3.0 L Model

The reservoir tank is mounted on the vehicle body.
The fluid pump is belt-driven by the engine. The fluid flow is controlled according to the engine speed so that an adequate steering resistance is given during high-speed operation.
The fluid pump is a variable capacity type vane pump whose delivery rate per rotation decreases as the engine speed increases. The pump is integral with a pump control valve and relief valve.

Fig 1: Identifying Fluid Flow 3.0 L Model
Courtesy of SUBARU OF AMERICA, INC.
The oil pump is driven by the engine and its flow is controlled in accordance with the engine speed to maintain an adequate steering effort at high speeds.
Also, to improve fuel efficiency and limit temperature rise of the system, a variable displacement vane pump whose output flow per rotation is controlled in accordance with the engine speed is used, and a control valve and a relief valve are built in the pump.

Fig 2: Identifying Output Flow Per Rotation Of Oil Pump
Courtesy of SUBARU OF AMERICA, INC.
The vane pump consists of a rotor, cam ring, adapter, and eleven vanes. When the rotor rotates, the vane in each slot of the rotor is moved out by centrifugal force in radial directions and pressed against the cam ring. The tip of the vanes slide along the inner wall of the cam ring and the oil that has entered the chamber formed by the rotor, cam ring and vanes is discharged continuously.

Fig 3: Identifying Vane Pump Consists Of Rotor, Cam Ring, Adapter & Eleven Vanes
Courtesy of SUBARU OF AMERICA, INC.
Flow control
Consists of a control valve, orifice, adapter and cam ring, and regulates the amount of oil supplied to the gearbox by changing the eccentricity of the cam ring to control the discharge rate per rotation.
1. In phase (a), the pressure difference is small between both sides of the orifice, and the control valve is pressed to the set position by the control valve spring. Tank pressure is led to pressure chamber A. Pressure after the orifice is led to pressure chamber B. The cam spring is pressed to the set position by the differential pressure between pressure chambers A and B (A < B) and the force of the cam spring itself, and maximum discharge rate is achieved.
  
  Fig 4: Identifying Flow Control Graph
  Courtesy of SUBARU OF AMERICA, INC.
  
  Fig 5: Identifying Discharge Flow Graph
  Courtesy of SUBARU OF AMERICA, INC.
2. In phase (b), the pressure difference between both sides of the orifice increases, and the control valve moves against the valve spring force. Pressure before the orifice is led to pressure chamber A, and the cam ring moves against the cam spring force by the differential pressure between pressure chambers A and B (= differential pressure across the orifice). When the cam spring moves, the delivery rate per rotation decreases and will not increase even when the engine speed increases.
  
  Fig 6: Identifying Pressure Difference Between Both Sides
  Courtesy of SUBARU OF AMERICA, INC.
3. In phase (c), the cam spring moves further due to the effect of the orifice, and the flow is controlled as shown in the previous figure.
  
  Fig 7: Moving Cam Spring & Flow Controlled
  Courtesy of SUBARU OF AMERICA, INC.
4. Relief valve operation
  When the discharge pressure exceeds the specified value, the relief valve opens to release the discharged oil to the low pressure side and limit the maximum pressure.
  
  Fig 8: Identifying Relief Valve Operation
  Courtesy of SUBARU OF AMERICA, INC.