(a)  
Rotate the engine-2 fire handle counterclockwise.
1) Check for 0 volts dc between D590 pins 2 and 1.
2) Check for 28 volts dc between D592 pins 2 and 1.

(b)  
Rotate the engine-2 fire handle clockwise.
1) Check for 0 volts dc between D590 pins 2 and 1.
2) Check for 28 volts dc between D592 pins 2 and 1.

(c)  
Release the engine-2 fire handle.

(8)  
Reset the engine-2 fire handle to the normal position.

D.  Restore the system to normal.
(1)  
Set the engine bleed switches to OFF.

(2)  
Keep switch S2 on M400 module set at position B.

(3)  
Close the boost pump circuit breakers on the P6-3 panel.

(4)  
Close the igniter circuit breakers on the P6-2 panel.

(5)  
Remove electrical power if it is not necessary (Ref 24-22-0 MP).

(6)  
Open the left and right fire extinguisher bottle circuit breakers.

(7)  
Connect these connectors to the engine fire bottles:

(a)  
D592 to V25, engine-2 fire valve.

(b)  
D584 to M195, right bottle squib.

(c)  
D590 to V24, engine-1 fire valve.

(d)  
D582 to M194, left bottle squib.

(8)  
Close the left and right fire extinguisher bottle circuit breakers.

(9)  
Push the EXT TEST switch on the fire control panel.

(a)  Make sure the L and R lights come on.
500 
26-21-41 Page 502  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  May 01/98 

APU FIRE EXTINGUISHING SYSTEM - DESCRIPTION AND OPERATION
1. General
A.  The APU fire extinguishing system is a fixed system providing fire extinguishing capability to the APU. The extinguishing system is a gaseous smothering system designed to flood the APU shroud with an inert gas in case of a fire. The fire extinguishing system is electrically controlled by a fire switch.
B.  The APU fire extinguishing system consists of the fire protection system module, fire extinguisher bottle and bottle fittings, remote APU fire control panel, indicators and associated tubing (Fig. 1). The fire extinguisher bottle fittings are connected by airplane wiring to the APU fire switch and bottle discharge light on the fire protection system module. The fire protection system module contains the APU fire switch, discharge light, test light and circuitry necessary to shut down the APU and discharge the extinguishing agent into the APU shroud. For engine coverage, refer to 26-21-0, Engine Fire Extinguishing System.
C.  When a fire is sensed in the APU, the APU fire switch handle is unlocked. The fire switch handle may then be pulled arming the fire extinguishing system. The APU is automatically shut down by APU fire warning circuit. After the system is armed, the handle may be rotated either clockwise or counterclockwise to discharge the APU fire extinguisher bottle. The fire extinguishing system for the APU may also be controlled from the remote APU fire control panel.
D.  The fire extinguishing system can be tested in flight or on the ground by operation of the fire extinguisher test switch (Fig. 2). The fire extinguisher test circuit shows continuity through the bottle discharge squib. The indicator disks on the lower skin on the lower skin of the empennage section may be checked for system discharge or thermal relief. The bottle pressure may be checked by monitoring the bottle discharge lights.
513 
Aug 01/94  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  26-22-0 Page 1 

L20417H28226
APU Fire Extinguishing System Component Location  5C8 
26-22-0  Figure 1  Jun 20/83 
Page 2 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

500  APU Fire Extinguishing System Circuit 
May 15/67  Figure 2  26-22-0 
Page 3 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

2.  Fire Protection System Module Components
A.  The fire protection system module components described in the following paragraphs will be only those associated with the APU fire protection system. The fire protection system module (P8-l) is located on the aft electronic panel (P8). For further information on the fire protection system module, refer to 26-00, Fire Protection.
B.  Fire Switch
(1)  
The fire switch is a combination seven-pole, double-throw, push-pul1 and a four-pole single-throw rotary switch with a holding solenoid and lighted handle. (See figure 2.) Two poles of the rotary switch are actuated by clockwise rotation of the handle and the other two poles are actuated by counterclockwise rotation of the handle. The handle is spring-loaded to the normal position. The solenoid is provided with a mechanical override to be used in event of a malfunction.
　
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