Power Steering Description; Neutral (Straight-Ahead) Position - Toyota Hilux Surf 1993 Owner's Manual

SR–36
POWER STEERING
Description
Two types of power steering are the standard type
and the PPS (progressive power steering) type. Both
these types have a recirculating ball system and rotary
type hydraulic control valve.
PPS TYPE
Vehicle speed is detected by a speed sensor and fluid
pressure acting on the piston is varied accordingly.
When the vehicle is stopped or when moving at low
speed, fluid pressure is increased to lighten the force
required for steering. At high speed, pressure is re-
duced to lessen the amount of assist and provide ap-
propriate steering wheel response.
PRINCIPLES OF POWER STEERING
Power steering is one type of hydraulic device for
utilizing engine power to reduce steering effort.
Consequently, the engine is used to drive a pump
to develop fluid pressure, and this pressure acts
on a piston within the gear box so that the piston
assists the sector shaft effort. The amount of this
assistance depends on the extent of pressure act–
ing on the piston. Therefore, if more steering
force is required, the pressure must be raised. The
variation in the fluid pressure is accomplished by
a control valve which is linked to the intermediate
shaft and the steering main shaft.
STEERING – Power Steering (Description)
NEUTRAL (STRAIGHT–AHEAD)
POSITION
Fluid from the pump is sent to the control valve.
If the control valve is in the neutral position, all
the fluid will flow through the control valve into
the relief port and back to the pump. At this time,
hardly any pressure is created and because the
pressure on the power piston is equal on both
sides, the piston will not move in either direction.
WHEN TURNING
When the steering main shaft is turned in either
direction, the control valve also moves, closing
one of the fluid passages. The other passage then
opens wider, causing a change in fluid flow vol–
ume and, at the same time, pressure is created.
Consequently, a pressure difference occurs be–
tween both sides of the piston and the piston
moves in the direction of the lower pressure so
that the fluid in the cylinder is forced back to the
pump through the control valve.