On Tue, 6 Jul 2004 19:05:22 -0400, "Jeremy Lew"
wrote:

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?

External RF is not the cause.

Does your plane have a voltmeter? If not, you can get a cigarette
lighter VM to monitor the bus voltage. A VM is a helpful tool to
verify whether or not the alternator/charging sytem is operating.
When the charging system is active, the voltage should be around 14V,
when inactive, the bus voltage will quickly (within a few seconds)
taper off to 13V and then slowly decay to 10V (over the course of an
hour or so) as the battery loses charge. The decay time is dependent
on the battery and the electrical load.

Anyways, if you see a zero on the ammeter, and the bus voltage has
dropped below 14V, there is a good chance the charging system has
failed.

Also, don't be surprised that the system came back to life after a few
minutes. Intermittent contacts are very common on the 30 year old
wiring, circuit breakers, and switches in our planes.

-Nathan