Rear light stops working when front is on CB125s 1973
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  1. #1
    Junior Member maralions's Avatar
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    Rear light stops working when front is on CB125s 1973

    Hi everyone,

    I'm going mad with this issue so hopefully someone has some wisdom for me!

    I've finished restoring a Honday CB125s 1973. I'm having an issue with the rear light.

    When the light switch is off, the brake light works when I use both the rear and front brake. So the brake light switches are both functioning. However, when I turn the lights on, the front light illuminates but the rear does not. I can't get anything from the rear and so it also doesn't work as a brake light whereas it works fine when the light switch is off.

    I've done the following:

    - taken the wiring loom out, checked connections and connectivity to the all wires in the rear light - all fine
    - checked the light switch - all seems to be working
    - checked my earth to the rear light - it's connected between the light casing and the not sure if that's correct. It's had a new paint job so I've filed back to the metal on all surfaces to have a contact.

    I've also go an intermittent problem with the neutral light - sometimes this works, sometimes it only works if I touch the bulb casing to the body.

    I just don't get why putting the front light on stops the rear light from working.

    Any ideas??? I can only ride the bike during the day because I can't use the front light and have no brakes!

  2. #2
    Sensei 66Sprint's Avatar
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    Phone me at number below....
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."
    Contact info: E-mail; [email protected] Phone; 540-525-5199

  3. #3
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    Quote Originally Posted by 66Sprint View Post
    Phone me at number below....
    Ahh.. I'm in the UK! I could do messenger or Whatsapp or something? Not sure if that works for the US but I'll have a look.

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  5. #4
    Sensei 66Sprint's Avatar
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    OK then...Go to rear of bike (remove/open seat) to where the Green with Yellow stripe, solid Brown, and Dark Green wires are....Verify ALL three are plugged into the same color wires at BOTH ends of the rear fender ( Green to Green, Brown to Brown, Green/Yellow to Green/Yellow )......Also, remove and test the dual filament tail/brake light bulb verifying both filaments work and can work independently......
    Last edited by 66Sprint; 05-15-2019 at 03:57 PM.
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."
    Contact info: E-mail; [email protected] Phone; 540-525-5199

  6. #5
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    Quote Originally Posted by 66Sprint View Post
    OK then...Go to rear of bike (remove/open seat) to where the Green with Yellow stripe, solid Brown, and Dark Green wires are....Verify ALL three are plugged into the same color wires at BOTH ends of the rear fender ( Green to Green, Brown to Brown, Green/Yellow to Green/Yellow )......Also, remove and test the dual filament tail/brake light bulb verifying both filaments work and can work independently......
    They're all connected correctly and I've tested their connectivity with a multimeter.

    Do you know how I can test the dual filament? Not sure how to do that!

  7. #6
    Sensei 66Sprint's Avatar
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    Test continuity from each bottom solder "button" to the casing shell......Then across the two buttons.....
    Also test continuity from harness side of Green wire at each connection point to Battery NEGATIVE terminal.....
    Last edited by 66Sprint; 05-15-2019 at 04:25 PM.
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."
    Contact info: E-mail; [email protected] Phone; 540-525-5199

  8. #7
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    Quote Originally Posted by 66Sprint View Post
    Test continuity from each bottom solder "button" to the casing shell......Then across the two buttons.....
    Also test continuity from harness side of Green wire at each connection point to Battery NEGATIVE terminal.....
    Thanks. I'm not sure what you mean about harness side of green wire to battery negative - as in to the point where the battery is connected (with it disconnected)? Sorry, I'm not great with electrics!

  9. #8
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    Sorry and just to be clear, disconnect each earth when testing? So disconnect completely from bike and battery and test between those points?

  10. #9
    Sensei 66Sprint's Avatar
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    Nope.... as you follow the Green wire along the fender insert one probe into each Green to Green connection and the other probe on Battery negative....
    No need to disconnect anything....

    EVERY dark Green wire on the bike should show a direct (low ohms) connection to B- .......
    Last edited by 66Sprint; 05-15-2019 at 04:37 PM.
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."
    Contact info: E-mail; [email protected] Phone; 540-525-5199

  11. #10
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    Ok I've tested the bulb, seems fine - reading is 0 between the buttons and from each button to the case.

    I've tested connectivity (using this setting on the multimeter) for each green wire and it seems fine. Where it's mounted to the bike I've also tested from near this area too to the earth wire to make sure that it's touching bare metal (e.g. the headlight earth bolts through the headlight case to the indicator, so I've tested between the indicator casing and the connected neutral wire to make sure it's grounding.

    I really don't know what to do next. There are no breaks, everything is where it should be, I've removed the new paint back to the bare metal for each neutral wire. I don't get it.

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