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747-400 MAINTENANCE MANUAL
 D. Return the airplane to its usual condition
 S 865-117
 (1)  
Do the "Put the Air/Ground Relay System Back in the Ground Mode" task (Ref 32-09-02/201).

 S 415-118

 (2)  
Install the left floor panel of the forward cargo compartment if necessary.

 S 865-119

 (3)  
Remove electrical power if it is not necessary (Ref 24-22-00/201).

 TASK 21-58-00-735-026
 7.  System Test - Differential Pressure Switch__________________________________________
 A. Standard Tools and Equipment
 (1)  Vacuum source with gauge, capacity: 0 to 2.0 inch (50 mm) water minimum, commerically available.
 B. References
 (1)  
24-22-00/201, Manual Control

 (2)  
32-09-02/201, Air/Ground Relay System

 C. Access
 (1)  Location Zone 121 Forward Cargo Compartment Forward Half, LH
 D. Prepare for the Test
 S 865-027
 (1)  Supply electrical power (Ref 24-22-00/201).
 S 865-031
 WARNING: YOU MUST CAREFULLY FOLLOW THE STEPS IN THE SPECIFIED TASK TO
_______ PUT THE AIR/GROUND RELAY SYSTEM IN THE AIR MODE. FAILURE TO FOLLOW THE STEPS CORRECTLY CAN CAUSE THE AUTOMATIC OPERATION OF THE AIRPLANE SYSTEMS. THIS CAN CAUSE INJURY TO PERSONS AND DAMAGE TO EQUIPMENT.
 (2)  Do the "Put the Air/Ground Relay System in the Air Mode" task (Ref 32-09-02/201).
 E. Do the Test
 S 715-017
 (1)  Do a test of the differential pressure switch.
 (a)  Find the differential pressure switch on the E1/E2 rack plenum.
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 (b)  
Remove the tubing from the differential pressure switch.

 (c)  
Connect a vacuum source to the plenum port tube connection of the differential pressure switch.

 CAUTION: DO NOT KEEP THE EQUIP COOLING SWITCH IN THE OVRD POSITION
_______ FOR MORE THAN FIVE MINUTES. EXTENDED OPERATION OF AIRPLANE SYSTEMS WITHOUT COOLING AIRFLOW CAN RESULT IN DAMAGE TO EQUIPMENT.
 (d)  
Set the EQUIP COOLING switch on the P5 overhead panel to OVRD.

 (e)  
Make sure the caution message EQUIP COOLING is shown on the EICAS display.

 NOTE: This EICAS message has a four minute time delay.
____
 CAUTION: MAKE SURE YOU APPLY THE VACUUM SLOWLY AND NOT ALL AT ONCE.
_______ IF YOU APPLY THE VACUUM ALL AT ONCE YOU CAN DAMAGE THE DIFFERENTIAL PRESSURE SWITCH.
 (f)  Slowly apply a vacuum of 1.5 inches (38 mm) of water to the tube connection of the differential pressure switch at a maximum rate of 10,000 feet per minute.
 NOTE: By reducing the pressure in the tube connection so that
____ the pressure drops by an amount of 1.5 inches (38mm) water that is equivalent to simulating the airplane being pressurized in the air to a 4 psi differential pressure.
 (g)  
Make sure the caution message EQUIP COOLING is not shown on the EICAS display.

 (h)  
Set the EQUIP COOLING switch on the P5 overhead panel to NORM.

 (i)  
Make sure the caution message EQUIP COOLING is not shown on the EICAS display.

 (j)  
Remove the vacuum source from the differential pressure switch.

 (k)  
Install the tubing on the differential pressure switch.

 F. Put the Airplane to its Usual Condition
 S 865-032
 (1)  Do the "Put the Air/Ground Relay System Back in the Ground Mode" task (Ref 32-09-02/201).
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