Alternator whine

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.

There do seem to be some variations - the noise is worse when
transmitting on the pilot side than the copilot side. Worse when using
the installed PTT and headset jacks than the mic.

I've got an an old KMA-20 and a Sigtronics SPA400 intercom installed.

Nothing has 'changed' recently. The alternator was replaced by an OH
about 18 months/100 hours ago. The battery was replaced 4 months/25
hours ago at annual. Things were fine for the first 20 hours after
annual.

Because it was quick and easy, I cleaned the battery connections and
the pins on the back of the KMA-20. No improvement.

I'm time-constrained, and so would rather not have to make multiple
trips to my AP and/or radio shop. Any informed guesses are more than
appreciated.

Thanks.

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.

On 2007-03-11 14:20:46 -0400, "RST Engineering" said:

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.

Thanks much Jim.


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

Heavens to Betsy, I'd never even think of such a thing...

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.

A coup'la questions & comments:

1. Why would you disconnect the battery to replace the alternator?

2. One presumes certain basic troubleshooting procedures prior to throwing
parts at the problem, like back all heavy nuts grounding the alternator,
squirt in a little magic juice, and retighten.

3. You are correct. Most general aviation shops don't have the
sophisticated tools to diagnose problems with alternators and regulators. A
fairly large shop dedicated to nothing but alternators, regulators, and the
like will. Most of these are in the automotive trade.

4. If the whine is coming mostly from the right front seat of the airplane,
you might want to think about how long it has been since you took her to
dinner {;-)

Jim



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.

"RST Engineering" wrote in message
...
A coup'la questions & comments:

1. Why would you disconnect the battery to replace the alternator?

I suspect it was in an attempt to reseat all connections that could
contribute to the problem. I had already done this as well as "moving" all
of the alternator mounting bolts. I didn't try any magic juice, however.


2. One presumes certain basic troubleshooting procedures prior to
throwing parts at the problem, like back all heavy nuts grounding the
alternator, squirt in a little magic juice, and retighten.

I'll emphasize that I did my own diagnosis and concluded that it was an open
diode. I took it to the shop not to have them diagnose but to swap the
alternator - which they did quite well and it (or the process of doing it)
solved the problem.

-------------------------------
Travis
Lake N3094P
PWK

"Travis Marlatte" writes:

1. Why would you disconnect the battery to replace the alternator?

I suspect it was in an attempt to reseat all connections that could
contribute to the problem. I had already done this as well as "moving" all
of the alternator mounting bolts. I didn't try any magic juice, however.

A wise move...

And I always pull the battery ground before I swap an alternator or
such. Then, when I drop the wrench or such; that precaution cuts
down on the "sparky-do's" as an old cow orker used to called it.

I've no doubt it's now a Terrorist Act to take an aircraft alternator
to an autoparts store for testing; but if it were from an airboat,
[YES, that's it, an IO-540 in an airboat....] then you might go that
route. The trouble is, will Al Autozone know how to hook up an
alternator sans regulator?

I wonder if aircraft will ever get internal regulators as cars have
had for 20+ years. It really cuts down on the number of wires to
come lose & cause weird grief...

--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

On Mar 12, 6:17 pm, "Travis Marlatte"
wrote:
"RST Engineering" wrote in message

...

A coup'la questions & comments:

1. Why would you disconnect the battery to replace the alternator?

I suspect it was in an attempt to reseat all connections that could
contribute to the problem. I had already done this as well as "moving" all
of the alternator mounting bolts. I didn't try any magic juice, however.



2. One presumes certain basic troubleshooting procedures prior to
throwing parts at the problem, like back all heavy nuts grounding the
alternator, squirt in a little magic juice, and retighten.

I'll emphasize that I did my own diagnosis and concluded that it was an open
diode. I took it to the shop not to have them diagnose but to swap the
alternator - which they did quite well and it (or the process of doing it)
solved the problem.

-------------------------------
Travis
Lake N3094P
PWK

We once had an alternator whine in a 172. The alternator
wasn't the problem; there was a ground loop somewhere. After messing
endlessly with ground straps and filters and other connections, I
finally got fed up and ran the alternator using our table saw motor--
wheeled the saw up to the front of the airplane and used the belt to
spin the alternator while I got under the panel wearing the headset
and used a jumper to try grounding/ungrounding various jack points.
Managed to minimize the whine by moving a groundpoint to a point where
a bunch of other avionics grounded. Old airframes develop resistances
at riveted joints, causing a voltage drop across certain components
and introducing noise that's supposed to be filtered out in the radios
or audio panel. I think. Anyway, it worked.

Dan

On 2007-03-12 09:11:59 -0400, "Travis Marlatte"
said:

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.

Yup, that is pretty much it.

Turns out my AP has a spare alternator in stock, so we can do a quick
switch as a test.

"RST Engineering" wrote:

but I've got this friend Ernie ...

Thats my dad... GRIN