L322 Range Rover System Description and Operation
STEERING
DESCRIPTION AND OPERATION
57-19
The steering gear comprises the mechanical steering rack, the valve and an integrated hydraulic power cylinder.
The steering rack uses a rack with an integrated piston which is guided on plain bearings within the rack housing. The
pinion, which is attached to the valve runs in bearings and meshes with the rack teeth. The rack is pressed against
the pinion by a spring loaded yoke which ensures that the teeth mesh without any play. The pinion is connected to
the valve rotor via a torsion bar.
The rotary motion of the steering wheel is converted into axial movement of the rack by the pinion and is initiated by
the valve. This motion is transferred into movement of the wheels by adjustable track rod arms.
The rotary valve is used to control the pressurised fluid required for power assistance. The valve comprises a valve
body, a control bush and a torsion bar. The valve body has eight control groove in its bore. The control bush also has
eight radial grooves which are matched to the valve. The control bush is positively attached to the pinion. The torsion
bar is the connecting element between the valve body, the pinion and the control bush.
Torque input from the steering wheel is passed to the valve body and causes a rotary motion of the torsion bar. The
valve body changes its relative position to the control bush and subsequently the relative positions of the control
grooves is also changed. This allows pressurised fluid to pass via the mismatched grooves to the rack piston and
provide the required assistance in the selected direction.
The piston is located at one end of the rack housing. Each side of the piston is connected to fluid pressure or fluid
return via a metal external pipe which is connected to the valve housing.
Each end of the rack has a threaded hole which provides for the fitment of an inner track rod joint. The external ends
of the rack are sealed with gaiters which prevent the ingress of dirt and moisture. The inner track rod joints has a long
outer threaded shank which screws into the outer track rod. The steering toe can be adjusted using the threaded end
of the inner track rod. When the correct toe is achieved, a locknut on the inner tie rod is tightened to prevent further
inadvertent movement.
Servotronic Transducer Valve
The Servotronic transducer valve is located in a port on the side of the steering rack valve housing. The valve is sealed
in the housing with an O-ring seal and is secured with two long screws into threaded holes in the housing.
The Servotronic valve is a transducer controlled valve which responds to control signals supplied from the Body
Control Unit (BCU). The BCU contains a microprocessor which receives road speed signals from the ABS ECU and
calculates the correct controlling signal for the Servotronic valve. The Servotronic software within the BCU has a
diagnostic capability which allows TestBook/T4 to check the tune of the steering.
The Servotronic valve determines the hydraulic reaction at the steering rack rotary valve and controls the input torque
required to turn the steering wheel. The Servotronic system allows the steering to be turned with minimum effort when
the vehicle is stationary or manoeuvred at slow speed. The hydraulic reaction changes proportional to the vehicle
speed, with the required steering effort increasing as the vehicle moves faster. At high speeds, the Servotronic system
provides the driver with a good feedback through the steering providing precise steering and improved stability.
A major advantage of the Servotronic system is that fluid pressure and flow through the rotary valve remains constant
and allows full steering pressure to be available in an emergency where a sudden and unexpected steering correction
may be required.
