AMM 24-00 General Information

Cirrus aircraft have a 28 volt, direct-current (DC) electrical system. The system generates and stores DC power,
distributes electrical loads, and provides access for external power. During normal operation, the system provides
uninterrupted power for avionics, flight instruments, lighting, and other electrically operated and controlled systems.
The Master Control Unit (MCU) is mounted to the forward side of the firewall. The MCU provides alternator voltage
regulation and overvoltage protection, fault protection for the starter and battery (1), and power distribution.
The external power receptacle is located forward of the firewall on the left side. External power may be applied to the
aircraft for cold weather starting or maintenance procedures. External power must be regulated to 28 volts (DC). The
BAT 1 switch in the cockpit must be ON for external power to supply the aircraft.
Alternators are self-exciting (not self-starting); the field must be energized with battery power to generate power.
Single Power Distribution – MCU – No maintenance permitted on this item – remove and replace only.
Serials 20-1005 through 20-1267 (Single or Dual Alternator(s) and Single Battery)
Constant charging current for the battery and power for the electrical system is provided by the alternator(s).
The MCU regulates and distributes power to the battery and system loads. In the event of an electrical failure, the
system is designed to allow for simplified load shedding.
Dual Power Distribution (Essential & Main Busses) – MCU 100, MCU 110, MCU 120, MCU 130 & MCU 140
Serials following 20-1268 & 22-0002 (Dual Alternators and Dual Batteries)
Constant charging current for the batteries and power for the electrical system is provided by the alternators. Each
alternator provides constant charging current for its corresponding battery and powers its distribution bus for the
electrical system. The MCU regulates and distributes power to the batteries and system loads (alternator 1 charges
battery 1 and powers the main distribution bus; alternator 2 charges battery 2 and powers the essential bus).
Distribution busses are interconnected by two (2) 50 amp fuses and a diode. In the event of a failure of any electrical
power component, the system automatically directs available power to the essential bus. The system is also designed
to allow for simplified load shedding.

Diodes prevent alternator 2 from supplying power to the main distribution bus.
Alternator 1 voltage is regulated to 28.0 volts and alternator 2 is regulated to 28.75 volts so that
alternator 1 will only supply power to the essential bus when alternator 2 is inoperative.

AMM 5-20 Master Control Unit Inspection

AMM 24-30 Serials following 20-1268 & 22-0002 (MCU 100, MCU 110, MCU 120, MCU 130 & MCU 140)
Turn BAT 1, BAT 2, & AVIONICS switches OFF.
Disconnect battery 1.
Remove MCU cover.
Inspect for moisture. Dry if necessary.
Apply Mil-C-81309 Type III corrosion inhibitor to circuit
board, internal components, and connectors.
Inspect security of mounting.
Inspect physical condition of cables and connectors.
MCU 100, 110, 120, & 130:
Inspect sealant on housing and around cable exit holes.
Replace sealant as necessary. Install MCU cover.
Connect battery 1.

USE CIRRUS TECHNICAL PUBLICATIONS FOR MAINTAINING CIRRUS AIRPLANES.

THIS TRAINING DOCUMENT IS FOR REFERENCE AND REVIEW.
Created by Doug Larson

SR2X 1 of 12 ©2009 CIRRUS DESIGN CORPORATION

Electrical System – A English
 AMM 24-30 Alternator 1
AMM 5-10 Overhaul alternator 1 when engine is overhauled.
Serials 20-1005 and subsequent
75A belt driven alternator
Belt tension is correct when the belt slips when a force of 11-13 ft lb (14.9-17.6 Nm)
is applied counter-clockwise to the alternator pulley nut.
Serials 22-0002 through 22-1820 and subsequent
with Entegra Avionics but no cabin fan or air conditioning
60A engine driven alternator
Serials 22-1821 and subsequent
with cabin fan, air conditioning, or Perspective Avionics
100A engine driven alternator

AMM 24-30 Alternator 2

AMM 5-10 Overhaul alternator 2 after 2000 hours in service.
Serials 20-1005 through 20-1267 with optional alternator 2
Serials including and following 20-2016 with Perspective Avionics
40A engine driven alternator
Serials including and following 20-1268 & 22-0002 with Entegra Avionics
20A engine driven alternator
Look for gray dust residue around rear housing
cooling slots which may indicate bearing failure.
Clean casting webs at base and look for cracks.
Replace alternator if cracks are found.
Serials including and following 22-3026 with Perspective Avionics
70A belt driven alternator
Belt tension is correct when the belt slips when a force of 11-13 ft lb
(14.9-17.6 Nm) is applied counter-clockwise to the alternator pulley nut.

