Is the whine worse at low RPM? Does the whine change pitch and/or volume
with RPM?

If no to either one, it's not the alternator.

Classic symptom of an open alternator diode is fairly loud, low pitch whine
at idle. Increased frequency and lessor volume with increased RPM. Usually
to the point where it is hardly detectable (at least to my non-dog ears) at
high RPM. However, a bad connection to the battery or a bad battery will
cause similar symptoms.

More constant pitch that changes with electrical load is probably the
regulator or, again, a bad connection to the battery.

There are lots of nuances that can confuse the diagnosis. Most shops (that
I've been to) are not skilled at trouble shooting. The complex relationship
of the alternator, regulator, electrical grounding and battery cause most to
just start replacing parts until the problem goes away.

I had a whine last fall. Advice here was that it was not an open diode. I
convinced myself that it was. Swapped out the alternator with a rebuilt and
the problem went away. Of course, in the process, they disconnected the
battery and all the connections to the alternator. I never will know for
sure.

-------------------------------
Travis
Lake N3094P
PWK
"RST Engineering" wrote in message
...
Best chance is an open diode in the rectifier bridge. There are a
thousand more automobile electrical repair shops than there are aviation
electrical repair shops, and the replacement parts are mostly the same.

I would NEVER suggest having an automobile shop work on an airplane part,
but I've got this friend Ernie ...

JIm

"Dave Anderer" wrote in message
news:2007031113571416807-danderer@udeledu...
I'm looking for advice to help shorten the trouble-shooting process here.

My Arrow has developed an alternator whine over the past few hours. It
is *barely, barely" noticable on both radios when receiving, but loud and
obnoxious (both to me and other aircraft) when transmitting. No doubt it
is the alternator - switching the alternator off fixes the problem.