Description
Description
General
Rack and pinion steering is introduced on Range Rover for the first time. A ZF power assisted steering rack is fitted on the front subframe. The rack is a conventional end take-off rack and pinion power assisted unit with the addition of ZF Servotronic 2 assistance.
Servotronic 2 adds electronic control and speed sensitive steering to the steering rack. The Servotronic 2 feature provides easy and comfortable steering operation when parking, improved 'road feel' at increased road speeds and adds an integrated, positive centre feel feature which optimizes steering wheel torque during high speed driving.
The Servotronic 2 system is controlled by software which is incorporated into the Body Control Unit (BCU). The software responds to steering torque inputs and road speed signals and controls the assistance via a transducer valve located on the steering rack valve.
The steering column features fully electrical adjustment for tilt and reach. On vehicles fitted with memory seats, the steering wheel position control is incorporated into the memory seat ECU.
The column also features an electronically controlled steering lock. The lock is operated when the ignition switch senses that the correct key has teen inserted. An ECU, located inside the steering column, controls a motor operated lock, releasing the steering lock when appropriate.
Steering Column
The steering column comprises the upper column assembly which is connected to the steering rack via several shafts which are connected via flexible couplings and universal joints.
The lower column comprises upper and lower shafts which are connected together as a sliding fit. The sliding fit allows the shafts to collapse in the event of a frontal collision, preventing column intrusion into the vehicle interior. The upper shaft is connected to a flexible coupling and provides the attachment to the upper column. The lower shaft has internal splines which connect with splines on the steering rack valve input shaft.
The upper column also comprises two shafts which are also connected together as a sliding fit. The shafts are a double 'D' section to prevent rotation. The sliding fit allows the shafts to collapse in the event of a frontal collision further preventing intrusion into the vehicle interior. The lower shaft is connected to the flexible coupling which transfers torque to the lower column and the steering rack. The upper shaft has a splined internal bore which connects with the adjustable upper column assembly.
The upper column assembly comprises two extruded aluminum sections which connect together and can slide on each other in the event of a frontal impact, further preventing column intrusion into the vehicle interior. The column is attached to the cross-car tube with four bolts. A strap is bolted to the upper of the two aluminum sections. The strap is coiled around a bush on the upper section and has a hook which locates on the lower section. In the event of a frontal impact, the strap can unwind, allowing the upper column to collapse in a controlled manner absorbing energy.
The upper column also provides the location for the steering angle sensor which is used by the ABS system. Refer to the Braking section for details.
The upper column provides the locations for the column adjustment motors and the steering lock mechanism and steering lock ECU. The steering lock operates via a locking bolt which engages with a locking sleeve located around the column shaft. The locking sleeve has a tolerance ring which is located between the sleeve and the column. The tolerance ring allows the locking sleeve to slip on the upper column shaft if a high torque is applied to the steering wheel when the lock is engaged. This prevents the locking bolt from being sheared by someone forcibly turning the steering wheel while the steering lock is engaged, yet still effectively locking the steering.
Column Adjustment
The steering column is adjusted using a four way switch located on the LH side of the steering column, below the LH column control switch.
The steering column adjustment is achieved by two electric motors and a column adjustment control unit on vehicles without memory seats or by the memory seat ECU on vehicles with memory seats. Both motors are attached to the lock housing/tilt head assembly and move the column via worm drive screws.
The reach adjustment motor drives a screw shaft which moves the column in and out for reach adjustment. The adjustment is achieved by the two extruded aluminum sections which slide on one another. The tilt adjustment motor also drives a screw shaft. This shaft is connected to a cam, which causes the tilt head to pivot, adjusting the column angle.
On vehicles without memory seats, inputs from the column adjustment switch are received by the adjustment control unit located under the column. The control unit interprets the signals from the switch and operates the requested adjustment motor in the required direction.
On vehicles with memory scats, the adjustment control unit is not fitted. Inputs from the column adjustment switch are received by the memory seat ECU which is incorporated into the driver's seat switch pack. The ECU interprets the signals from the switch and operates the requested adjustment motor in the required direction. The motors used on the memory vehicles have Hall effect sensors known as 'Ripple counters'. These counters output the position of the column when the seat memory store function is used and applies this setting to the applicable ignition key.
Column Electronic Steering Lock
With the ignition switch located in the centre console, a conventional steering lock mechanism cannot be used. An electronic system was designed which comprises an ignition switch assembly with a position switch and anti-rotational lock and a steering column assembly locking unit with integrated steering lock ECU.
The upper steering column assembly houses the column lock mechanism and ECU. The components are assembled with non-removable pins and is therefore non-serviceable. Failure of any steering lock components will require replacement of the upper steering column assembly.
The steering column lock comprises a locking motor, locking bolt and a safety release solenoid actuator. The actuator is required to hold the locking bolt in the unlocked position in the event of power loss during operation. The locking motor drives a cam which moves the locking bolt into and out of engagement with the locking sleeve on the steering column. The locking motor is fitted with a Hall effect sensor which informs the steering lock ECU of the position (locked/unlocked) of the steering lock mechanism.
The ignition switch assembly also has a locking mechanism which works in conjunction with the immobilization ECU and the steering lock ECU. The ignition switch mechanism comprises an ignition switch lock solenoid actuator which prevents the ignition switch being rotated unless the key has been recognized by the immobilization ECU. A Hall sensor is located in the ignition switch body and senses the rotation of the ignition switch. This signal is transmitted on the K bus and is used by the steering lock ECU to confirm the key status with the immobilization ECU. Refer to Operation later in this section for functional description.
Steering Rack
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 pressurized 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 pressurized 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 Test Book/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 maneuvered 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.
Power Assisted Steering (PAS) Pump and Reservoir
The PAS fluid pumps used on both the Td6 and V8 models are of the vane type and are similar in their operation. The pumps are driven by a polyvee belt from the engine crankshaft pulley. A tensioner is used to maintain the correct tension on the belt.
V8 PAS Pump and Reservoir
The V8 PAS pump comprises a body which houses the internal components of the pump. A pressure relief valve, which also incorporates a flow control valve, is installed in the housing. The pressure relief valve limits the maximum pressure to between 124 and 132 bar (1800 and 1915 inch lbs.2). The flow control valve limits the maximum flow to between 9.75 and 11.25 l/min (2.14 and 2.47 UK gal/min).
A shaft runs longitudinally through the pump. One end of the shaft has a drive flange which accepts the drive pulley. The opposite end is closed by a cover. The shaft runs in bearings located in the body. Oil seals at each end of the shaft prevent leakage.
An oval cam ring is located in the body. Ten vanes are housed in a carrier and rotate within the cam ring. The carrier is mounted in the centre of the shaft and receives positive drive from the shaft via a drive pin. The carrier is seated against an end plate which is located in the cover. The front of the carrier is covered by a port plate which is located against a seal plate in the body. The port plate controls the fluid flow into and out of the vanes during their cycle.
The fluid reservoir used on the V8 model is mounted remotely on the LH side of the engine compartment, adjacent to the engine coolant reservoir.