Ray Andraka wrote in message ...
The master switch has two poles, one for the contactor which handles about a
quarter amp, and one for the alternator field circuit, which has to handle a
couple of amperes. The switch is not really suitable for high DC current
applications, so the contacts tend to burn with time. When they get old, you can
get fluctuations on the field circuit, which in turn causes the output current to
fluctuate.

In most airplanes the alternator switch controls only a relay in the
regulator, not the field current itself. The relay connects the
voltage regulator relay to the "A" line, which runs to the alternator
output which in turn is connected to the aircraft's electrical system.
The small current required to run the relay might not be enough to
keep the master/field switch contacts burned clean, and the relay
contacts themselves can get worn.
The voltage regulator contacts can burn and start cutting out or
sticking shut intermittently, too.

A bad ground on the regulator can also cause
similar problems. Check the ground path from the regulator case to the battery.
Be especially vigilant checking the riveted pieces, as oxidation under a rivet in
the ground path can cause the alternator output to fluctuate. You might try
running a separate ground wire from the regulator directly to the battery to see
if it eliminates it (use 16 ga or bigger). I'd bet it is one of the two. If
not, then check the circuit breaker to make sure it has low resistance.

Bad grounds are certainly most likely. As our airplanes get older
and older, the oxidation begins to provide many hours of
entertainment.

Dan

wrote:

On Tue, 3 Feb 2004 16:29:09 -0500, "Bob Chilcoat"
wrote:

On a possibly related subject, the load meter on our '74 Archer develops St.
Vitus dance about 25% of the time. Battery seems to charge fine, and you
can't hear any noise in the headphones, but the meter just jumps up and down
a couple of time a second. No one can figure out what the problem is. The
shop boys say that they see it a lot in Cherokees, and that they have seen
replacing the master switch cure it. Any other ideas?

I don't suppose you have Strobes or Beacon that flashes at twice per
second. Just a daft thought!

David

--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email
http://www.andraka.com

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