Cessna 172P - Page 151

CESSNA
MODEL 172P
SECTION 7
AIRPLANE
&
SYSTEMS
DESCN,IPTIONS
high-low voltage control
unit mounted
on
the engine
side
of the
firewall
and a
red warning tight,
Iabeled LOW
VOLTAGE,
on
the
left side
of
the
instrument
panel
below the ammeter.
In
the event an over-voltage
condition
occurs,
the alternator
control
unit automaticalty
removes alternator
field current
which
shuts down
the
alternator. The battery
will
then
supply system
current
a,s
shown by a,
discharge
rate
on
the ammeter. Under
these conditions,
depending
on
electrical
system load, the
low-voltage warning
light
will
illuminate
when
system
voltage drops below
normal. The
a,Iternator control
unit may
be
reset
by
turning the master switch
off and
back on
again.
If the
warning
light
does not illuminate,
normal alternator
charging
has resumed;
however, if the light does illuminate
again, a malfunction
has occurred,
and
the
flight should be terminated
as soon
a,s
practicable,
NOTE
Illumination of the low-voltage
light
and ammeter
dis-
charge indications
ma,y
occur
during
low
RPM
conditions
with an electrical
load on the system, such as
during a
low
RPM taxi. Under these conditions,
the
light will
go
out
at
higher RPM.
The master switch
need not be
recycled since
an over-voltage condition
has not occurred
to
de-activate
the alternator
system.
The
warning light may be
tested by turning
on the
landing lights and
momentarily turning off
the
ALT
portion
of
the master switch
while
leaving
the BAT
portion
turned on.
CIRCUIT BREAKERS AND FUSES
Most
of the
electrical circuits
in
the airplane are
protected
by
"push-to-
reset" type
circuit breakers mounted on the left slde of the switch and
control
panel.
Elowever, circuit
breakers
protocting
the
alternator output
and the strobe light/ avionic
cooling fan
circuits
are the
"pull-off"
type. In
addition to
the individual circuit breakers, a
toggle switah/ circuit
breaker,
labeled
AVIONICS POWER, on the left
side of the switch and control
panel
also
protects
the avionics systems.
The cigar Iighter
ls
protected
by a
manually-reset
type cirouit breaker on the
back of the lightor, and
a fuse
behind
the instrument
panel.
The control
wheel
map light
(if
installed) is
protected
by the NAV
LT circuit breaker
and a
fuse behind the
instrument
panel,
Eleotrical
circuits which
are not
protected
by circuit breakers are
the
battery contactor
closing
(external power)
circuit,
clock
circuit,
and
flight hour
recorder
circuit.
These circuits
are
protected
by fuses
mounted
adjacent to
the
battory.
30
May 1980
7-27