On 11/30/2021 2:29 PM, John Gotschall wrote:
In my Cessna 172 the master solenoid isolated everything. Really,
everything. Including the starter solenoid. The only thing not isolated
by the master solenoid was one lonely fuse feeding the master switch
contact that fed only the master solenoid coil.
That way when the master was off, there was no battery connection to
anything but the master switch contacts the control the solenoid coil.
The other half of the master switch disconnected the only other current
source, the generator.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Your 172 has Magnetos and gravity feed fuel so you could turn off your
"master" switch and keep right on flying. Just don't turn off the mag
switch. My KR is wired that way but I need electric for fuel pumps or
it's glide mode. After considerable thought in the building stage I
installed the small backup battery. At 10 hours in to phase 1 testing I
had an electrical failure that was the equivalent of turning off the
"master" switch. I continued on to the nearest airport on the backup
system. After some trouble shooting I rolled the dice and flew it home
20 minutes on the backup battery being charged by the alternator. No
problem except I had no backup. At home base if both systems are not
100% it is a "no-go". In a system like mine I had to ensure the coil for
the relay that ties the alternator output to the backup battery is
energized (switched on) with power from the backup battery, not the main
buss. Duuhhhh....................
Larry Flesner
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