.  
flight spoilers (one on each wing)

.  
leading edge flaps and slats

.  
normal brakes

.  
No. 2 thrust reverser

.  
autopilot B

.  
alternate nose wheel steering (as installed)

.  
landing gear transfer unit

.
autoslats

.  
yaw damper

.  
trailing edge flaps

A and B Hydraulic System Pumps
Both A and B hydraulic systems have an engine–driven pump and an AC electric motor–driven pump. The system A engine–driven pump is powered by the No. 1 engine and the system B engine–driven pump is powered by the No. 2 engine. An engine–driven hydraulic pump supplies approximately four times the fluid volume of the related electric motor–driven hydraulic pump.
The ENG 1 (system A) or ENG 2 (system B) pump ON/OFF switch controls the engine–driven pump output pressure. Positioning the switch to OFF isolates fluid flow from the system components. However, the engine–driven pump continues to rotate as long as the engine is operating. Pulling the engine fire warning switch shuts off the fluid flow to the engine–driven pump and deactivates the related LOW PRESSURE light.
The ELEC 2 (system A) or ELEC 1 (system B) pump ON/OFF switch controls the related electric motor–driven pump. If an overheat is detected in either system, the related OVERHEAT light illuminates.
Note: Loss of the system A, engine-driven hydraulic pump and a heavy demand
on system A may result in an intermittent LOW PRESSURE light for the
remaining electric hydraulic pump. The system A flight controls LOW
PRESSURE light, Master Caution light, and the FLT CONT and HYD
system annunciator lights also illuminate.
Copyright . The Boeing Company. See title page for details.
13.20.2   December 1, 2000

Hydraulics -System Description
737 Operations Manual
Hydraulic fluid used for cooling and lubrication of the pumps passes through a heat exchanger before returning to the reservoir. The heat exchanger for system A is located in main fuel tank No. 1 and for system B is in main fuel tank No. 2.
CAUTION: Minimum fuel for ground operation of electric pumps is 760 Kgs (1675 Lbs) in the related main tank.
Pressure switches, located in the engine–driven and electric motor–driven pump output lines, send signals to illuminate the related LOW PRESSURE light if pump output pressure is low. A check valve, located in each output line, isolates the related pump from the system. The related system pressure transmitter sends the combined pressure of the engine–driven and electric motor–driven pumps to the related hydraulic system pressure indication.
System A Hydraulic Leak
If a leak develops in the engine–driven pump or its related lines, a standpipe in the reservoir prevents a total system fluid loss. With fluid level at the top of the standpipe, the reservoir quantity displayed indicates approximately 22% (1/4 on non–EIS equipped airplanes) full. System A hydraulic pressure is maintained by the electric motor–driven pump.
If a leak develops in the electric motor–driven pump or its related lines, or components common to both the engine and electric motor–driven pumps, the quantity in the reservoir steadily decreases to zero and all system pressure is lost.
System B Hydraulic Leak
The system B reservoir has two standpipes. One standpipe supplies fluid to the engine–driven pump and the other to the electric motor–driven pump. If a leak develops in the engine–driven pump or its associated lines, the system B quantity decreases until it indicates approximately 40% or (1/2 on non–EIS equipped airplanes) full. System pressure is maintained by the electric motor–driven pump. If a leak develops in the electric motor–driven pump or its associated lines, system B pressure is lost. However, fluid remaining in the system B reservoir is sufficient for power transfer unit operation.
 A leak in system B does not affect the operation of the standby hydraulic system.
Copyright . The Boeing Company. See title page for details.
June 4, 2004  13.20.3
Hydraulics -System Description 737 Operations Manual

Power Transfer Unit
The purpose of the PTU is to supply the additional volume of hydraulic fluid needed to operate the autoslats and leading edge flaps and slats at the normal rate when system B engine–driven hydraulic pump volume is lost. The PTU uses system A pressure to power a hydraulic motor–driven pump, which pressurizes system B hydraulic fluid. The PTU operates automatically when all of the following conditions exist:
.  
airborne

.  
system B engine–driven pump hydraulic pressure drops below limits
　
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