Figure 12-29.  Variable displacement pump. The pressure compensator is a spool valve that is held  in the closed position by an adjustable spring load.  [Figure 12-30]  When pump outlet pressure (system pressure)  exceeds the pressure setting (2,850 psi for full flow), the spool  moves to admit fluid from the pump outlet against the yoke  actuator piston. In  Figure 12-30,  the pressure compensator  is shown at cracking pressure; the pump outlet pressure is  just high enough to move the spool to begin admitting fluid  to the actuator piston.  The lower displacement results in a corresponding reduction  in pump flow. The pump delivers only that flow required to  maintain the desired pressure in the system. When there is no  demand for flow from the system, the yoke angle decreases  to nearly zero degrees stroke angle. In this mode, the unit  pumps only its own internal leakage. Thus, at pump outlet  pressures above 2,850 psi, pump displacement decreases  as outlet pressure rises. At system pressures below this  level, no fluid is admitted through the pressure compensator  valve to the actuator piston and the pump remains at  full displacement, delivering full flow. Pressure is then  determined by the system demand. The unit maintains zero  flow at system pressure of 3,025 psi. When the solenoid valve is energized, the electrical  depressurization valve (EDV) solenoid valve moves up  against the spring force and the outlet fluid is ported to  the EDV control piston on the top of the compensator  (depressurizing piston).  [Figure 12-32]  The high-pressure  fluid pushes the compensator spool beyond its normal  metering position. This removes the compensator valve from  the circuit and connects the actuator piston directly to the  This control pressure overcomes the yoke return spring force  and strokes the pump yoke to a reduced angle. The reduced  angle of the yoke results in a shorter stroke for the pistons  and reduced displacement.  [Figure 12-31]  The yoke is supported inside the pump housing on two  bearings. At pump outlet pressures below 2,850 psi, the  yoke is held at its maximum angle relative to the drive shaft  centerline by the force of the yoke return spring. Decreasing  system flow demand causes outlet pressure to become high  enough to crack the compensator valve open and admit fluid  to the actuator piston. 12-21 Normal Pumping Mode Depressurized Mode Inlet pressure Case pressure Control pressure Outlet pressure IN OUT Coupling shaft Cylinder block Piston Keepage drain Yoke Actuating piston Steel sleeve Housing Blocking valve SCL engine valve Compensator valve Attenuator Case relief valve Impeller cover Impeller Case drain Piston shoe Drive shaft Baffle Rotary seal Mating plug Shaft seal subassembly Yoke spring EDV piston Check valve