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Strange electrical issue after changing rear light wiring harness

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  • Strange electrical issue after changing rear light wiring harness

    Hello everyone! I have a tiny issue and I'm hoping someone here might point me in the right direction.

    I bought a Roadster a couple of days ago - I absolutely love it - but it came with a small problem. The rear left indicator and light weren't working (brake light was working). Long story short, I discovered a corroded and partially broken off connector for the rear light and replaced the whole left wiring harness with an OEM part to avoid merely splicing the wires to change the failed connector. The lights, apart from reversing light, were now working fine.

    Today I went for a drive and things got interesting.

    First, when braking, the front position lights came on and the illuminated speedo display went dark. Then it went to normal whenever I got off the brake. It still does this.

    Second, when I turned on the lights as it got dark, the gearbox error signal came on the dash along with the ABS light and a generic warning light (picture below). The ABS and gearbox continued to work normally despite this, but then the gearbox suddenly jumped out of gear at motorway speed and reverted to Automatic mode despite me being in top gear and cruising steadily. It then worked perfectly fine in that mode, but the symbol remained. It did this twice, after I once again put it in Manual.

    Lastly, the front left light suddenly went out and, as I stopped to check, I noticed that the brake lights - all three of them - were constantly on. I then did a small experiment, where my friend confirmed that the brake lights stayed on even as I drove away but worked perfectly fine if I turned off the lights - as if driving by day. So if the lights are off, the brake lights work as intended but if I turn the lights on the brake lights are also on.

    Since none of this was happening before I changed the wiring harness I assume that's what caused it. The question is, where did I go wrong and how do I fix it? Disconnect the battery to reset the ECU? Check the fuses? Summon Cthulhu?

    PS: the only light actually not working at the moment is the reversing light as I didn't realise the bottom of the bulb there was corroded as well, but I'm not sure if one malfunctioning bulb could be the cause.

  • #2
    I've heard of this happening on 450s when the (double filament) brake/sidelight bulb is forced in the wrong way and the filaments connect with the wrong wires.
    Possibly you have mixed up brake and sidelight wires. I'd fix the reversing light bulb first though in case it is that.


    • #3
      check the earth


      • Thrumbleux
        Thrumbleux commented
        Editing a comment
        Good point.

    • #4
      The earth connection would be my thought too. When the nearside tail light on my Roadster fails (or needs a thump to make it work) the radio back light dims when I press the brake pedal .... :-?


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