When the OFF light comes on, the BLEED OFF APU advisory message usually shows. This message does not show if the APU is off. See the duct leak and overheat detection system section for more information (SECTION 26-18).
Isolation Bleed Switches
The isolation bleed switches have an annunciator light that comes on when the related isolation valve is closed because of a non-normal condition. The switches have two positions:
*
In (usual position)

*
Out (non-normal position.)

The AUTO position (switch in) lets the related isolation valve, left, center, or right OPEN/CLOSE as necessary to control the direction of air flow in the pneumatic distribution system.
The switch out position sets the related isolation valve to close. The CLOSED annunciator light in the switch comes on when the valve is closed.
The AUTO position with CLOSED indication (switch in) can occur when the related valve is closed and any of the these non-normal conditions occurs:
*
Wing or body duct overheat

*
Related isolation valve failed closed

*
Left (right) bleed source loss.

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PNEUMATIC -OPERATION

When the CLOSED light comes on because the related valve is failed closed or the related switch is set to out, the advisory message, BLEED ISLN CLOSED L (C, R) shows.
See the air supply distribution section for more information on the isolation valve OPEN/CLOSE position control (SECTION 36-12).
Training Information Point

Annunciator lights and EICAS messages on the bleed air/ pressurization panel usually relate to valve positions that the left and right ASCPCs control and monitor.
The isolation valves and APUSOV have RVDTs that the ASCPCs use to calculate when to turn on the related CLOSED or OFF lights. If a RVDT fails, the ASCPCs calculate when to turn on or show the related indications without the use of the failed RVDT. The calculation for the lights is based on the ASCPC set position for the related valve. The calculation for the message is based on switch position for the related valve.
The PRSOVs do not have any direct feedback to the ASCPC, no RVDTs, no limit switches and no proximity switches. The ASCPCs calculate the related valve position for all indications based on set position.
The primary control (digital) part of the ASCPCs usually gives control and indications for the related engine bleed air system, the isolation valves and APUSOV. If primary control fails in one controller, the other controller controls the isolation valves and APUSOV. All indications that are related to the ASCPC that has the failed primary control are lost. These are the indications lost related to the bleed air/pressurization panel:
BLEED AIR/PRESSURIZATION PANEL 
FAILED PRIMARY CONTROL  INDICATIONS LOST FOR THESE VALVES 
LEFT ASCPC  L ISLN 
C ISLN 
RIGHT ASCPC  R ISLN 
APUSOV 

The backup control (analog) part of the ASCPCs that has a failed primary control gives limited engine bleed air control and engine OFF indications.

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PNEUMATIC -OPERATION

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PNEUMATIC -INDICATIONS -EICAS DISPLAY AND SECONDARY ENGINE DISPLAY

General

You can see duct pressure for the left and right parts of the pneumatic distribution system on these displays:
*
EICAS display

*
Secondary engine display

*
Air synoptic display (not shown)

*
Air supply maintenance page (not shown)

*
Ice protection maintenance page (not shown)

*
Performance maintenance page (not shown).

The synoptic display and the maintenance pages are described later in this section.
The units of duct pressure are psig. The color of the pressure readout changes from white to amber when pressure is less than 11 psig. No pressure will show if any of these conditions occurs:
*
Related ASCPC primary (digital) control fails

*
Fault in the indication circuit.

EICAS Display

Duct pressure shows in the ECS data block part of the EICAS display. This block shows if any of these conditions occur:
*
Loss of landing altitude, FMC to ASCPCs

*
Loss of automatic pressurization control

*
Loss of automatic fwd or aft pressurization outflow valve control

*
MAN selected for outflow valve control

*
Exceedance for cabin altitude or cabin differential pressure