AMM 24-30 Battery 1

AMM 5-10 Battery 1 is a 24 volt, lead-acid battery located forward right side of the firewall.
Replace Concorde batteries, original equipment on serials including and following
20-2016 & 22-3026, after 3 years or 1800 hours in service.

AMM 24-30 Battery 2

AMM 5-10 Battery 2 is comprised of two (2) 12 volt, sealed, lead-acid batteries connected in series.
Battery 2 is located in the empennage, aft of bulkhead 222.
Replace battery 2 after 2 years or 500 hours in service.

USE CIRRUS TECHNICAL PUBLICATIONS FOR MAINTAINING CIRRUS AIRPLANES.

THIS TRAINING DOCUMENT IS FOR REFERENCE AND REVIEW.
Created by Doug Larson

SR2X 2 of 12 ©2009 CIRRUS DESIGN CORPORATION

Electrical System – A English
 AMM 24-30 System Indications & Annunciators
AMM 31-50 Serials 20-1005 through 20-1267 (MCU)
POH SR20 Low Volt annunciator illuminates red if system voltage is approximately 24.5 volts or less.
POH SR22 Voltmeter displays main bus voltage.
Ammeter indicates alternator 1 (and alternator 2) current generation and battery charging status.
Aircraft equipped with optional, second alternator include:
Alt 1 annunciator illuminates amber if alternator 1 is not generating power.
Alt 2 annunciator illuminates amber if alternator 2 is not generating power.
Voltmeter Ammeter
Serials 20-1268 through 20-1581 & 22-0002 through 22-1662 (MCU 100, 110 & 120)
Low Volt annunciator illuminates red if system voltage is approximately 24.5 volts or less.
Alt 1 annunciator
Flashes amber approximately 40 times per minute when alternator 1 is overloaded.
Illuminates amber when alternator 1 generates approximately 2 amps or less.
Alt 2 annunciator
Flashes amber approximately 40 times per minute when alternator 2 is overloaded.
Illuminates amber when alternator 2 generates approximately 2 amps or less.
Voltmeter displays essential bus voltage.
Ammeter indicates alternator 1 and alternator 2 current generation and battery 1 charging status.
Serials 20-1582 & 22-1663 and subsequent with Entegra Avionics (MCU 130)
Low Volt annunciator illuminates red if system voltage is approximately 24.5 volts or less. Annunciator Panel
(Entegra)
Alt 1 annunciator
Flashes amber 40 times per minute when alternator 1 is overloaded.
Illuminates amber when alternator 1 generates approximately 2 amps or less.
Alt 2 annunciator
Flashes amber 40 times per minute when alternator 2 is overloaded.
Illuminates amber when alternator 2 generates approximately 2 amps or less.
Primary Flight Display (PFD) displays main and essential bus voltages below normal values.
Multi-Function Display (MFD)
Engine page displays main and essential bus voltages.
Engine page displays alternator 1, alternator 2, and battery 1 charging status. MFD Engine Page
Displays crew advisories about system status. (Entegra)

Serials 20-2016 & 22-3026 and subsequent with Perspective Avionics (MCU 140)
Primary Flight Display (PFD) displays MFD ribbon functions (below) in reversionary mode.
Multi-Function Display (MFD)
Left side engine ribbon displays essential bus voltage and battery 1 charging status.
Displays crew advisories about system status.
Engine page displays alternator 1, alternator 2, and battery 1 charging status. MFD engine ribbon
Engine page displays voltage for essential and both main distribution busses. (Cirrus Perspective)

AMM 5-30 Dual Power Distribution Bus Functional Inspection (MCU 100, 110, 120, 130 & 140)
At every aircraft operational/functional inspection performed before and after a 100 hour or annual inspection, the
diodes preventing essential bus voltage from powering the main bus must be functionally inspected.
Turn battery 2 master switch ON.
Battery 2 or essential bus indicates 24 volts or more.
Flap position indicator remains off.
Turn battery 1 master switch ON.
Flap position indicator turns on.
Turn battery 2 master switch OFF.
Battery 1 or main bus indicates 24 volts or more.
Turn battery 1 master switch OFF.
USE CIRRUS TECHNICAL PUBLICATIONS FOR MAINTAINING CIRRUS AIRPLANES.
THIS TRAINING DOCUMENT IS FOR REFERENCE AND REVIEW.
Created by Doug Larson

