Cessna SkyHawk SP 172S NAV III 2005 Pilot Operating Handbook page 105

SECTION 3
EMERGENCY PROCEDURES
CESSNA
MODEL 172S NAV III
SYSTEM SUPPLY
ELECTRICAL POWER
MALFUNCTIONS
(Continued)
EXCESSIVE RATE-Of-CHARGE (Continued)
Electronic components in the electrical system can be adversely
affected by higher than normal voltage. The alternator control unit
includes an overvoltage sensor circuit which will automatically
disconnect the alternator if the charge voltage increases to more
than approximately 31.75 volts. If the overvoltage sensor circuit
does not operate correctly, as shown by voltage more than 31.75
volts on the main battery bus voltmeter, the MASTER switch ALT
section should be set to the OFF position. Unnecessary electrical
equipment should be de-energized and the flight terminated as soon
as practical.
INSUffiCIENT RATE-Of-CHARGE
When the over-voltage sensor circuit, or other fault, opens the
alternator (ALT FI ELD) circuit breaker and de-energizes the
alternator, a discharge (-) current will be shown on the main battery
ammeter and the low voltage annunciator (LOW VOLTS) will come
on. The ACU can de-energize the alternator due to minor
disturbances in the electrical system, resulting in a
II
nuisance"
opening of the ALT FIELD circuit breaker. If this happens, an
attempt should be made to energize the alternator system.
To energize the alternator system
1. MASTER Switch (ALT Only) - OFF.
2. Alternator Circuit Breaker (ALT FIELD) - CHECK IN.
3. MASTER Switch (ALT Only) - ON.
If the problem was a minor ACU disturbance in the electrical
system, normal main battery charging will start. A charge (+)
current will be shown on the main battery ammeter and the LOW
VOLTS annunciator will go off.
If the LOW VOLTS annunciator comes on again, there is an
lalternator system problem. Do not repeat steps to energize the
lalternator system. The electrical load on the battery must be
minimized (by de-energizing nonessential electrical equipment and
avionics) because the battery can supply the electrical system for
only a short time. Reduce electrical load as soon as possible to
extend the life of the battery for landing. Land as soon as practical.
(Continued Next Page)
3-34 U.S.
172SPHAUS-03