(2)  
TT1000A Test Set - Digital Turbine Temperature, Lead and Insulation Resistance Measurements 21844 Barfield Instrument Corp. 4101 N.W. 29th Street, P.O. Box 7705, Miami, FL 33142-7705

 B.  
References

 (1)  
AMM 49-41-05/401, Ignition Cable

 C.  
Access

 (1)  
Location Zones 221 Control Cabin - Left 222 Control Cabin - Right 310 Fuselage - Station 2360 to Station 2792

 (2)  
Access Panels
 317AL APU Access Door - Left
 318AR APU Access Door - Right

 D.  
Procedure

 (1)
Do this task: Ignition Cable Removal (AMM 49-41-05/401).

 S 765-005

 (2)  
Do the ignition cable test:

 (a)  
Use the ohmmeter to do a continuity check of the two ignition cables from one terminal end to the other terminal end. 1) If there is no continuity for each ignition cable, replace

 S 025-008
 the ignition cable(s).
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747-400 MAINTENANCE MANUAL
 WARNING: DO NOT TOUCH THE OPEN PART OF THE TEST PROBES OR THE
_______ COMPONENT DURING THE TEST. INJURY TO PERSONS CAN OCCUR.
 (b)  Use the digital turbine temperature test set to measure the insulation resistance to ground for the two ignition cables. 1) If the insulation resistance is less than 5000 ohms at 45V
 dc or ac, replace the ignition cable(s).
 S 425-009
 (3)  Do this task: Ignition Cable Installation (AMM 49-41-05/401).
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747-400 MAINTENANCE MANUAL
 APU COOLING AIR SYSTEM - DESCRIPTION AND OPERATION
__________________________________________________
 1.  General_______
 A.  The APU cooling air system supplies cooled air for APU component and APU compartment cooling. The APU cooling air system has a cooling air inlet screen, a cooling fan which supplies cooled air to the two APU generators and air-cooled oil cooler and a short cooling air exhaust duct for compartment cooling.
 2.  Cooling Fan (Fig. 1)___________
 A.  The cooling fan has an inlet housing, axial flow type fan rotor and an exit duct. The inlet housing is installed on the load gearbox and supports the bearings for the spur gearshaft. The fan rotor is installed on the spur gearshaft and is driven by the load gearbox. Intake air from the inlet housing is compressed by the cooling fan and discharged into the exit duct. Vanes on the exit duct change the air flow to the manifold and supply cooling air to the two ac generators, air-cooled oil cooler and APU compartment.
 3.  Operation_________
 A.  The cooling fan draws air from the inlet plenum, through the fan inlet duct. The air is compressed by the fan and discharged into the fan exit manifold. The four-way manifold distributes cooling air to both electrical generators, the air cooled oil cooler (ACOC) and to the APU compartment for compartment cooling. Exhaust air from the generators and ACOC is discharged into the APU compartment.
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747-400 MAINTENANCE MANUAL 
SEE  A 
FWD APU COMPARTMENT  SEE  B  INLET SCREEN COOLING AIR 

COOLING FAN
FWD
A
APU Cooling Air System Components Figure 1 (Sheet 1)
F58406
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