Service Manuals, User Guides, Schematic Diagrams or docs for : . Car Manuals Subaru Legacy 2000-2003 Approved Subaru Legacy 2000 2000 Service Manual MECHANISM AND FUNCTION SECTION Steering System MSA5TCD00L18218

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MSA5TCD00L18218


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              LEGACY (W2260BE_SGML Complete)



                                 MECHANISM AND FUNCTION                                   [M2A0]      4-3
                                                                                 2. Power Steering System


2. Power Steering System
A: HYDRAULIC SYSTEM
D Oil pump is belt-driven from the engine to discharge oil under pressure.
D When the steering wheel is not being turned, the pressure-sensitive valve operates to drain oil,
relieving the pressure in the pump.
D Oil under pressure is controlled by the oil pump in response to engine speed and is delivered
to control valve via hose A.
D When the steering wheel is turned, control valve connected to the pinion shaft activates to form
an oil flow circuit corresponding to the rotation direction of the steering wheel. Oil will then be deliv-
ered to chamber A or B via pipe A or B.
D Oil in chamber A or B acts on rack piston to produce the force required to move rack shaft to
the left or the right. This helps reduce the effort required to operate the steering wheel.
D Movement of rack piston in turn causes oil in the other chamber to return to tank via pipe A or
B, control valve and hose B.
   D If the hydraulic system becomes inoperative, the steering shaft will then be connected to the
   pinion shaft mechanically via control valve. Thus, the steering shaft can act as one similar to a
   manual steering system to move the rack and pinion.
   D To control the maximum oil pressure setting, relief valve is built into the oil pump to release
   excess oil pressure.




                                                                                                B4H1719A




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                       LEGACY (W2260BE_SGML Complete)



4-3     [M2B1]                 MECHANISM AND FUNCTION
2. Power Steering System


B: GEARBOX ASSEMBLY
1. POWER CYLINDER
The gearbox is integrated with a built-in control valve and power cylinder. The rack shaft is used
as a power cylinder piston and a rotary control valve is located in such a manner as to enclose
the pinion shaft.
The control valve and power cylinder are connected to each other by two pipes through which
hydraulic oil flows.




                                                                                         S4H0024B

      (1) Piston                                        (4) Power cylinder
      (2) Rack shaft                                    (5) Pinion shaft
      (3) Cylinder                                      (6) Control valve




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             LEGACY (W2260BE_SGML Complete)



                                MECHANISM AND FUNCTION                                  [M2B2]      4-3
                                                                               2. Power Steering System


2. CONTROL VALVE
The control valve consists of a rotor (which rotates together with the steering shaft), a pinion (which
is connected to the rotor and torsion bar), and a sleeve (which rotates together with the pinion).
Oil grooves C and D are located in the rotor and sleeve to form oil flow passages V1 through V4.
The pinion and rotor are meshed with adequate clearance. They utilize a fail-safe design.




                                                                                             S4H0025B




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                      LEGACY (W2260BE_SGML Complete)



4-3     [M2B2]                MECHANISM AND FUNCTION
2. Power Steering System


D Operating principle
When the torsion bar twists in relation to the steering force, a relative rotational displacement oc-
curs between the rotor and sleeve. This displacement changes the cross-sectional area of oil pas-
sages V1, V2, V3 and V4, which in turn switches oil passages and controls oil pressure.
  D When no steering force is applied:
  The rotor and sleeve are held at the neutral position. Oil passages V1, V2 and V3, which are
  formed by valve grooves C and D are open equally. Under this condition, oil delivered from the
  oil pump returns to the oil reservoir so that neither oil pressure builds up nor does the power
  cylinder activate.




                                                                                          G4H0011




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             LEGACY (W2260BE_SGML Complete)



                               MECHANISM AND FUNCTION                                 [M2B2]      4-3
                                                                             2. Power Steering System


  D When steering force is applied:
  When the steering wheel is turned to the right, for example, oil passages V1 and V3 open while
  oil passages V2 and V4 nearly close.
  At this point, oil under pressure in chamber A increases in response to the throttle position of
  oil passages V2 and V4 so that the rack piston moves to the right. Oil in chamber B, on the other
  hand, is discharged through oil passage V3 returning to the oil reservoir.




                                                                                             G4H0012

D Fail-safe function
If oil pressure fails to build up due to a broken oil pump drive belt, torque is transmitted from the
valve rotor to the pinion by way of the fail-safe function.




