If the oil is cold during engine starting, the pressure will be high which will cause the oil to be bypassed.  A thermal lockout prevents the button from popping when the oil temperature is below 38 oC.  The bypass valve assembly opens when the oil pressure is too high.  This means the filter element is dirty or the oil is still cold.  The bypass valve will close when the oil temperature warms up to operating temperature and pressure decreases.
Oil Filter Bypass Indicator Systems (Normal Position):  Differential pressure between the filter inlet and outlet act to move a piston against a spring when the pressure reaches 44 to 60 psi.  The piston contains a magnet, which normally attracts a red button assembly and holds it seated against its spring.  When the piston moves, the button is released and it extends 3/16 inch to visually indicate an impending bypass condition.  The button is physically restrained from tripping by a cold lockout bimetallic latch if temperature is less than 100 to 130 oF to prevent a false trip during cold starts.
Oil Filter Bypass Indicator Systems (Tripped Position):  As the button is released, a small ball also moves out of position to latch the button and block reset.  The internal piston assembly automatically resets on shutdown; however, the indicator remains latched out.  When the bowl is removed from the gearbox and the filter element is removed, a spring-loaded sleeve around the indicator moves aft and pulls the piston assembly to a tripped position.  This makes the button trip if operation is attempted with no filter in the bowl. To reset the indicator, the bowl is held vertical so that the button latch ball can roll out of the latched position.  The button is then manually reset after the filter element has been replaced.
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Oil Pressure and Temperature Transmitters- The oil pressure transmitter monitors engine oil pressure and forwards the signal to the CDU through the SDC.  Normal oil pressure may range from 20 to 100 psi.  Minimum oil pressure is 20 psi below 90% Ng or 35 psi above 90% Ng.  The ENGINE OIL PRESS caution light illuminates at 20 psi.  The oil temperature transmitter monitors engine oil

temperature and forwards the signal to the CDU through the SDC.  When oil temperature reaches 150o C, the ENGINE OIL TEMP caution light illuminates.

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Oil Cooler- The fuel-oil cooler is a tube and shell design which cools the combined output of the scavenge discharge oil that is ported through gearbox cored passages to the cooler.  The cooler is mounted adjacent to the fuel boost pump on the forward side of the gearbox.  The oil cooling process heats the fuel, which improves fuel atomization.  A counter parallel flow multi-pass cooler design is used in order to minimize pressure drop while obtaining maximum cooler effectiveness.  Fuel flows through the tubes, while the oil flows over the tubes, resulting in the counter-parallel flow arrangement.  If the oil cooler pressure becomes too high, the oil cooler relief valve will open to dump scavenge oil directly into the oil tank.
　
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