Power Steering

Power Steering

COMPONENT LOCATION

NOTE:
RHD (right-hand drive) 4.2L naturally aspirated installation shown, other installations similar.









INTRODUCTION
The power steering system comprises a ZF manufactured rack and pinion Servotronic 2 steering gear, a power steering pump, a fluid reservoir, a fluid cooler and fluid hoses. The steering gear is an end take-off rack and pinion power assisted unit with the addition of road speed proportional ZF Servotronic 2 assistance.
The steering rack is connected to the front wheel knuckles by adjustable tie-rods. The tie-rods allow for adjustment to centralize the steering wheel and also adjust the toe setting of the front wheel geometry.
The steering gear has a variable ratio rack. This provides conventional response when the steering is in the centre, straight ahead position, but provides more direct and faster steering as the turning angle increases. The variable ratio provides precise and rapid steering response at high speeds and provides optimized steering of the vehicle when maneuvering into parking spaces, turning in tight areas and when cornering in extreme conditions.
Fluid is supplied to the steering gear by a fixed displacement vane pump, belt driven from the crankshaft pulley. The pump is mounted on the LH (left-hand) side of the engine, above the A/C (air conditioning) compressor.
A fluid reservoir is positioned at the front LH (left-hand) side of the engine compartment, forward of the front suspension housing. A fluid cooler is located in front of the engine cooling radiator.
Servotronic 2 adds electronic control and speed sensitive steering to the steering gear. 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 instrument cluster. The software responds to road speed signals and controls the power assistance via a transducer valve located on the steering gear valve housing.

STEERING GEAR









The steering gear is located at the rear of the engine and attached to the front sub-frame. The gear is secured to the sub-frame with three bolts and washers which screw into threaded tubes which are integral with the sub-frame.
The steering gear comprises an aluminum, cast, valve housing which contains the hydraulic valve unit and Servotronic valve. The mechanical steering rack and the hydraulic actuator are located in a steel cylinder which is attached to the cast valve housing.
The steering gear uses a rack with an integrated piston which is guided on plain bearings within the cylinder and the valve housing. The pinion, which is attached to the valve unit, 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 with the minimum of play. The pinion is connected to the valve unit via a torsion bar. The rotary motion of the steering wheel is converted into linear movement of the rack by the rack and pinion mechanism and is initiated by the valve unit. This movement is transferred into movement of the road wheels by adjustable tie-rods.
The teeth angles at the centre of the rack vary from those at the end sections. It is this variation in teeth angles which provides the variable ratio.
The piston of the hydraulic actuator is located on the rack bar. Each side of the piston is connected to fluid pressure or fluid return via external metal pipes which are connected to the valve unit.
Each end of the rack bar has a threaded hole which provides for the fitment of the tie-rod. The external ends of the gear are sealed with boots which prevent the ingress of dirt and moisture. The tie-rod has a long threaded area which allows for the fitment of the tie-rod end. The thread allows for the adjustment of the steering toe. When the correct toe setting is achieved, a locknut is tightened against the tie-rod end preventing inadvertent movement.
The gear has a central hole machined along most of its length. The hole allows the air in the boots to be balanced when the steering is turned. The boots are serviceable items and are retained on the gear housing and the tie-rod with clips.

