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Thread: Gauge cluster - big headache

  1. #1

    Gauge cluster - big headache

    My LH fuel gauge is currently being replaced for the third time in
    two months the new ones keep getting fried. Last time around, I
    checked for myself that it passed the ops check with the master
    switch on when reinstalled, but apparently never worked in the air (I
    wasn't flying it). This time around, the oil temp gauge has also
    fried. The RH gauge has been soldiering on all this time without a
    problem, including when it's hooked up to the LH fuel tank sender.
    Source, load, and across-the-lugs voltages are identical for the two
    fuel gauges. All three gauges get their power along the same wire
    from the same circuit breaker.

    Air Parts of Lock Haven, who are getting sick of doing warranty
    replacements, suggest there might be voltage spikes in the plane
    somehow, but the rest of the electrical system seems normal. The
    alternator and voltage regulator were replaced last year, the battery
    had just been charged at the time of the last failure, and no other
    anomalies have been noted. Neither the alternator nor the gauge
    circuit breakers are popping I'm considering replacing the latter
    preventively. Even if that is the problem, the mystery would remain
    why didn't the RH gauge get lunched, too?

    When I get the gauges back, and before I hook them up, I'm taking the
    plane to Lock Haven to let them have a look at it. Meantime, anybody
    have ideas, or experience with such a problem? My only other idea is
    that maybe the LH sender is intermittently shorting to ground, but
    that hardly makes sense either.

    Sierra MC-472, Rochester gauges, Kelly Aerospace overhauled
    alternator, 14V. Don't know the m/m of the voltage regulator or O/V
    relay, but I believe they're Zeftronics.

    BTW, where can I get a specs table for the fuel senders? Have one for
    the oil temp sensor.

    Craig MacCallum
    Sierra N525SB
    Montclair, NJ






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  2. #2

    Gauge cluster - big headache

    Two key questions:



    What does "fried" mean, in this case? Just quits working? Some component
    smells like it has burned?



    Can AirParts tell you exactly what component has repetitively failed? Is a
    meter coil winding burning out? Diodes failing open? Or shorting?
    Resistors burning open? They should be able to identify the probable
    failure mode by the nature of the failing components.



    The Rochester gauge system (as opposed to the SW system) reads empty when
    the transmitter lead is grounded. It reads full when the transmitter lead
    has 32 Ohms of resistance. A transmitter lead shorted to ground will read
    empty, and an open circuit in the transmitter lead will read full.



    It sounds like competent people are working on the system. Even so, the
    connections on the back of the cluster can be a bit confusing; it is not
    hard at all to get a couple mixed up. AirParts should be able to tell you
    whether the failure mode implies a wrong connection. Grounding or opening
    the transmitter line won't hurt anything. Connecting it to power may be a
    very different story. For example, crossing the 14V and transmitter wires.
    Or even reversing the 14V and Ground wires.



    At 70 degrees ambient, the MS28034-1 oil temp bulb will have about 84 Ohms
    resistance. I will post the Mil-spec chart on BAC.





    _____

    From: musketeermail@yahoogroups.com [mailto:musketeermail@yahoogroups.com]
    On Behalf Of craigmaccallum
    Sent: Friday, October 28, 2005 12:41 PM
    To: musketeermail@yahoogroups.com
    Subject: [musketeermail] Gauge cluster - big headache



    My LH fuel gauge is currently being replaced for the third time in
    two months - the new ones keep getting fried. Last time around, I
    checked for myself that it passed the ops check with the master
    switch on when reinstalled, but apparently never worked in the air (I
    wasn't flying it). This time around, the oil temp gauge has also
    fried. The RH gauge has been soldiering on all this time without a
    problem, including when it's hooked up to the LH fuel tank sender.
    Source, load, and across-the-lugs voltages are identical for the two
    fuel gauges. All three gauges get their power along the same wire
    from the same circuit breaker.

    Air Parts of Lock Haven, who are getting sick of doing warranty
    replacements, suggest there might be voltage spikes in the plane
    somehow, but the rest of the electrical system seems normal. The
    alternator and voltage regulator were replaced last year, the battery
    had just been charged at the time of the last failure, and no other
    anomalies have been noted. Neither the alternator nor the gauge
    circuit breakers are popping - I'm considering replacing the latter
    preventively. Even if that is the problem, the mystery would remain -
    why didn't the RH gauge get lunched, too?

    When I get the gauges back, and before I hook them up, I'm taking the
    plane to Lock Haven to let them have a look at it. Meantime, anybody
    have ideas, or experience with such a problem? My only other idea is
    that maybe the LH sender is intermittently shorting to ground, but
    that hardly makes sense either.

    Sierra MC-472, Rochester gauges, Kelly Aerospace overhauled
    alternator, 14V. Don't know the m/m of the voltage regulator or O/V
    relay, but I believe they're Zeftronics.

    BTW, where can I get a specs table for the fuel senders? Have one for
    the oil temp sensor.

    Craig MacCallum
    Sierra N525SB
    Montclair, NJ






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  3. #3

    Gauge cluster - big headache

    --- In musketeermail@yahoogroups.com, "Mike Rellihan" <rellihan@r...>
    wrote:
    >
    > What does "fried" mean, in this case? Just quits working? Some
    component
    > smells like it has burned?


    What I mean by "fried" is: Dead. Non-functional. Deceased. Passed
    away. (Bereft of life, it rests in peace! It's rung down the curtain
    and joined the Choir Invisibule!. If it weren't screwed into my
    panel, it would be pushing up the daisies! What we have here is a
    late fuel gauge!) Nothing is implied about olfactory observations
    at the time of demise none were made.


    > Can AirParts tell you exactly what component has repetitively
    failed? Is a
    > meter coil winding burning out? Diodes failing open? Or shorting?
    > Resistors burning open? They should be able to identify the
    probable
    > failure mode by the nature of the failing components.


    Some good questions here I'll be sure to ask the shop, but
    whichever component first failed or broke down, I'm guessing that it
    led directly to the consequent failure of the meter coils. Last
    time, they returned to me the red-tagged (fried! dead! deceased!
    demised!) meter movement without any of the electronics attached.
    This implies that the rest of the unit checked out ok on the bench.
    They also said that meter movements, like lightbulbs, can be expected
    to burn out eventually.


    > The Rochester gauge system [. . . ] reads full when the transmitter
    lead
    > has 32 Ohms of resistance. A transmitter lead shorted to ground
    will read
    > empty, and an open circuit in the transmitter lead will read full.


    Do you happen to know its resistance at intermediate fuel levels? Is
    it a linear relationship, by any wild chance? Since the float is on a
    pivot, I suppose it's some kind of sinusoidal relationship. Not that
    it matters, just curious.


    > It sounds like competent people are working on the system. Even
    so, the
    > connections on the back of the cluster can be a bit confusing; it
    is not
    > hard at all to get a couple mixed up. AirParts should be able to
    tell you
    > whether the failure mode implies a wrong connection. Grounding or
    opening
    > the transmitter line won't hurt anything. Connecting it to power
    may be a
    > very different story. For example, crossing the 14V and
    transmitter wires.
    > Or even reversing the 14V and Ground wires.


    You only have my word for it, but I am absolutely certain that the
    connections were correct. I connected them myself according to a
    pretty foolproof procedure, rather than leaving it to some A&P who
    may or may not have been up on his electrical systems. I checked all
    wires for proper voltage to ground before and after connecting them,
    and again after the failure to boot. And don't forget, they worked
    properly in the ops check. Not to beat it to death, but believe me,
    that ain't the problem.

    - CAM






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