>I have a 75 AA5 which will not disengage the master relay in flight when
>the alternator is charging (you cannot turn off the master switch and
>disconnect the buss during flight). It works ok on the ground when the
>engine is not running.
Without seeing wiring for the airplane, I'll have to make
some assumptions. Many TC and amateur built airplanes
have classic split rocker switch for battery and alternator
control which is INTENDED to prevent battery disconnection
any time the alternator is operating. A mechanical internconnect
allows the alternator to be shut off with battery still on
and shuts of battery any time the alternator is shut off.
The reason for this is because most alternators do not run
well with the battery off line.
If you have two separate switches for altenrator and battery,
it may be that the alternator switch is two-pole with one
side used to switch alternator field, the other used to make
sure the battery contactor stays closed any time the alternator
is on.
>There is a diode from the battery terminal to the
>solenoid energize terminal. Do you suppose that this could be a factor if
>it has failed? Or do oyou think that maybe somebody has done some wiring
>changes such that the alternator is feeding voltage to the master relay
>solenoid terminal so that it cannot be disengaged.
Most diodes were added to battery contactors as a spike
catcher for the little bit of engergy that's stored in
the contactor's magnetic circuit. I cannot deduce from your
words exactly how this diode is wired but I suspect you're
speaking of a three-terminal contactor with a (+) or
banded end of a diode attached to the big "BAT" terminal
and the other end attached to the small coil terminal.
This is a conventional use of a diode for spike catching.
Failure of the diode (either open or shorted) would not
produce the symptoms you describe.
In any event, opening the battery contactor in flight without
also shutting down the alternator is generally not a good
idean.
Bob . . .
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