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                      LEGACY (W2260BE_SGML Complete)



4-3     [M2C0]                MECHANISM AND FUNCTION
2. Power Steering System


C: OIL PUMP & TANK
The oil pump is belt-driven from the engine.
The reservoir tank is mounted on the body.
The oil pump is the vane type.
The oil pump incorporates the flow control valve, pressure-sensitive valve, and relief valve that
control the flow rate and pressure of the oil.
D The flow control valve controls the flow rate of discharged oil corresponding to an engine speed.
D The pressure-sensitive valve helps to decrease the pressure in the pump when the steering
wheel is not being turned.
D The relief valve relieves the pressure when the pressure in the system becomes too high such
as when the steering wheel is turned all the way.




                                                                                          B4H1720A




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LEGACY (W2260BE_SGML Complete)



                 MECHANISM AND FUNCTION            [M2C0]      4-3
                                          2. Power Steering System




                                                        B4H1764A




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                      LEGACY (W2260BE_SGML Complete)



4-3     [M2C1]                 MECHANISM AND FUNCTION
2. Power Steering System


1. VANE PUMP
The vane pump consists of a rotor, cam rings, and ten vanes.
When the rotor rotates, the vane located in each groove of the rotor is radially swung out by centrif-
ugal force and pressed against the cam ring. The tip of the vane slides along the inner oval wall
of the cam ring so that oil is delivered to the chamber formed by the rotor, cam ring and vane by
way of a pea-shaped groove. Oil from the chamber is discharged into the oil circuit via the dis-
charge port.




                                                                                            H4H1188B



2. FLOW CONTROL VALVE
D The flow control valve consists of a sub-spool which is pushed to the right by the fluid pressure
as the pump discharge rate becomes higher with the engine speed increase. When the sub-spool
is shifted to the right, some area of the variable orifice is covered by the spool, thus the discharge
rate being reduced.




                                                                                             B4H1721A




                                                 12
             LEGACY (W2260BE_SGML Complete)



                               MECHANISM AND FUNCTION                                 [M2C3]      4-3
                                                                             2. Power Steering System


3. PRESSURE-SENSITIVE VALVE
D The pressure-sensitive valve is exposed on its left end directly to the vane pump discharge pres-
sure and on its right end to the fluid pressure which has just passed through the flow control valve
and will be led to the gear box.




                                                                                           B4H1722A

D When the steering wheel is not being turned, the fluid which has passed through the flow control
valve is led to the gear box but returned to the reservoir tank without acting on the control valve
in the gear box. Accordingly, the pressure does not build up.
For this reason, the vane pump discharge pressure acting on the left end of the pressure-sensitive
valve assembly is higher than the pressure acting on the right end (the pressure just having
passed through the flow control valve). This causes the pressure-sensitive valve assembly to move
to the right. Accordingly, the drain port which has been closed by the outer spool is opened, the
pressure inside the pump thereby being reduced.




                                                                                           B4H1723A




                                                13
                      LEGACY (W2260BE_SGML Complete)



4-3     [M2C3]                MECHANISM AND FUNCTION
2. Power Steering System


D When the steering wheel is turned, the fluid which has passed through the flow control valve and
flown into the steering gear builds up its pressure, because it is used as a power source assisting
the steering effort.
The inner spool of the pressure-sensitive valve assembly is kept pressed to the right by the vane
pump discharge pressure acting on its left end. In this condition, as the fluid pressure acting on
the right end is increased by the steering operation, the outer spool is moved to the left, closing
the drain port. Therefore, the pump internal pressure is increased, and a higher pressure required
for assisting the steering effort is supplied to the gear box.




                                                                                          B4H1724A




                                                14
             LEGACY (W2260BE_SGML Complete)



                               MECHANISM AND FUNCTION                                 [M2C4]      4-3
                                                                             2. Power Steering System


4. RELIEF VALVE
The relief valve is composed of a check ball and a tension spring. The check ball is exposed, on
the right side, to the fluid pressure regulated by the flow control valve (fluid pressure supplied to
the steering gear box). At the left side, it is subjected to the spring tension and normally closes
the passage for the said fluid pressure.
If the pressure in that passage is increased abnormally high for some reason (for example, the
steering wheel has been fully turned to its stopper) and overcomes the spring tension, the ball is
pushed to the left, and the oil is drained into the reservoir tank, thus the pressure in the passage
being relieved.




                                                                                            B4H1778A




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       LEGACY (W2260BE_SGML Complete)



4-3            MECHANISM AND FUNCTION


MEMO




                                 16



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