David Lesher wrote:
Jonathan Goodish writes:

I know that this is another frequent topic, but I'm hoping that
someone
can help narrow this down for me.

I have a rebuilt Chrysler alternator (from Electrosystems) on my
Cherokee that was installed about 400 hours ago. However, I am
experiencing alternator whine that varies with engine speed, but
which
disappears completely when I turn off the alternator side of the
master
switch. In addition, increasing electrical load seems to make it
worse.

You could have lost a diode. There are 6 main diodes, usually molded
into a unit.

If you have a friend with a portable oscilloscope, it's an easy test:
load the alternator and look at the waveform. You'll see the chunk
missing as compared to http://www.medar.com/images/3phase5.gif

Or pull the alternator [disconnect the battery ground FIRST,
please..]
and take it to an alternator shop. [NOT a chain auto parts store
who has an alleged tester.] They should be able to spin-test it.
AFAIK, having a test done by a non-FAA-anointed shop would be legal.
(Having them fix it, of course will get you sent to Gitmo..)

Alternator noise is often caused by an electrical "ground loop," as
the engineers call it, and it's not related to the taildragger
groundloop some of us are familiar with. The alternator generates noise
even if all the diodes are working, and in some airplanes even if all
the filters are in place. If there's a bad ground anywhere in the
alternator or power supply circuitry or radio and intercom circuitry,
resistance forms at that point and the ground current seeks another
path for a portion of the load. If this alternate path happens to be
through avionics, the noise will be picked up.
For example, if the grounding of the engine mount is imperfect, and
the alternator has no separate ground wire from it to the firewall,
some of the alternator's noisy current might seek a path through, say,
the electric oil temperature wire and through the gauge to the panel.
Now we have electrical noise through the panel to the rest of the
airframe, and radios grounded to various points of the panel, and since
gound connections between sections of the panel might be old and dirty
we could get voltage drops between those points and the radios pick
them up. I have also seen erratic oil temperature readings caused by
the spurious current.
That's my take on it anyway, and I have managed to eliminate noise
on occasion by cleaning all the ground connections I can find and
bridging suspect panel sections with ground wires to eliminate the
voltage drops. If you have a portable intercom, try making sure it's
NOT grounded except for its power supply. I've seen noise picked up as
a result of different potentials between the power line ground and a
screw that was holding the intercom to the pedestal. Velcro fixed that
one.
Some alternator noise comes from the field. There is magnetic
interaction with the stator when the thing is working, and noise can
come back up the alternator field line into the bus. Many alternators
have a filter capacitor on the field terminal.

Dan