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 (b) Bleed Air Temperature Regulation
 1)  The precooler is sized to limit the maximum normal supply temperature to 450°F. To conserve fan airflow during normal operation and to maintain the temperature of the supply air high enough to provide cabin heating, the precooler control valve sensor modulates the precooler control valve to control the amount of fan airflow passed through the precooler. The temperature setting of the sensor is 415° |25°F.
 (c) Supply Air Pressure Regulation and Overpressure/Overtemperature Protection 1) Supply bleed air passes through the pressure regulator and shutoff valve (PRSOV) and the precooler. The bleed on/off function is provided by a momentary pulsed latching solenoid. 2) The nominal pressure regulation range is 34-50 psig. During descent and holding conditions, bleed pressure is usually less than that range, and the valve is normally wide open. The bleed air regulator incorporates a pressure relief valve, which provides additional actuator pressure relief during transient conditions.
 3)  Overpressure protection is provided by the overpressure switch in the bleed air regulator. If bleed pressure reaches 180 |10 psig, the overpressure switch activates the PRSOV shutoff solenoid which causes the PRSOV to close.
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737-300/400/500MAINTENANCE MANUAL
 4)  In the event that bleed flow rates exceed the capacity of the precooler to limit downstream temperature to 450°F, the 450°F Thermostat (overtemperature sensor) will cause the PRSOV to throttle back on pressure. The degree of throttling depends on the bleed temperature and altitude, but in general will reach maximum throttling capability by 490°F. The overtemperature feature is primarily intended to prevent bleed trip off in the event WTAI flow is inadvertently turned on under high temperature ambient conditions. Throttling will reduce the amount of bleed flow, thus reducing the bleed air temperature and possibly averting 490°F Overheat Switch bleed trip-off. The overtemperature feature performs a similar function in the event of single failures which may lead to excessively high bleed temperatures.
 (d) Reverse Flow Check Operation 1) The reverse flow check mechanism provided in the high-stage regulator is designed to shut off the high-stage valve prior to a flow reversal. This is accomplished by sensing high stage port pressure upstream of the high-stage valve and comparing it to the pressure downstream of the precooler. As long as there is flow in the forward direction, the high stage port pressure upstream of the high-stage valve will be higher than the pressure downstream of the precooler because of the pressure drop between the high stage sense port and precooler outlet. The reverse-flow check mechanism is designed to vent the opening chamber of the high-stage valve, causing it to close when the pressure drop between the two sense taps is less than 1.0 psi.
 2)  Additional protection against flow reversal is provided by the reverse flow check device in the bleed air regulator. This device senses pressure upstream of the pressure regulator and shutoff valve and downstream of the precooler. Upon a flow reversal, a switch is actuated which sends a signal to the valve's shutoff solenoid. Switch actuation occurs at a maximum differential pressure of 0.3 psi in the reverse airflow condition.
 (e) Bleed Trip Off Light 1) The bleed trip off light on the overhead panel indicates an abnormal system operation. The light is illuminated for any of the following conditions: a) Air temperature out of the precooler is 490° |10°F. b) Pneumatic pressure upstream of the PRSOV is 180 |10
 psig.
 (f) Lockup Pressure Relief
 1)  A pressure relief valve is located on the downstream side of the high stage valve flow body. The relief valve is set to relieve at 160 |10 psig in the event of pressure buildup with the PRSOV closed.
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737-300/400/500MAINTENANCE MANUAL
 D. Starting operation (Fig. 11)
 (1)  
When starting the engine (AMM 71-00-00/201), compressed air will flow in the reverse direction through the pneumatic system to the engine start system. The bleed switches are positioned to OPEN and the isolation valve switch is positioned to CLOSE. If compressed air from the APU is used, the APU bleed switch is positioned to OPEN. When the APU is used for engine starting, a dual bleed situation will be sensed and the dual bleed light will illuminate.
　
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