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"

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

Alternator problem. Possibly a filter, but my guess is a diode is going bad.

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.

Alternator speed depends on engine speed.

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.

Adding load makes the alternator work harder. Cutting it all out lets it loaf
until the battery charge drops a bit, then the alternator has to work again.

George Patterson
If a tank is out of ammunition, what you have is a sixty ton portable
radio.

I'd say George is dead on. I also thought filter, but the fact that the
amps oscillate makes me think diode. I've never heard of a bad filter
causing an ammeter oscillation, the filters that have failed for me just
complain loudly.
Jim

Could it be a very loose alternator belt ? The symptoms don't all jib
though.

See: http://www.sacskyranch.com/altnoise.htm

Jim

I third the thought. I have a capacitor filter across the output of the
alternator and ground. I have also cooked an alternator in the summer .
The fan on the back was not enough forced air for a high load when it is
hot. I added a blast tube form the front of the cowling to the "Blast
adapter" on the back on the Alternator. The hole in my cowling just
below the engine air intake is for the alternator. Works great. No more
over heating problems.
Michelle

George Patterson wrote:

Jay Honeck wrote:

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


Alternator problem. Possibly a filter, but my guess is a diode is
going bad.

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.


Alternator speed depends on engine speed.

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.


Adding load makes the alternator work harder. Cutting it all out lets
it loaf until the battery charge drops a bit, then the alternator has
to work again.

George Patterson
If a tank is out of ammunition, what you have is a sixty ton
portable
radio.

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.


Jay,

Is this not the "Cherokee oscillating Ammeter" thing? Mine has it. Exact
same symptoms. Had it for 11 years and 4 different alternators. Either
the split master switch (alternator side) or the alternator breaker
having a high resistance connection (either the wires or the
switch/breaker itself).

Google it. You should see lots of hits. A common problem if I recall.

Mike

See: http://www.sacskyranch.com/altnoise.htm

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

Is this not the "Cherokee oscillating Ammeter" thing? Mine has it. Exact
same symptoms. Had it for 11 years and 4 different alternators. Either the
split master switch (alternator side) or the alternator breaker having a
high resistance connection (either the wires or the switch/breaker
itself).

Google it. You should see lots of hits. A common problem if I recall.

It may be -- but it just started happening in mine. *Something* had to
change, and the fact that my mechanic was digging around in the magnetos
last week seems to point to the all-to-common "mechanic-induced" squawk.

Or, it could be the sudden, severe cold? Or, it could be coincidence.

Next nice day I'm going to de-cowl the engine and tighten all electrical
connections. That has cured other electrical gremlins for us...and it's
cheap!
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Jay - If you get desperate, you might be able to create this problem on
the ground using a hand held or jury rigged electric motor to spin the
alternator rather than having to run the engine. This would allow
better diagnostic capability using an oscilloscope.

However, if the problem only occurs only at higher amperage loads, a
fractional hp electric motor won't be able to spin the alternator.
Likewise the RPM may be a problem. Alternators spin pretty fast at
cruise - like over 10,000 rpm.