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Zener Diode location

Discussion in 'Norton Commando Motorcycles (Classic)' started by Wolfman, Sep 19, 2016.

  1. Wolfman

    Wolfman

    Joined:
    Sep 11, 2016
    Can some one show me where to mount the zener diode. Maybe a picture or good description 73 / 750 commando
     
  2. L.E.N.

    L.E.N.

    Joined:
    Dec 12, 2012
    The "backside" of the big right side aluminum Z-plate
     
  3. Wolfman

    Wolfman

    Joined:
    Sep 11, 2016
    Awwh I saw that hole & wondered. Thanks a bunch.
     
  4. J. M. Leadbeater

    J. M. Leadbeater

    Joined:
    Apr 21, 2015
    Zener diodes dissappait heat thus need a large lump to dissapait the heat. I assume someone looked at the two ******* great vastly over engineered rediculously heavy footrest support plates and thought 'Ah now there is a real heat sink!. Now whether they also thought to run an earth wire from the heat sink back to the main earth point on the frames......it would NOT suprise me if they relied upon the plate making contact with the frame to do this but I could well be wrong. Paint does not make a good electrical connector as many a car and motor cycle owner has learnt over the years as bulbs give out little light etc. Copper wire is a good electrical conductor....
    Have you never heard of those U.S.A. made Tympanium regulators? (Look them up on the web). Small lumps that take the place of the rectifier/s and zener/s. They were / are available positive or negative earth along with single or three phase. I and friends fitted them a great many years and they have to date, as far as I am aware, worked perfectly. Mind you in those days when we first fitted them the Pound against the Dollar was vastly different ..........
     
  5. Onder

    Onder

    Joined:
    May 11, 2010
    Mind you in those days when we first fitted them the Pound against the Dollar was vastly different ..........


    But on this list the higher the price the better off you are so as the pound sinks the quality of your
    entire bike goes up!
     
  6. L.A.B.

    L.A.B. Moderator VIP MEMBER

    Joined:
    Nov 20, 2004
    Those "ridiculously heavy support plates" weigh about 24 ounces each! :roll:

    You would be wrong.
    All standard Commando harnesses have a positive return (red) wire for the Zener (Mk3s have two Zeners so have two returns).

    The return wiring going to the battery positive terminal regardless of any other earth/return connections on the frame (the exception being the Mk3 where the final return to battery positive should only be through the heavy gauge positive starter cable).
     
  7. maylar

    maylar VIP MEMBER

    Joined:
    May 13, 2007
    The right side footrest attaches to the Z plate with 2 studs (front 2) and one through bolt (rear hole). The red ground wires for the zener attach to that bolt on the back side, secured with a nut and washer. Those red wires go back to the battery and I think to the engine ground at the head steady.

    The factory even saw fit to machine a flat spot on the Z plate for the zener, to insure it had the best possible heat transfer rather than rely on the casting to be flat. Aluminum is a great heat sink.
     
  8. Wolfman

    Wolfman

    Joined:
    Sep 11, 2016
    Seems to me years ago I saw a thing for sale that replaced both the zener & the rectifier. Never looked into it because my Triumph always worked good & if it aint broke, don't fix it.
     
  9. bluto

    bluto

    Joined:
    Jul 15, 2010
    A few companies make boxes that do that and they are inexpensive, about $60 or so and also is a way to convert to negative ground if desired (they can be wired either way)....am still running the stock items on my Commando too as they work fine.

    FWIW can remember reading somewhere that the Mk3 dual zener diodes need to be replaced as a matched set...makes sense, if the diode breakdown curves were much different one would be doing all the work dissipating excess power and probably would not live long.

    [​IMG]
     
  10. L.A.B.

    L.A.B. Moderator VIP MEMBER

    Joined:
    Nov 20, 2004
    Not the 850 Mk3 charging system, as the diodes are on the AC side, they don't need to be a matched pair.

    [​IMG]



    If two Zeners were to be connected to the DC side of the rectifier they would have to be a matched pair, however, this system wasn't used on Commandos but was a feature of the Lucas 3-phase RM 24 'Powerbase' conversion kits.

    http://atlanticgreen.com/images/lucasrm24.jpg
     
  11. aceaceca

    aceaceca

    Joined:
    Dec 18, 2004
    I like the Zener as well. Mine quit after 40 or so years and I put a new one on there. They are not cheap though. The one from Canada was $40 or so. You can get them much cheaper in the correct voltage range but have to add the 1/4 inch or so stud to mount them per original. If you run the stock set up, be aware that if the Zener fails in the closed position, shunt to ground mode, you will immediately blow your trusty fuse. All forward motion will cease at this point. I am talking now about a 71 Commando with one fuse. It behooves you to understand how to hot wire the ignition system so you can avoid the tow truck ride home. I have a spare spade terminal tipped wire permanently hooked to the negative battery post. In the event of a dead short or zener failure, I merely pull the white wire from the ignition switch and plug it into the negative battery wire to power the ignition system. Problem solved, ride home on battery power only. Been there done that. Never been stranded and nothing fallen off in many years of Norton riding.
     
  12. steelcap

    steelcap VIP MEMBER

    Joined:
    Jul 13, 2015
    Could someone explain why this is the case please.
     
  13. BillT

    BillT

    Joined:
    Jan 27, 2008
    I'm no electrical expert, but it may have to do with the 'path of least resistance'. Current flow will always choose the lowest resistance path, which would be (generally, barring corrosion) the fattest wire with the most surface area.
     
  14. L.A.B.

    L.A.B. Moderator VIP MEMBER

    Joined:
    Nov 20, 2004
    Although there is a battery positive wire included in the Mk3 harness and one is also shown on the wiring diagram, if there is a working starter motor then it is not a good idea to also have a light gauge positive return wire connected to the battery as under certain conditions (a loose or corroded heavy gauge positive cable terminal) the 100+ Amp starter motor current will attempt to return to the battery through the harness return wiring (and the fuse won't blow because it's on the negative side of the battery) which can result in a burnt-out harness or even cause the bike to catch fire!
     
  15. fiatfan

    fiatfan VIP MEMBER

    Joined:
    Aug 9, 2015
    No worries, when the harness has cooled off, you just refill it with this genuine Lucas service part :mrgreen:

    [​IMG]
    Tommy
     
  16. L.E.N.

    L.E.N.

    Joined:
    Dec 12, 2012
    That jar is not nearly so large as I would have thought.
    Lance
     
  17. kommando

    kommando

    Joined:
    May 7, 2005
    Wonder how the Wassell Lucas smoke will fair ;)
     

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