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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