Valve Unit









The valve unit is an integral part of the steering gear. The principle function of the valve unit is to provide power assistance (i.e. when parking) to optimize the effort required to turn the steering wheel.
The pinion housing of the valve is an integral part of the main steering gear casting. The pinion housing has four machined ports which provide connections for pressure feed from the power steering pump, return fluid to the reservoir and pressure feeds to each side of the cylinder piston.
The valve unit comprises an outer sleeve, an input shaft, a torsion bar and a pinion shaft. The valve unit is co-axial with the pinion shaft which is connected to the steering column via the input shaft. The valve unit components are located in the steering gear pinion housing which is sealed with a cap.
The outer sleeve is located in the main bore of the pinion housing. Three annular grooves are machined on its outer diameter. PTFE (polytetrafluoroethylene) rings are located between the grooves and seal against the bore of the pinion housing. Holes are drilled radially in each annular groove through the wall of the sleeve. The bore of the outer sleeve is machined to accept the input shaft. Six equally spaced slots are machined in the bore of the sleeve. The ends of the slots are closed and do not continue to the end of the outer sleeve. The radial holes in the outer sleeve are drilled into each slot.
The input shaft has two machined flats at its outer end which allow for the attachment of the steering column intermediate shaft yoke. The flats ensure that the intermediate shaft is fitted in the correct position. The inner end of the input shaft forms a dog-tooth which mates with a slot in the pinion shaft. The fit of the dog-tooth in the slot allows a small amount of relative rotation between the input shaft and the pinion shaft before the dog-tooth contacts the wall of the slot. This ensures that, if the power assistance fails, the steering can be operated manually without over stressing the torsion bar. The central portion of the input shaft has equally spaced longitudinal slots machined in its circumference. The slots are arranged alternately around the input shaft.
The torsion bar is fitted inside the input shaft and is an interference fit in the pinion shaft. The torsion bar is connected to the input shaft by a drive pin. The torsion bar is machined to a smaller diameter in its central section. The smaller diameter allows the torsion bar to twist in response to torque applied from the steering wheel in relation to the grip of the tires on the road surface.
The pinion shaft has machined teeth on its central diameter which mate with teeth on the steering gear rack. A slot, machined in the upper end of the pinion shaft mates with the dog-tooth on the input shaft. The pinion shaft locates in the pinion housing and rotates on ball and roller bearings.

Servotronic Valve
The Servotronic transducer valve is located in a port in the side of the steering gear 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 Servotronic software in the instrument cluster.
The Servotronic valve determines the hydraulic reaction at the steering gear rotary valve and controls the input torque required to turn the steering wheel. The Servotronic system allows the steering to be turned with the optimum 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.
The instrument cluster receives road speed signals from the ABS (anti-lock brake system) module and calculates the correct controlling signal for the Servotronic valve. The Servotronic software within the instrument cluster has a diagnostic capability which allows a Jaguar approved diagnostic system to check the tune of the steering and retrieve fault codes relating to the Servotronic valve. Two fault codes are stored relating to the valve for positive connection short to ground or battery and negative connection short to ground or battery.
The Servotronic software within the instrument cluster also contains a number of steering maps which are selected via the car configuration file depending on the vehicle model and tire fitment.
If a failure of the Servotronic valve or software occurs, the system will suspend Servotronic assistance and only a default level of assistance will be available. Fault codes relating to the fault are stored in the instrument cluster. No warning lamps are illuminated and the driver may be aware of the steering being 'heavier' than usual.

