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One of our Comanche drivers has sent this in:
My 530WAAS would work fine 99% of the time. I would power down when reducing power for the approach. Just what you need on final in instrument conditions. After many hours trying to find the problem my A&P figured it out. He said while running at cruise power the 530 gets power from the alternator. When reducing engine power the (with landing lights gear and flaps) 530 gets power from the battery through the amp meter. He found one terminal on the amp meter warm the other burned his finger. Just a loose connection. I drive a pa24-260b. Something worth checking if you have the top of the panel open
I have also seen very similar items with other equipment. The battery is connected to the primary bus and the generator to the secondary bus. When you pull the engine back and the RPM drops the generator output drops. Therefore, the power must come from the primary/battery side of the bus.
The power flows back and forth through the ammeter connections and if those are loose or corroded there is a voltage drop. If the voltage to a Garmin gets to 9 volts or less that is outside the datasheet and can cause it to shut down.
A similar thing can happen with old circuit breakers and the connections on the circuit breakers. Same thing with the battery contactor and ground strap although that affects all devices on the aircraft. This can also inhibit charging to the battery and potentially cause gear up circuit breaker pops as well.
