c172 alternator

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CESSNA AIRCRAFT COMPANY
MODEL 172
MAINTENANCE MANUAL
ELECTRICAL POWER - GENERAL
1. Scope
A. This chapter gives the electrical units and components which control and supply electrical power for
the airplane systems. This includes the alternator, batteries, and relays.
B. Electrical energy for the airplanes is supplied by a 28-volt, direct current, single primary bus, negative
ground electrical system. A single 24-voltmain battery supplies power to the starting system and gives
a reserve source of power if an alternator failure were to occur. Airplanes that have the Garmin G1000
system have a second battery known as the Standby Battery. The Standby Battery is controlled and
monitored by the Standby Battery Controller and supplies power to the G1000 Essential Bus if there
is a failure of the main battery and alternator. A power junction box, also referred to as a Master
Control Unit (MCU), is attached to the forward left side of the firewall and includes electrical relays, an
alternator control unit (ACU), an ammeter sensor, an external power receptacle, fuses and/or circuit
breakers in a single box. An engine-driven alternator is the normal source of power during flight and
maintains a battery charge controlled by the ACU. The external power receptacle is used for ground
operation of the electrical equipment and helps the main battery during ground starts.
C. Electrical power is supplied to the two primary electrical busses through two 30A fuses, two 30A circuit
breakers, or two 40A circuit breakers in the junction box. These electrical busses supply power to two
avionics busses through 15A circuit breakers. The two avionics busses are controlled by an avionics
master switch.
D. The operation of the main battery and alternator system is controlled by the MASTER ALT BAT switch.
The switch is an interlocking split rocker and is found on the left side of the switch panel. The right
half of the rocker controls the main battery and the left half controls the alternator. It is possible
in this configuration for the main battery to be online without the alternator. However, operation of
the alternator without the main battery is not possible. The BAT MASTER switch, when operated,
connects the main battery contactor coil to ground so that the contacts close and supply power to the
system from the main battery only. The ALT MASTER switch, when ON, applies positive voltage to
the ACU and to the alternator contactor coil at the same time, which then applies field voltage to the
alternator field and supplies power to the electrical system from the alternator.
E. The operation of the Standby Battery, if installed, is controlled by a three-position STDBY BATT switch.
Normal flight operation is with the switch in the ARM position that lets the standby battery charge from
the G1000 Essential Bus. If there is an alternator failure, the standby battery controller will not let the
standby battery discharge to the G1000 Essential Bus until the depletion or failure of the main battery.
It is necessary during preflight to do an "energy level" acceptance test. Refer to the Pilot's Operating
Handbook, Chapter 4, Starting Engine, for details of the "energy level" acceptance test.
F. The main battery ammeter is controlled by a sensor found in the power junction box. In flight, without
the use of external power, the meter shows the quantity of current that flows to or from the battery.
With a low battery and the engine at cruise speed, the ammeter will show a large positive output and a
charge of the main battery. When the main battery is fully charged, the ammeter will show a minimum
charge rate.
G. The main battery is a 24-volt, 12.75 Amp-hour (5-hour rate), flooded lead-acid type. The battery is
installed in the front-left side of the firewall.
2. Tools, Equipment and Materials
NOTE: Equivalent substitutes can be used for the following items:
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© Cessna Aircraft Company Jul 3/2006
CESSNA AIRCRAFT COMPANY
MODEL 172
MAINTENANCE MANUAL
ALTERNATOR CONTROL UNIT - MAINTENANCE PRACTICES
1. General
A. The Alternator Control Unit (ACU) is found inside the power junction box, also referred to as a Master
Control Unit (MCU) or J-Box. The alternator system includes the ACU, Alternator Contactor, and
alternator field circuit. The ACU functions are as follows:
(1) Alternator Voltage Regulation - The ACU controls the alternator field circuit to supply a main bus
voltage of approximately 28.5 volts.
(2) Low Voltage Annunciation - The ACU monitors the main bus voltage in the power junction box
and supplies an output for low voltage (less than 24.5 +0.35 or -0.35 volts) for the annunciation.
(3) Over-voltage Protection - The ACU monitors the main bus voltage in the power junction box and
disengages the aircraft ALT FIELD circuit breaker. This removes the power from the alternator
system if there is an over-voltage condition greater than 31.75 +0.5 or -0.5 volts.
(4) Reverse Alternator Current Protection - The ACU monitors the alternator output current and
disengages the aircraft ALT FIELD circuit breaker. This removes the power from the alternator
system if there is a reverse alternator current.
(5) Excess Field Current Protection - The ACU monitors the alternator field current and disengages
the aircraft ALT FIELD circuit breaker. This removes the power from the alternator system if
there is an excessive field current.
2. Alternator Control Unit Removal/Installation
A. Remove the Alternator Control Unit. Refer to Power Junction Box - Maintenance Practices,
Component Removal/Installation.
B. Install the Alternator Control Unit. Refer to Power Junction Box - Maintenance Practices, Component
Removal/Installation.
3. Over-voltage Protection Circuit Test
A. General.
(1) The ACU Over-voltage Protection Circuit must be tested in accordance with the time limits in
Chapter 5, Inspection Time Limits. Use one of the two procedures that follow to do the test of
the Over-voltage Protection Circuit. The recommended procedure uses the Lamar TE04 MCU
Test Set. The external battery procedure can be used if a TE04 test set is not available.
B. Over-voltage Protection Circuit Test with the Lamar TE04 MCU Test Set
(1) Use a Lamar TE04 MCU Test Set and do steps 4.2, 4.3.A, 4.3.B, and 4.3.I in the Lamar’s TE04
MCU Test Set instructions LI-0021(refer to Electrical Power - General, Tools, Equipment, and
Materials).
(2) If the ACU TRIP indicator on the TE04 MCU Test Set does not illuminate in step 4.3.I, the Overvoltage
Protection Circuit is not operational.
(a) Replace the ACU.
(b) Do this test again.
(3) If the ACU TRIP indicator does illuminate in step 4.3.I, the Over-voltage Protection Circuit is
operational.
(a) Complete the Lamar procedure 4.3.I.
(b) Remove the TE04 MCU Test Set.
(c) Continue with step D in this section.
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© Cessna Aircraft Company Jan 1/2008