Alternator problem

Wow! A very informed response. I appreciate it immensely.

Thank you!

"Nathan Young" wrote in message
...
On Fri, 23 Apr 2004 13:28:40 GMT, "Tom Jackson"
wrote:

The other day, I was flying and noticed that my ammeter guage was reading
0
(it had been reading fine for a long portion of the flight.) Also, I had
been running a lap-top from the cigar lighter plug for the first time
ever.
I turned-off virtually all electric devices and continued to my
destination.
I also shut-off the alternator switch. Later in the flight, I tried
recycling the alternator switch, and the guage would read full
deflection,
so I then would shut the alternator switch off again.

Later in the day, I fired-up the plane - could tell that the battery was
weak because it could hardly pull the prop through. The ammeter guage,
however seemed to look ok - appeared to be charging the battery. I
figured
that somehow it had recycled itself, so I took off and headed home
without
incident (I watched the ammeter guage very closely throughout the flight,
and it appeared normal throughout.)

The plane is a 1974 Piper Warrior - 151.

Any thoughts? Was it merely a failsafe due to too much current being
drawn
from the lap-top? Should I have it looked at?

No way it was the laptop. Laptops run on a few amps at 12V, much less
than many accessories (pitot heat or landing light).

If the alternator had been off (and the plane running on battery) for
a long time the battery might be down at 9-10 volts. When the
alternator is re-energized, the current will go to close to max
deflect for a minute or two until the battery gets back up to 12-13V,
then it will slowly ramp down to normal as the battery gets back to
14V. The behavior you saw was normal.

Now the real question is why did your alternator go offline? The
Cherokee electrical system has about 5-6 components and all can be
failure points. Most pilots/mechanics replace the alternator, but
that rarely solves the problem.

In the Cherokee, the alternator output is wired directly to the master
bus. The field circuit breaker taps off the master bus, and then
provides current to the field switch, the field switch is connected to
an overvoltage regulator, which is connected to the voltage regulator.
The output of the VR controls the field current on the alternator, and
hence controls the output of the alternator.

Common culprits to the Cherokee electrical system include:
1. Field current breaker develops corrosion (it is probably 30 years
old, so not surprising), and begins to intermittently introduce a
resistance into the circuit. This allows a voltage drop across the
breaker, so the VR turns up the bus voltage until it sees 14V. The
problem here is that because there is voltage drop across the breaker,
the master bus might be sitting at 15,16,17V. Eventually the plane
hits some turbulence and the breaker gets moved a slight amount, the
resistance goes back to zero, and the overvoltage regulator sees 16V,
so it goes open circuit, which cuts the voltage to the VR, which then
has no power to give to the altenator field, so the alternator goes
offline. Solution: Replace the field breaker.

2. The Cherokee field current switch gets worn with age and will
begin to exhibit strange behavior. This is commonly seen as 'pulsing'
of the electrical system as the contacts in the switch heat, expand,
lose contact, cool, and then make contact again. Solution: Replace
the switch.

3. The overvoltage regulator in most older Cherokees is an
electromagnetic monstrosity, and the magnet/relay can fail to hold the
circuit open. This causes intermittent random electrical failures.
Solution: Repalce the OVR.

4. The VR can go bad.

5. In addition to all these components that can fail, the wiring
between them (after 30 years) can develop loose contacts, corrosion,
or cracks in the wire that allow open circuits or shorts to ground.

All of these can be really difficult problems to track down,
especially when the problem can't be reproduced on the ground.

Be patient, and be careful. Flakey electrical systems don't matter
much to a VFR pilot, but it is no-go issue for an IFR plane.

Also, if you use a voltmeter to probe around under the panel, be
careful especially if you have the master engaged - it is super easy
to short the bus while probing around the master bus.

-Nathan