I had the same problem with my Cessna and was cured by cleaning every
contact and replacing the Cessna with a more robust military switch.

David Lesher wrote:

"DoodyButch" writes:

There is a usual problem that causes this on Cessnas.

Cessna solid state voltage regulators on pre-1980 alternator equipped
Cessnas had three wires. One for the aircraft bus, one for the alternator
field and one for ground.

THIS IS THE PROBLEM:

The "Aircraft bus" lead on the regulator serves both as the voltage sense
lead and as the power lead to ultimately provide power to the alternator
field. When energized, the field draws about 2 amps.
.....

This is obviously a big design defect. In about 1980, Cessna apparently
realized this and changed to a voltage regulator with four leads, the extra
one being a separate "sense" lead connected directly to the aircraft bus so
that the alternator field did not draw current through it.

This is one of those "damned if you do, damned if you don't" issues.
Many auto regulators also draw current down the sense lead to run
things and here is why.

You're zipping along, and the sense lead falls off. "Ooops, Zero
volts!" says Mr. Regulator... "Full Power, Scotty, the Romulans
are headed this way.." and it raises the field current to try to
get back to 14.4 vdc on the sense lead. No good, it's still zero.

Of course, meanwhile your avionics stack and battery are in rigor
mortis as the main bus voltage is not zero, but 30 or 40 or higher
volts. Did I mention the landing light? Or is it a landing {in the}
dark?

If you run the field supply off the sense lead, and it falls off,
then the alternator produces no output. Not great if in heavy IFR,
but a better deal then in the same situation with NO working
radios/navs.

There is another aspect. Ma Bell figured out eons ago that copper-copper
joints, say at binding posts/splices etc. develop intermittent
connections. [Little whiskers grow between 'em..] They perfected
"sealing current" -- run a small DC current and it cures the issue.
So you want to draw some current down the sense lead anyhow..

--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

--
Regards, Ross
C-172F 180HP