SR2X 3 of 12 ©2009 CIRRUS DESIGN CORPORATION

Electrical System – A English
AMM 24-30 MCU
The Master Control Unit (MCU) is a line replaceable unit that regulates and distributes alternator output. Alternators
are normally regulated at 28.0 volts. No maintenance is permitted inside of this unit.
Single Alternator System – Serials 20-1005 through 20-1267
The MCU contains electrical contactors for the alternator, battery, starter, and external power circuits as well as a
landing light relay, an ammeter transducer, an alternator control unit, and distribution fuses/circuit breakers.
Dual Alternator System – Serials 20-1005 through 20-1267
The MCU contains electrical contactors for the battery, starter, and external power as well as a landing light relay, two
(2) function modules, two (2) alternator control units, and distribution circuit breakers.
Normally, alternator 1 handles 60% of system loads and alternator 2 handles the remaining 40%. If one alternator has
an overvoltage condition, the MCU will open the corresponding circuit breaker and the other alternator will supply 100%
of the aircraft electrical system load.

AMM 24-30 MCU Installation

AMM 5-20 Turn Coordinator & Stall Warning Horn – Emergency Power Source
AMM 24-30 Serials 20-1005 through 20-1267
Turn coordinator emergency batteries are installed on the forward side of the pilot bolster panel. This power source is
independently controlled and wired to provide emergency power for the turn coordinator and stall warning horn when
connected with battery 1 and the alternator(s). Failure of the alternator(s) or battery will not affect the ability of the turn
coordinator batteries to provide necessary power. Diodes protect the battery and alternator(s) from a failure of the turn
coordinator emergency batteries.
Inspection requirements:
Check date stamped on batteries and replace as required.
All three (3) 9 volt batteries must be:
Manufactured by Duracell, marked MN1604
Marked with the same date stamp.
Changed at the same time.

PILOT BOLSTER PANEL

USE CIRRUS TECHNICAL PUBLICATIONS FOR MAINTAINING CIRRUS AIRPLANES.

THIS TRAINING DOCUMENT IS FOR REFERENCE AND REVIEW.
Created by Doug Larson

SR2X 4 of 12 ©2009 CIRRUS DESIGN CORPORATION

Electrical System – A English
 MCU CMM Schematic of Electrical System with MCU

75A
ALT 1
CONTACTOR
CURRENT
SENSOR
MCU
ALT 1 + (SINGLE ONLY)
FLD
Some early models used fuses
EXTERNAL FUNCTION instead of circuit breakers on
EXTERNAL POWER MODULE Power Bus Bar
POWER CONTACTOR
80A
CURRENT
SENSOR
POWER BUS BAR
BATTERY BATTERY 1
CONTACTOR FUNCTION 80A 25A 25A 25A 15A 15A
MODULE
LANDING
LIGHT
STARTER STARTER
RELAY
CONTACTOR

40A CURRENT
ALT 2 + SENSOR
CONSTANT POWER

FLD ALTERNATOR ALTERNATOR

CONTROL CONTROL
UNIT 1 – 28.0V UNIT 2 – 28.0V
5A
P111 J111

P109 J109
P108 J108

A B C D E F G H J K A B C D E F G H A B C
A B C D E F G H J K A B C D E F G H A B C STANDBY
VACUUM

NON-ESSENTIAL BUS
NON-ESSENTIAL

MAIN BUS 1

MAIN BUS 2
AVIONICS
RELAY

RELAY
ALT 1

ALT 2

AVIONICS
RELAY 1

15A
ON
20A 20A
AVIONICS ESSENTIAL

AVIONICS
RELAY 2

15A
ESSENTIAL

BAT 1 ALT 1 ALT 2 AVIONICS

MASTER

USE CIRRUS TECHNICAL PUBLICATIONS FOR MAINTAINING CIRRUS AIRPLANES.

THIS TRAINING DOCUMENT IS FOR REFERENCE AND REVIEW.
Created by Doug Larson

SR2X 5 of 12 ©2009 CIRRUS DESIGN CORPORATION

Electrical System – A English
AMM 24-30 MCU 100, 110 & 120
The MCU 100/110/120 contains electrical contactors for battery 1, the starter, and external power. A large printed circuit
board distributes control and power signals. The logic module, current sensors, power distribution circuit breakers,
landing light relay, and fuses are attached. Two (2) field control modules and the power diodes are mounted in the MCU
100/110/120 frame. The external power receptacle is integral to the MCU 100/110/120.
MCU 110 is an upgrade of MCU 100 for a higher capacity alternator and has an additional accessory bus.
MCU 120 is similar to MCU 110. Only the location and method of external electrical connections are different.

