OK, Thinking-Cap Time...

Of course, we have a new squawk shortly after coming out of the shop.
Perhaps someone here can point me in the right direction?

Here's the scenario with our 1974 Piper Cherokee Pathfinder, with a 530
hour, normally aspirated Lycoming O-540-B4B5 engine:

1) Flying to Janesville, WI today. Extremely cold -- around 15 degrees
Fahrenheit on the surface, near zero at altitude. The first real cold snap
of the year.

2) In steady-state flight, both of us hear a "woo--woo--woo..." softly in
the headphones. It sounds just like strobes that aren't grounded properly.

3) The noise slows down -- or speeds up -- in synch with the prop RPM. At
2300 RPM, it's going a bit faster than once per second -- maybe around three
times every two seconds.

4) I switch our digital voltage/amperage meter to AMPs, and watch as it
oscillates in time with the noise. It's alternating from 5 amps to 10-11
amps, in time with the "woo--woo--woo"... Voltage appears to be pretty
steady at around 13.8-ish volts.

5) Adding load to the system (landing lights, fuel pump, etc.) makes the
noise louder. Adding load to the system and then suddenly turning
everything off makes the noise and amperage oscillation go away -- for about
30 seconds. It then returns.

6) Turning the alternator side of the split master switch ALSO makes the
noise go away. It sometimes stays away when I turn the alternator back on.
Sometimes it doesn't.

7) The battery is new this past summer.

So, we've got a noise and amperage indication that can be corrected by
adding load to the system -- and then suddenly taking it away. The noise
is also fixed -- sometimes -- by cycling the alternator off and on.

The service last week was having the impulse couplings serviced (per service
bulletin), so electrical things have been tinkered with since our last
flight. (This is ALWAYS when some new squawk seems to crop up.)

Any ideas? Loose ground wire somewhere? Alternator failing? Any and all
input is welcomed!

Thanks!
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"