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B.  The right air conditioning system takes air from the No. 2 engine and the left air conditioning system from the No. 1 engine. (See figure 2.) One isolation valve is provided for the isolation of the systems. The isolation valve is automatic in operation and electrically controlled. Bleed air can be directed from one system to the other by opening of the isolation valve.
C.  Normally the APU check valve prevents engine bleed pressure from backing into the APU bleed system. Should the check valve fail open, with the APU bleed valve open, engine bleed pressure could be detrimental to the APU. The DUAL BLEED light on the overhead panel illuminates when the APU bleed valve is open and either engine No. 1 bleed valve or engine No. 2 bleed valve and isolation valve are open. The light would be on during an APU engine start. After engine start and the APU bleed valve switch is placed to OFF the light goes out indicating that the APU bleed valve has closed. (See figure 5.)
528 
May 15/70  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  36-11-0 Page 1 

Engine Bleed Air Distribution System Component Locations  543 
36-11-0  Figure 1 (Sheet 1)  Aug 01/74 
Page 2 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

F89202
567  Engine Bleed Air Distribution System Component Location 
Aug 15/78  Figure 1 (Sheet 2)  36-11-0 
Page 2A 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

525  Engine Bleed Air Distribution System Schematic 
May 15/73  Figure 2  36-11-0 
Page 3 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

2.  Bleed Valves
A.  
The engine bleed valves are motor-driven and electrically controlled. Each valve is located downstream of the heat exchanger (precooler) (Fig. 2 and 3). Control switches for the engine bleed valves are on the forward overhead panel. The engine bleed valves are provided to shut off bleed air from the respective engine bleeds to the main manifold. The control circuit includes an overtemperature switch downstream of the heat exchanger. The engine bleed overheat switch will cause the valve to close if the bleed air temperature exceeds 4900士100F. An indicator light on the forward overhead panel is illuminated concurrently with closure of the engine bleed valve and may be reset in flight after having been closed by the overtemperature switch, but only after the air temperature in the duct cools by approximately 200 to 300F. Cooling of the duct air allows the overtemperature switch to cool and reopen. A reset switch is provided for the bleed valves, and is located on the forward overhead panel. (Fig. 1). This reset switch is also used for the air conditioning pack valves.

B.  
A manual override is provided to permit positioning the valve manually. On some valves the manual override includes a lock out feature which disengages the valve motor. A decal on the actuator housing provides instructions for engaging and disengaging motor.

 

F89330
Bleed Valve Control Circuit  504 
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3.  Isolation Valve
A.  The right air conditioning system takes air from the No. 2 engine and the left air conditioning system takes air from the No. 1 engine. (See figure 2.) This separation of bleed air is accomplished by the isolation valve. The isolation valve remains closed unless a cross-feed of bleed air is required. The isolation valve is motor-driven and electrically controlled by a three-position switch on the forward overhead panel.
B.  The isolation valve may be manually controlled by the flight crew or it may be left in the automatic mode. In the automatic mode the isolation valve will be closed so long as both engine bleed switches and both pack switches are selected ON; this is the normal flight operating mode for the system. If any other combination of switch positions is selected the isolation valve will open. The isolation valve position is controlled by the position of the bleed and pack switches and not by the position of the bleed and pack valves themselves. In the event of an engine shutdown, or a 4900F overheat trip, the system will not automatically allow the remaining air source to supply both sides of the airplane. (See figure 4.)
　
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