2004 Thruxton 900 Tail Light Not Working

Discussion in 'Technical Help' started by Creaky, Jun 17, 2023.

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  1. Creaky

    Creaky Well-Known Member

    Feb 25, 2015
    100
    93
    Nottinghamshire
    Hi all. Wonder if someone could guide me in the right direction?

    My bike is fitted with an aftermarket Lucas style tail light and indicators.

    Owned the bike for 2 years, 8000 miles no problems then rear offside indicator stopped working. Long story short, wiring back to main loom plug was all a bit suspect so I have purchased and am in the process of fitting a new accessory loom from TEC bike parts which plugs into main bike wiring. Got everything working but tail light will not illuminate. Brake light works fine.
    Have tried 3 bulbs, tested okay for continuity with multimeter but still no joy. Think I'm missing something simple but can't see it.
    Be grateful for any pointers.
    Many thanks
     
  2. Creaky

    Creaky Well-Known Member

    Feb 25, 2015
    100
    93
    Nottinghamshire
    I should probably add that I've dismantled the mudguard, light assembly, etc. and everything is separate as I'm trying to get it all working before heat shrinking my connections and final reassembly.
     
  3. Eldon

    Eldon Elite Member

    Nov 14, 2018
    4,162
    800
    North Yorkshire
    Have you measured for 12v at the main harness plug pin?
    Have you proven continuity on the new TEC loom?
    Is there 12v at the tailight connection?
    Has the earth lead got continuuty to say the engine/frame?
     
    • Like Like x 1
  4. Eldon

    Eldon Elite Member

    Nov 14, 2018
    4,162
    800
    North Yorkshire
    Here is my Street twin with non-standard back end.
    Homemade tail tidy running a different tail light and aftermarket indicators spliced off a ( possibly Tec) short loom extension.

    20230617_232157.jpg
     
  5. Creaky

    Creaky Well-Known Member

    Feb 25, 2015
    100
    93
    Nottinghamshire
    Thanks for the prompt reply and photo - nice tidy wiring.
    I will double check continuity but
    think it may be an earthing issue and I'm going to explore further today.
    My rear light has only 2 wire feeds coming from it and the original loom I removed had an earth to the underside of the mudguard where the rear light fixing bolts protruded.
    When l look at the TEC website image of their Lucas tail light it has 3 wires including an earth exiting the back of the fitting whereas my non TEC unit only has 2. I can't see any earth point inside the light. As I've got everything apart I've lost my original earth and hope a dry reassembly before I heat shrink my connections with a trial earth to the frame may provide a solution.
    Will post on progress.
    Thank you.
     
  6. steve lovatt

    steve lovatt Something else

    May 12, 2014
    9,211
    1,000
    North Yorkshire
    Does sound like an earthing problem to me, it usually is :cool:
     
    • Agree Agree x 1
  7. Eldon

    Eldon Elite Member

    Nov 14, 2018
    4,162
    800
    North Yorkshire
    Run a crocodile clip cable directly from the battery earth to the tail light mounting bolt to temporarily prove either way.
     
  8. Creaky

    Creaky Well-Known Member

    Feb 25, 2015
    100
    93
    Nottinghamshire
    Hi Eldon
    Still struggling with this.
    I've got 3 wires in the TEC harness for the rear light, red, blue and black.
    Red I've connected to red wire from the light, blue I've connected to the black wire from the light. There is no earth point so I've attached the black earth wire to a temporary bolt through the light mounting point.
    The light is not yet fixed to the bike.
    Should I build up the rear mudguard temporarily and try again or use a lead from the battery negative as you suggest?
     
  9. Creaky

    Creaky Well-Known Member

    Feb 25, 2015
    100
    93
    Nottinghamshire
    Okay I've now reassembled the tail light, mudguard etc and connected up without heat shrinking over connectors. Still no rear light with earth as before linking to light mounting bolt under mudguard.
    Have run an extra earth from the same point back to battery and clamped onto negative post with small mole grips. Still no joy. Brake light works on both front and back brake switches.
    Have tested bulb with multimeter for continuity and checks out but swapped bulb as a precaution too.
    Thinking I may now order new Lucas light from TEC as it appears universal and website shows it with 3 cables exiting, red, blue and black so presume it has integral earth. Also appears to be LED.
    Not sure what else I can try but would like to test for 12V at lamp holder if someone could confirm correct way to do this with the multimeter as I'd prefer not to fry anything!
    Thanks in advance for any suggestions.
     
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