POWER STEERING PUMP

Power Steering Pump and Pulley - 4.2L Vehicles









Power Steering Pump and Pulley - 5.0L Vehicles









On 4.2L naturally aspirated and SC (supercharger) vehicles the power steering pumps are basically the same, but with different flow control valve mechanisms.
New power steering pumps are introduced with the 5.0L engine. On 5.0L naturally aspirated and SC (supercharger) vehicles the power steering pumps are the same.
All of the power steering pumps are positive displacement, vane type pumps which supply a constant fluid flow to the steering gear valve unit. On all engines the power steering pump is driven by a Poly Vee belt from the crankshaft pulley. A self-adjusting tensioner is fitted to maintain the correct tension on the belt.
The power steering pump has an internal pressure relief valve and a flow control valve. On the power steering pumps of 4.2L vehicles:
- The pressure relief valve limits the maximum pressure supplied to the steering gear to 110 bar (1595 lbf/in2) ± 4 bar (58 lbf/in2).
- The flow control valve limits the maximum flow to 7.5 l/min (1.98 US galls/min) ± 0.75 l/min (0.20 US galls/min) regardless of engine speed.
- The pump has a displacement of 10.5 cm3/rev (0.64 in3/rev).
On the power steering pumps of 5.0L vehicles:
- The pressure relief valve limits the maximum pressure supplied to the steering gear to 115 bar (1668 lbf/in2) ± 4 bar (58 lbf/in2).
- The flow control valve limits the maximum flow to 8.5 l/min (2.25 US galls/min) ± 0.50 l/min (0.13 US galls/min) regardless of engine speed.
- The pump has a displacement of 11 cm3/rev (0.67 in3/rev).
All of the power steering pumps are of a similar construction. A shaft runs longitudinally through the pump. One end of the shaft is fitted with a drive pulley, which is either a press fit (4.2L vehicles) or secured by bolts (5.0L vehicles); the opposite end of the shaft is closed by a cover. The shaft runs in bearings located in the body and oil seals at each end of the shaft prevent leakage of hydraulic fluid. The pump contains ten vanes which rotate within a cam ring and are driven by the shaft. As the vanes rotate, the cam ring causes the space between the vanes to increase. This causes a depression between the vanes and fluid is drawn from the reservoir via the suction hose into the space between the vanes.
As the shaft rotates, the inlet port is closed to the vanes which have drawn in fluid, trapping the fluid between the vanes. The cam ring causes the space between the vanes to reduce and consequentially compresses and pressurizes the hydraulic fluid trapped between them.
Further rotation of the shaft moves the vanes to the outlet port. As the vanes pass the port plate the pressurized fluid passes from the pump outlet port into the pressure hose to the steering gear.
The pressurized fluid is subject to control by the flow control and pressure relief valve. The flow control valve maintains a constant flow of fluid supplied to the steering gear irrespective of engine speed variations. The pressure relief valve limits the maximum pressure on the output side of the pump. A metering orifice is included in the discharge port of the pump. If the pressure in the orifice reaches a predetermined level, a spring loaded ball in the centre of the flow control valve is lifted from its seat and allows pressurized fluid to recirculate within the pump.
The pressure relief valve will operate if the discharge from the pump is restricted, i.e. steering held on full lock. If the output from the pump is blocked, all output is recirculated through the pump. In this condition, as no fresh fluid is drawn into the pump from the reservoir, the fluid temperature inside the pump will increase rapidly. Consequentially, periods of operation of the steering gear on full lock should be kept to a minimum to prevent overheating of the pump and the fluid within it.

RESERVOIR









The reservoir is located in the engine compartment, forward of the LH (left-hand) suspension turret. A bracket attached to the reservoir is bolted via 2 rubber mounts, to 2 brackets on the front body crossmember.
The reservoir is a plastic moulding with an integral 100 micron, non-serviceable filter. Two moulded ports at the base of the reservoir provide for attachment of the fluid supply hose to the power steering pump and fluid return hose from the fluid cooler.
The reservoir is fitted with a removable cap which is screwed into the reservoir body. The cap has a dip stick with different diameters which denote the minimum and maximum fluid levels.
The reservoir has a fluid capacity to the maximum level of 400 cc (0.85 US pints).

HOSES AND FLUID COOLER

Hoses and Fluid Cooler - 4.2L Vehicles

NOTE:
LHD (left-hand drive) installations shown, RHD (right-hand drive) installations similar.









Hoses and Fluid Cooler - 5.0L Vehicles

NOTE:
LHD (left-hand drive)
SC (supercharger) installation shown, other installations similar.









4.2L naturally aspirated and SC (supercharger) vehicles differ in the location of the fluid cooler and consequently the routing of the hoses. On 5.0L naturally aspirated and SC (supercharger) vehicles the design and location of the fluid cooler, and the routing of the hoses, is the same.

Hoses
A mixture of metal pipes and rubber hoses are used in the power steering system. The high pressure pipe from the pump and the return pipe to the cooler from the steering gear are attached to one mounting plate. The pipe connections are sealed with O-ring seals and the plate is secured with a bolt screwed into the steering gear.
On 4.2L naturally aspirated and SC (supercharger) vehicles, the pump high pressure pipe is attached to the pump with a threaded, flared, pipe fitting which is sealed with an O-ring seal.
On 5.0L naturally aspirated and SC (supercharger) vehicles, the pump high pressure pipe is attached to the pump with a special elbow, banjo bolt and washers.
A restrictor is installed in the return line from the fluid cooler to the reservoir. The restrictor optimizes power steering system performance in terms of hydraulic NVH (noise, vibration and harshness) (water-hammer, grunt, shudder etc.) and vehicle dynamics (steering feel/damping).

