I have found several cases where the 60 amp alt breaker would get old
and tired and not "trip" but would cause an intermittent open. This
will spike the electrical system and cause a over voltage trip. Also
an old electrolytic cap across the over voltage relay input will not
filter out the normal buss spikes and cause a false trip.

Here are the top problem areas that I have seen:
#1 Bonding ground strap from the engine case to the airframe ,or
engine mount, has high resistance due to corrosion. (This one almost
killed me one day, and would have too if I had pulled the gear up
before the flaps on a go around.)
#2 Intermittent master switch or alternator switch half on
Cessna’s.
#3 Old dried out voltage spike suppresser capacitor across the over
voltage relay on Cessna’s.
#4 Stainless steel washer between the battery cable terminal and
the battery post. It is OK if it is under the wing nut but is not OK
if it is between the terminal and the battery post.
#5 Worn out high resistance 60 amp alternator circuit breaker.
#6 Loose connections to the amp meter or inside the amp meter or
both. Generally tightening the terminals will fix the problem.
#7 Worn out master relay.

There is a quick safe way to test the resistance of each connection
without the engine running and it only requires a 12 or 24 ohm 50 watt
or bigger power resistor and a digital volt meter. The method used is
called "Kelvin 4 wire resistance measurement" By using an
independent current source and voltmeter that can read voltages to at
least one millivolt , you can measure the milliohms in each connection
while you bang on , tap or move the wiring or part. This will
uncover many vibration induced problems in a controlled method. You
are now looking for millivolt CHANGES in voltage across the part as
the part is moved or the wiring is moved. What you do is induce
about a one amp current in the wiring chain that you want to test with
the aircraft’s battery and the 12 ohm (24 ohm for 24 volts) resistor.
One amp is used because it makes the math easy, one millivolt drop is
equal to one milliohm of resistance (V=IR). With the one amp
flowing through what you want to measure use the voltmeter to measure
the voltage across the part or wire segment. In this way you can
measure directly the resistance of each crimp connection for each
terminal and each connection of the terminal to the stud and also the
resistance of each wire segment or you can read the resistance or the
whole circuit that may contain several items in series. As an example
connect the resistor between the battery terminal of the alternator
and the alternator case and turn on the aircraft master power relay.
Measure the voltage from the alternator case to the engine, from the
engine to the airframe and from the airframe to the battery negative
terminal. Remember whatever voltage that you measure will be 50
times greater if you have a 50 amp alternator while the alternator is
fully charging. The power dissipated in each connection is the
resistance times the current squared. For 0.1 ohms at 50 amps the
power in the 0.1 ohm resistance would be 250 watts and the voltage
drop would be 5 volts.

As strange as it appears I found one cessna that kept tripping the
over voltage that was caused by someone who had clear coated the
alternator rear case half and the ground stud did not make good
contact from the alternator diode stack to the case at all times.
This allowed the suppression cap to charge up to 90 volts or more then
discharge to ground which induced a big voltage spike on the
alternator wiring.


John


On 10 Nov 2004 22:18:14 GMT, (WinstonCup) wrote:

I have a 1978 C182 Q. I recently experienced my alternator turning off and the
high voltage light coming on. I cycled the switch (1/2 master) and got it to
reset every time, however, the interval between episodes varied. I had the
Voltage Regulator and the Alternator replaced (new) and flew a couple of
flights since with no problem... until my last flight. Last flight it happened
again. The Alternator switched off and the high voltage light came on. I
cycled the switch and it reset.

What else is causing this? Is it a bad switch? Loose connection at the bus?

Has anyone else experienced this and what was your solution?

Any feedback is appreciated.

K C
PP ASEL IA
N759JB