Alternator Field Circuit; Over-Voltage Warning System; Alternator Voltage Regulator; Removal And Installation - Cessna 1969-1976 172 Series Service Manual

nator mounting bolt, the alternator will then be free
for removal.
cedure.
pulley and adjust the belt tension
so
the belt slips
when the following torque value is applied.
g. To replace the alternator, reverse this pro-
h. Apply a torque wrench to the nut on alternator
TORQUE VALUES
FOR
CHECKING ALTERNATOR BELT TENSION
Used Belt
New Belt
Slips At
Slips At
7 to 9 Ft. Lbs. 11 t o 13 Ft. Lbs
-NOTE-
Whenever a new belt is installed, belt tension
should be checked within 10 to 25 hours of
operation.
i.
j
.
Tighten and safety wire upper and lower adjust-
Tighten alternator mounting bolt.
ing bolt
s.
16 -42.
ALTERNATOR FIELD CIRCUIT PROTEC-
TION. On all 172 models except the F172,
a
auto-
matic resetting circuit breaker located on the back
of the instrument panel is provided to protect the
alternator field circuit.
16-43. OVER-VOLTAGE WARNING SYSTEM.
16-44. DESCRIPTION. The over-voltage system
consists of an over-voltage sensor switch and
a
red
warning light labeled "HIGH VOLTAGE". The over-
voltage sensor
is
attached t o the wire bundle behind
the instrument panel and the light is located on the
right hand side of the instrument panel. When an
over-voltage tripoff occurs the over-voltage sensor
turns off the alternator system and the r e d warning
light comes on. The ammeter will show a discharge.
Turn off both sections of the master switch to recycle
the over-voltage sensor. If the over-voltage condi-
tion was transient, the normal alternator charging
will resume and no further action is necessary. If
the over-voltage tripoff recurs, then
a
generating
system malfunction has occurred such that the elec-
trical accessories must be operated from the aircraft
battery only. Conservation of electrical energy must
be practiced until the flight can be terminated. The
over-voltage light filament may be tested
at
any time
by turning off the "Alternator" portion of the master
switch and leaving the battery portion on. This test
does not induce an over-voltage condition on the elec-
trical system. On models prior to aircraft s e r i a l
17260318 and F17200826, should nuisance
trip-outs occur caused by voltage spiks
or
transient
voltage, Cessna Single-engine Service Letter SE72-15
dated April 21, 1972 should be complied with.
NOTE
The over-voltage sensor switch contains solid
state devices. Observe proper polarity before
supplying power. Grounding the orange lead
or
interconnecting orange and black leads will de-
stroy the device. When removal is required
for replacement, identify (tag) wiring and fol-
low the wiring diagram in Section 20 for re-
wiring.
16-45. ALTERNATOR VOLTAGE REGULATOR.
16-46. DESCRIPTION. The alternator voltage
regulator contains two relays. One relay is actu-
ated by the aircraft master switch and connects the
regulator to the battery. The second relay is a two-
stage, voltage sensitive device which is used to con-
trol the current applied to the field winding of the
alternator. When the upper
set
of contacts on the
voltage regulator relay a r e closed, full bus voltage
is
applied to the field. This condition will exist when
the battery
is
being heavily charged
or
when
a
very
heavy load is applied to the system. When the upper
contacts open, as the voltage begins to r i s e toward
normal bus voltage, the voltage to the alternator
field
is
reduced through
a
resistor network in the
base of the regulator, thus reducing the output from
the alternator. As the voltage continues t o r i s e ,
assuming
a
very light load on the system, the lower
contacts will close and ground the alternator field
and shut the alternator completely off. Under lightly
loaded conditions the voltage relay will vibrate be-
tween the intermediate charge r a t e and the lower
(completely off) contacts. Under a moderate load,
the relay will vibrate between the intermediate charge
rate and the upper (full output) contacts.
The voltage relay is temperature compensated
so
that
the battery is supplied with the proper charging volt-
age for all operating temperatures. With the battery
fully charged (ship's ammeter indicating
at or
near
zero) and
a
moderate load applied to the system
(a
taxi
light turned on) the voltage
at
the bus b a r should
be within the range shown according t o the air temp-
erature on the following chart:
TEMPERATURE BUS VOLTAGE
60
-
74°F.
. .
.
. . .
1 3 . 8
-
1 4 . 1
75
-
90°F.
.
. . . . .
1 3 . 7
-
1 4 . 0
91
-
1 0 0 ° F .
.
. .
.
.
1 3 . 6
-
13.9
The voltage regulator
is
adjustable but adjust-
ment on the airplane is not recommended. A
bench adjustment procedure
is
outlined i n the
Cessna Alternator Charging Systems Service/
Parts
Manual.
16-47. TROUBLE SHOOTING. For trouble shooting
the voltage regulator r e f e r to paragraph 16-39.
16-48. REMOVAL AND INSTALLATION. (Refer to
figure 16-5. )
a . Make sure that the master switch
is
off,
or dis-
16-12