A common failure point of our charging system is the Over Voltage
switch, a little known and hard to find device that has the power to
shut down the alternator at will, since it lives in the alternator field power
circuit. Let's take a look at this pesky
Nessled in the wire bundle of most of our aircraft is a small black
tube with three wires and a simple goal in life: to protect us from
a runaway alternator. Should the alternator start to produce higher
than desired voltages on the buss, this box will shut down the
alternator until it is reset.
It does this by interrupting the field voltage, and as such it
is in series with the alternator field. It senses the voltage
of this line as it's source for monitoring for over voltage as
well. It is reset by turning the master switch off and back
It is located in the bundle of wires behind the circuit breakers, over
the feet of the copilot. Often it is surrounded by the wires
in that bundle and is not visible without removing a large number
of wire wraps. The two pictures here are typical installations in
two different aircraft.
The failure mode of this device is simple: the alternator stops
producing current and the voltage drops to battery levels. If
the overvoltage switch is defective this will often be a time or
temperature driven event, occurring a certain
number of minutes after startup or once the cabin reaches a
A quick check of the overvoltage sensor can be performed in a
simple way: on the ground, with the engine off, stand beside
the left side of the airplane and turn on the master switch.
Listen for the click of the regulator. You may wish to turn
on the battery portion of the master switch first, then cycle
the alternator side while listening for the click.
If you do not hear a click, there are only a few possible
causes: either you have a very fancy solid state regulator,
the master switch is not working, the circuit breaker is not
passing current, there is a wiring problem or the overvoltage
relay is open. By far the most common reason is the last.
By the way, the master switch is another frequent source of
problems, but they are most often intermittent and can be
observed in the same way, listening for the click as you
cycle the switch.
What is the solution? A new overvoltage relay is one
possibility, but the most common solution is removal of the
switch altogether (connecting the field wires together to
remove the switch from the circuit) and replacement of the
old 'Ford-style' regulator with a more modem version that
has built-in overvoltage protection.
One example is the Zeftronics regulator shown here. It
performs the functions of the overvoltage switch internally,
and includes indications via it's LED when it does so.
When you remove the over voltage switch you might want to study
up on the over voltage wiring circuit so you know exactly what
you are working with.
Do we really need this protection? Perhaps we should simply
remove the overvoltage protection and let the alternator run?
Not a good idea: remember that certain cars use an unregulated
alternator to defrost their windshields, and these systems
operate at over 90 volts.
Even worse than loosing a charging system would be loosing all
your radios and the battery at the same time...
Over Taiwan, April '98
These pages are a collection of the ideas and impressions of the Cardinal pilots who frequent this site. This information is anecdotal, informal and may not be completely accurate. The Cardinal Flyers are not certified mechanics and do not guarantee the accuracy of the contents of these pages. Please research and confirm anything that is referenced on these pages with the experts appropriate to your situation.
As always, the Cessna maintenance, operations and flight manuals, and the advice of a certified mechanic and flight instructor, should be your primary sources of information regarding safe maintenance and operation of your aircraft.
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