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Thread: [BAC-Mail] Gauge electrical issue found

  1. #1

    [BAC-Mail] Gauge electrical issue found

    I'll toss out two possibilities; there may be more.

    1. Someone has mis-wired the power feed to the cluster instruments.
    Instead of getting 14V from the CB buss, they are getting variable voltage
    fed by the dimmer circuit. You didn't mention whether the lights in the
    instrument cluster were reacting to the dimmer too. If they were, someone
    may have both cluster lights and gauges working on the same circuit feed.
    If the cluster lights stayed bright, someone has the wires reversed.

    2. You have a bad ground between the panel and the airframe. There may be
    other bad grounds as well, such as wing-to-fuselage and fuel
    sender-to-fuselage. You can get some idea, if you have a heated Pitot
    tube, by watching the gauge needles while turning the Pitot heat on and off.
    If the gauge needles all "jump" and change readings, you have bad grounds.
    There are Beech Service Bulletins/Service Instructions/Service Letters that
    address the grounds; you can view the list on BAC.


    -----Original Message-----
    From: bac-mail-bounces@beechaeroclub.org
    [mailto:bac-mail-bounces@beechaeroclub.org] On Behalf Of Gary Hanchett
    Sent: Monday, November 14, 2005 11:44 PM
    To: 'bac-mail@beechaeroclub.org'; musketeermail@yahoogroups.com
    Subject: [BAC-Mail] Gauge electrical issue found

    Greetings all,

    I was out flying last night around dusk time and started bringing up my
    panel lights towards the end of the flight. A few minutes later noted
    that my fuel and oil temp gauges were reading zero when they were
    reading fine before???? Last thing I did was turn up the panel lights,
    so I turned them back off and all three gauges came back to life?!?!.
    In fact, the meter needles deflected in proportion to the intensity of
    the panel lights. As I brighten the panel, the gauge needles dropped,
    set the panel lights to full on the needles read zero. Decided to shut
    the panel lights off and land and not do my night quals until I get
    this sorted out. The amp meter (which seemed unaffected by the lights)
    was showing a slight positive charge which indicates there was no
    discharging going on. Has anybody seen this before? Very strange.....

    Thanks,

    Gary H
    Sport 150 MB-901

    _______________________________________________
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  2. #2

    [BAC-Mail] Gauge electrical issue found

    > Greetings all,
    >
    >> In fact, the meter needles deflected in proportion to the intensity of
    > the panel lights. As I brighten the panel, the gauge needles dropped,

    *** Sounds like a bad ground to me. A "resistive" ground to the meters.
    This ground is shared by the panel lights. When you turn on the lights,
    they pull current through the resistive ground, causing a voltage drop.
    That voltage drop - across the resistive ground - is subtracted from the
    supply voltage or sensor voltage to the meters. Probably their supply
    voltage, since they all react.

    This is what the electronics folks call a "ground loop".

    - Jerry Kaidor ( jerry@tr2.com ) KF6VB




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