-
APU Duct Overheat Relay Check.

 TASK 26-19-00-735-001
 2.  System Test - APU Duct Overheat Detection System
________________________________________________
 A.  Special Tools and Equipment
 (1)
H294 or H394 - Thermal Switch Tester, Howell Instruments, Inc.,

 3479 W. Vickery Blvd., Ft. Worth, Texas

 (2)  
BH28456 -Probe, Howell Instruments, Inc., 3479 W. Vickery Blvd., Ft. Worth, Texas

 NOTE: Howell Probe No. is dependent on the type of
____ thermal switch installed.
 (3)
Power Cable, BH 23859-115 or BH 23859-230, HOWELL

 (4)
Thermal Switch Cable, BH1338-1, Howell

 B.  Standard Tools and Equipment
 (1)
Ohmmeter - Capable of measuring 0 to 10 ohms

 (2)
Voltmeter - Capable of measuring 0 to 15VDC

 C.  References
 (1)
AMM 24-22-00/201, Manual Control

 (2)
AMM 26-19-01/401, Thermal Switch

 (3)
AMM 45-10-00/201, Central Maintenance Computer System

 (4)
AMM 49-61-07/401, APU Controller

 (5)
AIPC 26-14-10, Fig. 1

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747-400
MAINTENANCE MANUAL

 (6)  
AIPC 26-19-00, Fig. 1

 (7)  
WDM 49-52-21

 D.
Access

 (1)  
Location Zone
 142 Aft Cargo Container Compartment, LH
 145 Bulk Cargo Compartment, LH
 311 Area Aft of Pressure Bulkhead, LH
 313 Stabilizer Torsion Box Compartment, LH

 (2)  
Access Panel
 312AR Horizontal Stabilizer Jackscrew Access Door

 E.
Prepare for the Test

 S 865-002
 (1)  
Supply electrical power (AMM 24-22-00/201).

 S 865-003

 (2)  
Close these circuit breakers:

 (a)  P83 Panel
 1) APU DUCT OVERHEAT

 F.
APU Air Duct Detection Circuit Check

 S 015-004
 (1)
Access the thermal switch, S969 in the aft cargo compartment.

 S 485-005

 (2)
Install a jumper on terminals 1 and 2 of the thermal switch, S969.

 S 725-006

 (3)  
Make sure these indications occur:

 (a)  
The APU DUCT OVERHEAT IND LIGHT L6 on the P83 panel goes on after you install the thermal switch jumper.

 (b)  
The EICAS shows the "APU DUCT LEAK" status message.

 S 085-007
 (4)  
Remove the jumper from the S969 thermal switch.

 S 215-008

 (5)  
Make sure the APU DUCT OVERHEAT IND LIGHT L6 stays on and the EICAS shows the same message.

 (6)  
Push the APU DUCT OVERHEAT RESET SW S220 on the P83 panel.

 S 865-009
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747-400 MAINTENANCE MANUAL
 S 715-010
 (7)  Make sure the APU DUCT OVERHEAT IND LIGHT L6 is off.
 NOTE: If the APU DUCT OVERHEAT LIGHT L6 is not off and stays
____ off then one of the thermal switches has become inoperable or there is a short wire between the thermal switches and the R686 Duct Overheat relay on the P83 panel
 S 865-031
 (8) Remove the EICAS status message "APU DUCT LEAK" (AMM 45-10-00/201).
 NOTE: If above light did not go off, the APU DUCT LEAK message
____
 stays latched until problem is corrected.

 S 965-011
 (9)  
See the 36-CMCS Message Index, message 36301, if the system does not operate correctly.

 S 865-012
　
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