"Jeremy Lew" wrote in message ...
Was flying IFR in the clear on top of an overcast layer today. I noticed at
some point that the ammeter was indicating zero, and not moving. The Alt
warning light was not illuminated, except when I pressed the test button. I
started to shut down non-essential electriconics in case this was for real.
After a minute or two, the ammeter needle resumed normal indications, and
stayed that way for the rest of the flight. The Alt warning light was
properly illumated during low-RPM operations on the ground, as well as
during the engine runup split switch check.

The warning light/ammeter needle discrepancy suggests to me that problem was
in the ammeter, and that the alternator was functioning normally the whole
time. Anyone have any theories as to what would cause indications like
this? Is external RF interference possible?


I've had the same problem in my Mooney. Sometimes the buss voltage
drops down to just 13v sometimes to 12v but then comes back. It
appears the problem is an old master switch. The contacts on the
switch wear out and if the alternator does not get full battery
voltage from the field (going through the master switch) it will put
out less voltage. The contacts are a bit intermitant sometimes as they
get corrosion.

-Robert