MCU CMM MCU 110 Component Identification

AWM 24-30
Circuit Board

Logic Circuit
Module Breakers External
Power
Receptacle

Current Sensor
(Alt 2)
Fuses

Landing
Light Relay
Current Sensor
Fuses
(Alt 1) Current Sensor
(Bat 1)

Ext. Power
Contactor Circuit Board Removed

Starter
Contactor Field Field
Control Control
Module 1 Module 2 Diodes

Battery 1
Contactor

USE CIRRUS TECHNICAL PUBLICATIONS FOR MAINTAINING CIRRUS AIRPLANES.

THIS TRAINING DOCUMENT IS FOR REFERENCE AND REVIEW.
Created by Doug Larson

SR2X 6 of 12 ©2009 CIRRUS DESIGN CORPORATION

Electrical System – A English
MCU CMM Schematic of Electrical System with MCU (100), 110 & 120
AWM 24-30
Alternator 1 on SR20 aircraft with MCU 110 or 120 is only rated at 75 amps. Air conditioning is not an available option.

100A (75A)
CURRENT
SENSOR
MCU (100), 110 & 120
ALT 1 + 25A
FLD 100A

MAIN DIST BUS

(80A) 25A
EXTERNAL LOGIC 25A
POWER MODULE
EXTERNAL LANDING
CONTACTOR 15A
POWER LIGHT
RELAY

125A 40A
BATTERY 1 BATTERY 1 CURRENT
(100A) (X)
CONTACTOR SENSOR

ESSENTIAL DIST BUS

STARTER STARTER 50A 25A
CONTACTOR 50A 25A

CURRENT 40A
20A
SENSOR
ALT 2 +
FLD
CONSTANT
POWER

5A FIELD CONTROL FIELD CONTROL

MODULE 1 – 28.0V MODULE 2 – 28.75V
P111 J111

P757 J757
P108 J108

P115 J115

P109 J109
A B C D E F G H J K A B C D E F G H A B C A A B C
A B C D E F G H J K A B C D E F G H A B C A A B C

20A
AIR CONDITIONING BUS 1

ON
NON-ESSENTIAL BUS
ESSENTIAL BUS 1

ESSENTIAL BUS 2

MAIN BUS 1

MAIN BUS 2
BATTERY 2
RELAY

BAT 2 BAT 1 ALT 1 ALT 2 AVIONICS

MASTER 20A

30A 15A 15A

ESSENTIAL

ESSENTIAL
AVIONICS

AVIONICS

AVIONICS
NON-

BATTERY 2 AVIONICS
NON-
ESSENTIAL
ESSENTIAL
RELAY
RELAY

USE CIRRUS TECHNICAL PUBLICATIONS FOR MAINTAINING CIRRUS AIRPLANES.

THIS TRAINING DOCUMENT IS FOR REFERENCE AND REVIEW.
Created by Doug Larson

SR2X 7 of 12 ©2009 CIRRUS DESIGN CORPORATION

Electrical System – A English
AMM 24-30 MCU 130
The Master Control Unit (MCU)130 contains electrical contactors for battery 1, starter, and external power. A small
printed circuit board provides a mounting location for the landing light relay and MIDI fuses. The MCU 130 contains
current shunts for alternator 1, alternator 2, and battery 1. Two (2) field control modules, power distribution fuses, and
power diodes are mounted in the MCU frame. The external power receptacle is integral to the MCU 130.

MCU CMM MCU 130 Component Identification

AMM 24-30
AWM 24-30
Alt 2 Shunt Field
Control
Ext. Power
Module 2
Contactor

Fuses
External
Power
Battery 1 Landing Receptacle
Contactor Light Relay
Alt 1
Shunt

Fuses

Starter Circuit Board

Contactor Field
Control
Module 1

Diodes

Battery
Contactor
Shunt
Fuses

Fuses

Fuses

Circuit Board Removed

USE CIRRUS TECHNICAL PUBLICATIONS FOR MAINTAINING CIRRUS AIRPLANES.

THIS TRAINING DOCUMENT IS FOR REFERENCE AND REVIEW.
Created by Doug Larson

SR2X 8 of 12 ©2009 CIRRUS DESIGN CORPORATION

Electrical System – A English
MCU CMM Schematic of Electrical System with MCU 130
AMM 24-30
AWM 24-30 100A SHUNT
LANDING
LIGHT
MCU 130
ALT 1 +
FLD 100A RELAY

MAIN DIST BUS

15A

EXTERNAL 30A
POWER
EXTERNAL 30A
CONTACTOR
POWER 30A

30A
SHUNT

MODULE 2 – 28.75V
BATTERY 1 BATTERY 1 30A

FIELD CONTROL
CONTACTOR 125A

ESSENTIAL DIST BUS

STARTER STARTER 50A 30A
CONTACTOR 50A 30A

20A SHUNT
ALT 2 +
FLD 50A
CONSTANT
POWER

5A FIELD CONTROL
MODULE 1 – 28.0V
P137 J137

P135 J135

P136 J136

P138 J138
A B C D E F G H J K A B C D E A B C D E A B C D E F G H J K
A B C D E F G H J K A B C D E A B C D E A B C D E F G H J K

