(3)  
On takeoff, the ram air inlet and exit doors are no longer locked in the full open position. If a higher pack discharge temperature is demanded, the pack temperature controller signals the turbine bypass valve to open. If pack output temperature is too low with the turbine bypass valve full open, the ram air doors close until the temperature reaches the required point. If temperature requirements shift and call for cooler air, the ram air doors open, and then the turbine bypass valve closes.

 (4)  
If an overheat occurs in the pack discharge duct, the flow control and shutoff valve closes and the SYS FAULT light on the cabin temperature control module comes on. After the pack discharge overtemp switch has cooled, the pack may be restarted by pressing the PACK RST switch momentarily.

 (5)  
If a pack flow control and shutoff valve is closed due to SYS FAULT or any other reason, the turbine bypass valve and ram air doors are positioned by the pack temperature controller to the specified openings for pack shutdown. This is known as prepositioning and it prepares the pack for starting.

 (6)  
ACM fan surge is prevented by pack shutdown if improper sequencing occurs between the turbine bypass valve and either the ram air inlet or exit actuator. Shutdown occurs when the turbine bypass valve closes 70% and either the ram air inlet closes more than 70%, or the ram air exit closes more than 85%.

 B. Control
 (1)  Operation of the pack temperature control system is automatic. Refer to 21-00-00/201 for air conditioning system startup and shutdown procedures.
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 21-62-00
 ALL  ú ú 02 Page 9 ú Oct 18/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
 ACM TURBINE BYPASS VALVE - REMOVAL/INSTALLATION
_______________________________________________
 1.  General_______
 A.  This procedure has two tasks. The first task is the removal of the turbine bypass valve for the air cycle machine (ACM). The second task is the installation of the turbine bypass valve.
 B.  There are three turbine bypass valves for the ACMs in the air
 conditioning packs.

 C.  The removal of the turbine bypass valve has these steps:
 -Make sure the temperature of the ACM is at the ambient temperature
 -Remove the power from the turbine bypass valve
 -Get access to the air conditioning bay
 -Remove the turbine bypass valve.
 D.  The installation of the turbine bypass valve has these steps:
 -Install the turbine bypass valve
 -Do a test of the turbine bypass valve.
 TASK 21-62-01-004-001
 2.  Turbine Bypass Valve Removal (Fig. 401)____________________________
 A.  References
 (1)
AMM 06-09-03/201, Body Section 44 Access Doors and Panels

 (2)
AMM 45-10-00/201, Central Maintenance Computer System

 (3)
IPC 21-62-01 Fig. 1

 B.  Access
 (1)  
Location Zone
 133 Air Conditioning Equipment Bay, LH
 134 Air Conditioning Equipment Bay, RH

 (2)  
Access Panel
 191A-L Air Conditioning Access Door (Pack 1)
 191H-L Air Conditioning Access Door (Pack 2)
 192A-R Air Conditioning Access Door (Pack 3)

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 21-62-01
 ALL  ú ú 01 Page 401 ú Oct 18/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

PACK NO. 2 TURBINE BYPASS VALVE
SEE A
PACK NO. 1 TURBINE BYPASS VALVE
SEE A
PACK NO. 3 TURBINE BYPASS VALVE
SEE A
ELECTRICAL 
CONNECTOR 
VALVE POSITION 
INDICATOR 

ACTUATOR
MANUAL POSITIONING SHAFT (3/16" HEX)
COUPLING  SEE B CLAMP (2 PLACES) 
　
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