Fluid Cooler
The fluid cooler is located in the return circuit from the steering gear to the reservoir. The cooler is an aluminum fin and tube design. Cool air entering the front of the vehicle passes over the cooler and flows through the fins. The fins act as heat exchangers, conducting heat from the fluid as it passes through the tube.
4.2L naturally aspirated and SC (supercharger) vehicles have different designs of cooler. 5.0L naturally aspirated and SC (supercharger) vehicles have the same design of cooler.
On 4.2L naturally aspirated vehicles the cooler is located near the bottom of the cooling pack. On 4.2L SC (supercharger) vehicles the cooler is located higher up, near the top of the cooling pack, to allow for the fitment of the charge air cooler.
On 5.0L naturally aspirated and SC (supercharger) vehicles, the cooler is located near the top of the cooling pack.

PRINCIPLES OF OPERATION
The following hydraulic circuits show power steering operation and fluid flow for the steering in a straight ahead, neutral position and when turning right. The circuit diagram for turning left is similar to that shown for turning right.

Power Steering in Neutral Position

NOTE:
Circuit shows steering rotary valve in neutral position with the vehicle not moving.









When the engine is started, the power steering pump draws fluid from the reservoir down the low pressure suction line. The fluid passes through the pump and is delivered at pressure, via a hose, to the steering rack valve unit.
The pressurized fluid flows through a connecting bore in the valve and, via the feed fluid radial groove and the transverse bores in the valve sleeve, passes to the feed fluid control groove of the valve rotor.
In the neutral (straight ahead) position, the fluid passes over the open feed fluid control edges to all valve sleeve axial grooves. The fluid then passes through return fluid control edges and the return fluid grooves of the valve rotor, back to the reservoir passes via the fluid cooler.
Simultaneously, the radial grooves of the valve and their associated pipes provide a connection the left and right power assist cylinders.

Power Steering in Right Turn Position

NOTE:
Circuit shows steering rotary valve in right turn position, at high vehicle speed and with rapid steering corrections. The Servotronic transducer valve is fully open and the maximum hydraulic reaction is limited by the cut-off valve.









When the steering wheel is turned to the right, the steering rack and piston moves to the left in the piston bore. The valve rotor is rotated to the right (clockwise) and pressurized fluid is directed over the further opened feed fluid control edges and to the associated axial grooves, the radial groove and via an external pipe to the left power assist cylinder chamber. The pressure applied to the piston from the left power assist cylinder chamber provides the hydraulic assistance.
An adaptable pressure build-up is achieved by the partially or fully closed feed fluid control edges restricting or preventing a connection between the fluid pressure inlet and the other axial grooves connected to the radial groove.
Simultaneously, the fluid pressure outlet to the pressurized axial grooves are restricted or partially restricted by the closing return fluid control edges. The fluid displaced by the piston from the right power assist cylinder chamber, flows through an external pipe to the radial grooves. From there the fluid passes to the associated axial grooves and on to the return fluid control grooves, via the further opened return fluid control edges.
The return flow of fluid to the reservoir passes via interconnecting bores which lead to the return fluid chamber. When the steering wheel is turned to the left the operating sequence is as above but the pressure is applied to the opposite side of the piston.

Servotronic Operation
The Servotronic software contains a number of steering maps which are selected via the car configuration file depending on the vehicle mode and tire fitment.
If a failure of the Servotronic valve or software occurs, the system will suspend Servotronic assistance and only normal power steering wheel be available. Fault codes relating to the fault are stored, but no warning lamps are illuminated and the driver may be aware of the steering being 'heavier' than usual.
When the vehicle is manoeuvred into and out of a parking space (or other similar manoeuvre), the Servotronic software uses road speed data from the ABS (anti-lock brake system) module to determine the vehicle speed, which in this case will be slow or stationary. The Servotronic software analyses the signals and outputs an appropriate control current to the Servotronic transducer valve. The Servotronic valve closes and prevents fluid flowing from the feed fluid radial groove to the reaction chamber. An orifice also ensures that there is return pressure in the reaction chamber. This condition eliminates any 'reaction' ensuring that the steering is very light to operate, reducing the effort required to turn the steering wheel.
As the vehicle is driven and the road speed increases, the Servotronic software analyses the road speed signals from the ABS (anti-lock brake system) module and reduces the amount of control current supplied to the Servotronic valve which increases the reaction pressure. This modifies the input torque applied through the steering wheel and provides the driver with an improved 'road feel' allowing precise steering and directional stability.