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Zener failing?

Discussion in 'Norton Commando Motorcycles (Classic)' started by Douglass Harroun, Sep 13, 2013.

  1. Douglass Harroun

    Douglass Harroun

    Sep 12, 2008
    Found my headlight and pilot bulbs burned out at the same time. Noticed the blinkers cycle very fast at 4000 RPM. Put a volt meter on the battery and reved it up: a little over 13v up to 4000 RPM ( not excessive?). Should I just replace the zener? Has a newish gel battery- Scorpion brand
  2. comnoz

    comnoz VIP MEMBER

    Dec 10, 2008
    The 13 volt reading sounds correct. Not likely needing a zener. Jim
  3. B+Bogus


    Jun 28, 2009
    Is the Zener wiring OK?
    The symptoms you describe would lead me to the same conclusion, but perhaps it's an intermittent open circuit.
  4. Nater_Potater

    Nater_Potater VIP MEMBER

    Apr 7, 2013
    +1, and that would be on the wire leading down to the zener. The schematic shows this line originating off the rectifier as a "Medium-blue", but I'm not next to my '74 to verify that. It's prone to corrosion in its current location on the Zee plate, and, if it can't crowbar the excess current to earth, it's going to spike the system high.
  5. L.A.B.

    L.A.B. Moderator VIP MEMBER

    Nov 20, 2004
    The Zener (and battery negative) wire is coded "NU" for '71-on models, which is Brown/Blue. Pre '71, it's "NW" = Brown/White.
  6. Nater_Potater

    Nater_Potater VIP MEMBER

    Apr 7, 2013
    Hah! You beat me to it! I was just coming back to correct my mistake.
    So I can't read, either...
  7. daveparry


    Jul 4, 2009
    It's also worth checking that there's no corrosion where the zenner connects to earth in the Z plate.
  8. bluto


    Jul 15, 2010
    My own zener failed recently and I knew right away from a LED voltage monitor, flashed green for overvoltage so I was able to fix it before blowing bulbs, cooking the battery, etc...also a friend w/ a Triumph and the same monitor was able to limp home as he knew immediately when the charging system had failed...anyway definitely a good addition for old Lucas equipped bikes



    (tried to embed the video but no luck w/ that, looks like that feature is broken, made it work in the past))

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