20A

ON
AIR CONDITIONING BUS 1

AIR CONDITIONING BUS 2

AVIONICS ESSENTIAL

NON-ESSENTIAL BUS
ESSENTIAL BUS

MAIN BUS 1

MAIN BUS 2
BATTERY 2
RELAY

BAT 2 BAT 1 ALT 1 ALT 2 AVIONICS

MASTER
20A

30A 15A 15A

ESSENTIAL

ESSENTIAL
AVIONICS

AVIONICS

AVIONICS
NON-

BATTERY 2 AVIONICS
NON-
ESSENTIAL
ESSENTIAL
RELAY
RELAY

USE CIRRUS TECHNICAL PUBLICATIONS FOR MAINTAINING CIRRUS AIRPLANES.

THIS TRAINING DOCUMENT IS FOR REFERENCE AND REVIEW.
Created by Doug Larson

SR2X 9 of 12 ©2009 CIRRUS DESIGN CORPORATION

Electrical System – A English
AMM 24-30 MCU 140 – Used in Cirrus Perspective equipped airplanes
The Master Control Unit (MCU) 140 contains a large printed circuit board to distribute control and power signals. The
circuit board also provides mounting locations for electrical contactors (battery 1, starter, and external power), current
shunts, power distribution fuses, landing light relay, and power diodes. Two (2) field control modules are mounted in the
MCU 140 frame. The external power receptacle is integral to the MCU 140.

MCU CMM MCU 140 Component Identification

Ext. Power Field Control Module 1

Contactor

External
Power
Receptacle
Battery
Shunt Diodes
Contactor

Field
Control
Fuses Module 2

Starter
Contactor

Landing
Light Relay

USE CIRRUS TECHNICAL PUBLICATIONS FOR MAINTAINING CIRRUS AIRPLANES.

THIS TRAINING DOCUMENT IS FOR REFERENCE AND REVIEW.
Created by Doug Larson

SR2X 10 of 12 ©2009 CIRRUS DESIGN CORPORATION

Electrical System – A English
MCU CMM Schematic of Electrical System with MCU 140

100A SHUNT LANDING MCU 140

ALT 1 + LIGHT
100A

MAIN DIST BUS 1

FLD RELAY
7.5A
EXTERNAL 30A
POWER
EXTERNAL
CONTACTOR 30A
POWER
30A

ESSENTIAL DIST BUS

SHUNT 50A
BATTERY 1 BATTERY 1
CONTACTOR 125A

80A 30A

50A
STARTER STARTER
CONTACTOR 30A

MAIN DIST BUS 2

MODULE 2 – 28.75V
FIELD CONTROL
50A

30A
70A SHUNT
ALT 2 + 30A
FLD 80A
30A
FIELD CONTROL
CONSTANT
POWER

MODULE 1 – 28.0V
5A
P183 J183

P181 J181

P180 J180

P182 J182
A B C D E F G H J K A B C D E A B C D E A B C D E F G H J K
A B C D E F G H J K A B C D E A B C D E A B C D E F G H J K

20A
AIR CONDITIONING BUS 2

AIR CONDITIONING BUS 1

ON NON-ESSENTIAL BUS
ESSENTIAL BUS 1

ESSENTIAL BUS 2

MAIN BUS 3

MAIN BUS 1

MAIN BUS 2

BAT 2 BAT 1 ALT 1 ALT 2 AVIONICS

MASTER
20A 10A

BATTERY 2 AVIONICS AVIONICS

BATTERY 2
RELAY 30A RELAY BUS

USE CIRRUS TECHNICAL PUBLICATIONS FOR MAINTAINING CIRRUS AIRPLANES.

THIS TRAINING DOCUMENT IS FOR REFERENCE AND REVIEW.
Created by Doug Larson

SR2X 11 of 12 ©2009 CIRRUS DESIGN CORPORATION

Electrical System – A English
 Intentionally Left Blank

USE CIRRUS TECHNICAL PUBLICATIONS FOR MAINTAINING CIRRUS AIRPLANES.

THIS TRAINING DOCUMENT IS FOR REFERENCE AND REVIEW.
Created by Doug Larson

SR2X 12 of 12 ©2009 CIRRUS DESIGN CORPORATION

Electrical System – A English