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Thread: Trailer brake signal issue (Red trailer wire to CJB/BCM)

  1. 8th January 2023, 09:18 AM #1
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    Trailer brake signal issue (Red trailer wire to CJB/BCM)

    Morning...

    I have been investigating an issue with my non functioning trailer brakes that were disconnected at some point (wife's car).

    Basically I re joined the cut wires in back of the car and tail light assembly with new diode - as the D4 brake instructions but have this conundrum which there is no signal/power for the red wire as marked in yellow below :


    Trailer Brake Diag Cut .jpg

    This means the controller is not getting the trailer brake signal and not giving me the green light /or red when brake peddle touched. = no trailer brakes.

    Assuming the red line should be + in some power/volts ? - at the moment MM is not giving me any on the reading. ? I was sure the blue was giving me some power 4+ but need to re check.

    I traced back to the inside left of the car and the trailer 'socket' and tried to get a read - but seemingly 0 .

    At this point I can only assume the line is damaged somewhere back to the CJB? or the CJB/BCM is damaged?

    A replacement CJB/BCM feels bad and over the top .....but surely this can diagnosed? or if there is better area to access to test that cable ?

    On thinking back - this may have been damaged when the rear air con condenser was replaced? as that area got pulled out ... unfortunately I don't drive the car much and don't see these things until its an issue.

    Any help appreciated.
    Last edited by gotaflat; 8th January 2023 at 12:21 PM. Reason: Better description
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  2. 8th January 2023, 05:23 PM #2
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    Updated.Checked power in the fuse box for any trailer related feeds
    • Number 27 (trailer side lamps) = 0.0v ???????
    • 48 (Trailer connector Trailer feed) = 12.04v
    • 54 (Trailer connector Ignition feed) = 12.04v

    So looks odd . But i am also getting this on the reverse screen ... like a trailer is connected... and no reverse sensors (front yes). did not get this before I reconnected the brake controller
    Reverse screen.jpg

    However more disturbing is the taped wires in there... WTF is that???

    taped wires.jpg

    wire 1.jpg
    wire 2.jpg

    At this point happy take it some someone that can straighten this out...
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  3. 8th January 2023, 06:41 PM #3
    josh.huber Guest
    Do the brake lights work on the car?
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  4. 8th January 2023, 07:28 PM #4
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    Quote Originally Posted by josh.huber View Post
    Do the brake lights work on the car?
    Car yes - have not checked trailer when hooked up yet
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  5. 9th January 2023, 09:02 AM #5
    josh.huber Guest
    If there working the signal from the cjb should be fine
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  6. 9th January 2023, 10:14 AM #6
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    Quote Originally Posted by gotaflat View Post
    Updated.Checked power in the fuse box for any trailer related feeds
    • Number 27 (trailer side lamps) = 0.0v ???????
    • 48 (Trailer connector Trailer feed) = 12.04v
    • 54 (Trailer connector Ignition feed) = 12.04v

    So looks odd . But i am also getting this on the reverse screen ... like a trailer is connected... and no reverse sensors (front yes). did not get this before I reconnected the brake controller
    Reverse screen.jpg

    However more disturbing is the taped wires in there... WTF is that???

    taped wires.jpg

    wire 1.jpg
    wire 2.jpg

    At this point happy take it some someone that can straighten this out...
    The blue tape is factory over crimp joints.. Dont worry about it too much.
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  7. 9th January 2023, 11:29 AM #7
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    Quote Originally Posted by josh.huber View Post
    If there working the signal from the cjb should be fine
    yep ok understand,

    Thats for the car right - is the trailer plug a separate feed/plug? straight from the CJB (according the diagram)? Because that red line has 0.0v (may be an initial 0.02v) but zippo - i cant explain that - i am sure that needs to have a signal (can potentially run a wire with diode to the car brake light.. but ....)

    I can only speculate that there may be crossed /exposed wires down to the rear 6 pin round socket? that why the car thinks there is a trailer on once the trailer brake (blue to Brown trailer wire) is joined??
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  8. 9th January 2023, 12:08 PM #8
    josh.huber Guest
    Quote Originally Posted by gotaflat View Post
    yep ok understand,

    Thats for the car right - is the trailer plug a separate feed/plug? straight from the CJB (according the diagram)? Because that red line has 0.0v (may be an initial 0.02v) but zippo - i cant explain that - i am sure that needs to have a signal (can potentially run a wire with diode to the car brake light.. but ....)

    I can only speculate that there may be crossed /exposed wires down to the rear 6 pin round socket? that why the car thinks there is a trailer on once the trailer brake (blue to Brown trailer wire) is joined??
    What yr and model is it
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  9. 9th January 2023, 12:26 PM #9
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    Quote Originally Posted by josh.huber View Post
    What yr and model is it
    2011 - SDV6 3.0 HSE
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  10. 9th January 2023, 06:01 PM #10
    josh.huber Guest
    Turn the indicators on, one at a time and see which one if any bring on the flashing icon on the dash that lets you know the trailer lights are working.
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