On Sat, 23 Oct 2004 05:02:19 +0000 (UTC),
(Paul Tomblin) wrote:

I had to jump start my plane (using the Piper External Power port) (yes, I
stupidly ran down the juice when I thought a gas smell was coming from a
flooded engine when it wasn't). I turned on just the battery half of the
master while I did it, and the plane started ok.

There was an annoying whine in the intercomm, and it took me a good amount
of time (15-20 minutes flight time) to realize that the alternator guage
was pegged, and cycling the alterator half of the master switch off and on
again got the alterator guage down from being pegged, and got rid of the
whine. I suspect that I misread the sign language from the guy who was
helping with the jump start, and I turned on the alterator before he
unplugged the external power.

Does this damage the alternator in any way?

Alternators are flakey, they like to blow diodes for what seems like
inconsequential reasons. However, in this case, I suspect the
alternator has not failed. The alternator was probably pegged because
the battery needed a charge. I have jumped my Cherokee 1 time, and
was surprised at how long the ammeter stayed pegged afterwards.

When the master switch is on (so the master solenoid is engaged), it
is a direct connect between the output of the alternator and the
battery. When connected by a low resistance path - a lot of current
can flow between an alternator at 14V and a battery at 8 or 9 volts..
I = V/R, and in this case the R is small, so the I can be large.

All the same, I would put the battery on a charger before flying
again, monitor the ammeter current on your ground ops/runup, and make
sure that the next flight or two are VFR ones.

One last question - does your Piper have a Voltmeter? As long as the
bus is reading close to 14V, the electrical system is probably ok.

-Nathan