News

Consequences of momentary short circuit

Discussion in 'Technical Q&A' started by jselevan, Nov 16, 2003.

  1. jselevan

    jselevan Formula 3

    Nov 2, 2003
    1,849
    Some obvious words of advice. When working on electrics, especially under the instrument cluster, first disconnect your battery.

    Oh, by the way, does anyone know where I can find a new ampere meter for a 1983 Boxer...?

    In my next post I shall describe the internal workings of an ampere meter, and solicit suggestions on how to fix it.

    Thanks.

    Jim S.
     
  2. Verell

    Verell F1 Veteran
    Owner Consultant

    May 5, 2001
    6,967
    Groton, MA
    Full Name:
    Verell Boaen
    Ummm, Err, Ahhh,
    Can I deduce that the last post has an implied 'don't ask', or a 'been there..' ;-)
     
  3. jselevan

    jselevan Formula 3

    Nov 2, 2003
    1,849
    Verell - you have an uncanny grasp of the obvious.

    Jim S.
     
  4. jselevan

    jselevan Formula 3

    Nov 2, 2003
    1,849
    So this is what happens when one "fries" an ampere meter.

    While changing the instrument light in the ampere meter, I inadvertently touched one of the meter terminal posts to ground. Sparks flew for a moment, but no obvious damage. I changed the bulb, but when reinstalled in the instrument cluster, the ampere meter was reading +40 amperes with the ignition key off and battery disconnected.

    The following was found upon disassembly.

    There were no "fried" components. The needle is mounted on a shaft that has a counter weight and small magnet. The magnet slides (without touching) between a coil of 12 gauge shellac insulated transformer wire. It is clear that the ampere meter represents a short circuit through which charging current flows. As the current increases, so does the induced magnetic field, and this causes the needle to swing one way or the other.

    Here is what happened when the terminal post of the meter touched ground. The large instantaneous current likely caused a very large magnetic field that induced a new magnetic property to the needle magnet. It either cancelled the magnetic field of the needle, or increased the static field of the permanent magnet. In either case, I cannot fix it. I thought of de-Gaussing, but do not know how to recalibrate when finished.

    Fortunately, it appears that Maranello U.K. has new meters, albeit for a King's ransom.

    Lesson learned.

    Jim S.
     
  5. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    6,363
    Are you sure its not just a ground wire/fuse that blew?
     
  6. jselevan

    jselevan Formula 3

    Nov 2, 2003
    1,849
    Mitch - good thought. However, the needle shows +40 amps with ignition off and battery disconnected.

    Jim S.
     

Share This Page