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 4.  Operation (Fig. 2)
_________
 A.  Functional Description
 (1)  
During engine operation, the main oil filter removes contamination from the engine oil. If the buildup on the main oil filter becomes such that it restricts the oil supply to the engine bearings, the oil pressure differential across the main oil filter increases.

 (2)  
When the oil pressure differential across the main oil filter is 50 |2 psid, the differential pressure switch closes and the applicable advisory message shows on the main EICAS display. If the engine is operated during this condition, the pressure differential across the main oil filter further increases until, at 60 psid, the pressure relief valve for the main oil filter opens and lets the engine oil bypass the main oil filter.

 (3)  
When the oil pressure differential across the main oil filter decreases to less than 40 psid, the differential pressure switch opens and the EICAS advisory message goes off.

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////////////////////////A / PW4000 SERIES /747-400 / ENGINES /MAINTENANCE MANUAL ////////////////////////
SEE  A 
(REGULATOR) OIL SYSTEM PRESSURE RELIEF VALVE (REF)  FILTER (REF) MAIN OIL 

OIL FILTER
DIFFERENTIAL
PRESSURE SWITCH

FWD
VIEW IN THE UP DIRECTION
1 ENGINES WITHOUT PW SB 79-70
A1
2 ENGINES WITH PW SB 79-70
Oil Filter Differential Pressure Switch Location Figure 1 (Sheet 1)
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OIL SYSTEM PRESSURE 
(REGULATOR)  BRACKET 
RELIEF VALVE (REF) 
MAIN OIL 
FILTER (REF) 
OIL FILTER OUTLET 
PRESSURE TUBE 
BOLT, WASHER, NUT 
(3 LOCATIONS) 
OIL FILTER 
DIFFERENTIAL 
PRESSURE SWITCH 
OIL FILTER INLET 
PRESSURE TUBE 
ELECTRICAL 
CONNECTOR  FWD 
OIL PRESSURE 
TUBE NUT 
(2 LOCATIONS) 
VIEW IN THE UP DIRECTION 
A  2 

Oil Filter Differential Pressure Switch Location
Figure 1 (Sheet 2)

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E54359
////////////////////////A / PW4000 SERIES /747-400 / ENGINES /MAINTENANCE MANUAL ////////////////////////
PRESSURE PUMP

Oil Filter Bypass Warning System Simplified Schematic
Figure 2

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OIL FILTER BYPASS WARNING SYSTEM - ADJUSTMENT/TEST
__________________________________________________
 1.  General
_______
 A.  This test examines the system from the differential pressure switch to the EICAS. The test applies different air pressures on the differential pressure switch which will cause the EICAS message to go on or off. The the EICAS message, ENG (1, 2, 3, or 4) OIL FILT, will only show if the engine oil temperature is 25°C or more. An alternate method is shown for oil temperatures less than 25° C.
 NOTE: AIRPLANES POST-SB 747-79-2041;
____ The temperature threshold is increased from 25°C to 35°C.
 B.  The system test is used to do a test of the system performance. You can use the system test with the system fault isolation.
　
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