer this week to connect to the Ford mother ship for diagnosis.
  • noteven's avatar
    noteven
    Explorer III
    Apr 17, 2021
    curt12914 wrote:
    dodge guy wrote:
    Fords of that year have separate fuses for trailer taillight, left and right brake/turn lights.


    Correct answer.


    As mentioned in my original post the trailer circuit 15 amp fuses did not blow.

    There are also relays in the trailer circuits in the truck power center.
    Testing of them is next order.

    More info: the trailer right turn circuit was tested direct to a battery. Does not work - tiny spark when jumping to the terminal on the 7 way suggests a short in the trailer.

    Consumer OBD scanner does not show any codes.

    I suspect Nv Guy is on the right track.
  • curt12914's avatar
    curt12914
    Explorer
    Apr 17, 2021
    dodge guy wrote:
    Fords of that year have separate fuses for trailer taillight, left and right brake/turn lights.


    Correct answer.
  • dodge_guy's avatar
    dodge_guy
    Explorer II
    Apr 17, 2021
    Fords of that year have separate fuses for trailer taillight, left and right brake/turn lights.
  • byronlj's avatar
    byronlj
    Explorer
    Apr 17, 2021
    Years ago a friend from Wyoming with a new Ford p/u offered to tow my boat to Lake Havasu for me. I hooked the boat up to my p/u and all lights worked when I took it to meet him for the swap. Hooked up to his p/u and the right turn signal didn't work. I had read somewhere about the control module on the new Fords. I slammed the driver's side rear door and the turn signal worked. This had to be about 2009. And I am not making this up.
  • Nv_Guy's avatar
    Nv_Guy
    Explorer III
    Apr 17, 2021
    Not sure what I know about the Ford methods applies to 2012's, but here goes: The body tail / turn runs thru the Body Control Module (BCM). This module switches the various lights on and off and monitors the current load. If it senses excessive current, it shuts that circuit off. Now here's the lousy part- apparently it only does this a fixed number of times before it sets a permanent non resettable code and kills that circuit- and you have to replace the BCM to make everything work. Stupid way of doing things to me, but I am not the guy selling BCM modules.
    There are fuses for the BCM, so if you're lucky, one of them blew, but since the FT & mirror lights work I am not 100% sure about the fuses.
    Due to the mixture of one thing working and others not, I would verify that the 7 way is actually connected to the trailer lighting circuit, as that circuit is old school with fuses & relays.
    Ultimately I suspect the two trailers you pulled had some electrical issues that the BCM didn't like, hence the non op RT Turn.

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