CESSNA
SECTION 7
MODEL 172S NAV III
AIRPLANE AND SYSTEMS DESCRIPTION
ELECTRICAL SYSTEM
(Continued)
LOW VOLTAGE ANNUNCIATION
A signal from the Alternator Control Unit (ACU), located inside the
Power Distribution Module, provides the trigger for a red "LOW
VOLTS" annunciation shown on the PFD. "LOW VOLTS" is
displayed when the Main bus voltage measured in the junction box
is below 24.5 volts. This warning annunciation is an indication that
the alternator is not supplying the power that is required by the
aircraft. If the conditions causing the warning can not be resolved,
nonessential electrical loads should be eliminated and the flight
should be terminated as soon as practicable.
NOTE
During low RPM operation, with a high electrical
load on the system, such as during a low RPM taxi,
the "LOW VOLTS" annunciation may come on, the
bus voltage values may turn red, and Main battery
ammeter discharge indications may occur. Under
these conditions, increase RPM or decrease
electrical loads to reduce demand on the battery.
In the event an overvoltage condition (or other alternator fault)
occurs, the ACU will automatically open the ALT FIELD circuit
breaker, removing alternator field current and stopping alternator
output. The Main Battery will then supply current to the electrical
system as shown by a discharge (negative number) on the M BAT
ammeter. The LOW VOLTS annunciation will come on when the
system voltage drops below 24.5 volts. Set the ALT FIELD circuit
breaker to the ON position (push in) to energize the ACU. If the
warning annunciation extinguishes and the Main Battery (M BATT)
ammeter indicates positive current, normal alternator charging has
resumed. If the annunciation comes on again, or the ALT FIELD
circuit breaker opens again, an alternator malfunction has occurred.
If the circuit breaker opens again, do not SET it to the ON position
again. Have a qualified technician determine the cause and correct
the malfunction. Turn off nonessential electrical loads and land as
soon as practicable.
(Continued Next Page)
172SPHAUS-00
U.S. 7-51