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Posted for "Wally Bestgen" <whiskeyb@sbcglobal.net>:
Hamid,
The solenoid closes the fuel vent. The theory is: if the current draw is
zero the selonild has failed and the vent is open, making it possible under
certain filght conditions to allow air into the fuel system. This detector
tells the pilot that the solenoid is not producing any resistance and must be
assumed to have failed. Same with the pitot and prop heat.
For the prop and pitot heaters, that theory works. If you are drawing xxx amps of current, it is going somewhere and heating something. The chances that it is heating something other than the pitot or the prop are possible, but no very likely. Your level of certainty increases if you choose a narrow current band: 3A to 5A for example. Instead, if you go for "3A to when the 10A breaker pops" (which may not be till 15A), you are exposed to many more failure modes where you can get a false "OK" indication.
However, it is very different in case of the solenoid. If the solenoid is drawing the expected current, all it tells you is that the coil is energized. Did the energized coil actually pull the plunger? Did the pulled plunger actually actuate the mechanism to close the vent? Was there a twig or a bug stuck in the mechanism that keeps it from closing?
You are right, an incorrect current reading indicates a failed solenoid and hence a failed vent closing mechanism. However, the converse is not true. A correct current reading does not necessarily indicate that the entire mechanism is working and the vent is closed.
Regards,
Hamid
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