Steering System
STEERING SYSTEM DESCRIPTION
Steering Column Major Components
Integrated column assembly incorporating power, or manual, reach/tilt mechanism and lock.
Ignition switch.
Ignition interlock solenoid.
Key transponder coil.
Body attachment points.
Depending upon model, the steering column may be adjusted for tilt and reach, either by electrical or manual means. Power variants may be either automatically or manually adjusted and all types have the entry/ exit feature.
Steering Column Operating Principle
Power Adjust: Two independent motor / gearbox assemblies provide infinite adjustment for reach and height within approximate ranges of 35mm and 130 respectively. Adjustments may be automatically made in conjunction with the seat memory facility or manually when the adjustment switch is used. It should be noted that selection of 'Off' will disable the automatic entry / exit mode.
Manual Adjust: The cable operated reach adjustment is infinite within a range of 35mm, with the desired position being fixed by a rack and wedge. Tilt variations are stepped at approximately 30 intervals with 6 positions being available, the uppermost being unlatched.
From the uppermost position the column may be pulled down to engage the first detent without using the tilt lever.
WARNING: MANUAL ADJUST ONLY: TO AVOID PERSONAL INJURY, COLUMN UPWARD TRAVEL SHOULD BE MANUALLY RESTRAINED TO CHECK UPWARD SPRING ASSISTANCE. THIS IS ESPECIALLY IMPORTANT IF THE STEERING WHEEL HAS BEEN REMOVED FOR MAINTENANCE REASONS.
WARNING: ALL TYPES; DO NOT REMOVE THE STEERING COLUMN FROM THE VEHICLE WITH THE STEERING WHEEL ATTACHED UNLESS THE STEERING IS CENTERED AND THE COLUMN LOCK IS ENGAGED. IF THE LOCK IS TO BE RENEWED, 'LOCK-WIRE' THE ASSEMBLY TO PREVENT ROTATION. FAILURE TO OBSERVE THIS MAY RESULT IN AN INOPERATIVE AIRBAG SYSTEM. SEE LABEL ON STEERING WHEEL HUB.
Steering Hydraulic System Major Components
Engine driven rotary vane pump (belt driven 12 cylinder; direct drive from timing gear 6 cylinder) with falling flow characteristic (as pump speed increases fluid flow decreases) and integral pressure relief valve.
Remote fluid reservoir with integral 'return' side filter.
Steering rack (incorporating speed sensitive transducer).
Steering control module (SCM).
Fluid cooler integral with engine coolant radiator and associated pipe-work.
Hydraulic System Features
The 'Servotronic' system reduces steering input loads during parking and low speed maneuvers and progressively increases input loads as vehicle speed rises. This feature enhances steering feel.
Hydraulic System Operating Principle (see illustrations)
Rotary motion of the steering wheel is converted, via the steering gear pinion to lateral motion of the rack. Hydraulic assistance is provided by pressurized fluid being directed against the rack bar piston in the rack cylinder. The pressure applied to each side of the rack piston is controlled by the pinion valve which varies the restriction through which the flow for each side of the rack piston must pass.
With the vehicle travelling straight ahead, the valve restrictions are balanced, thus providing equal pressure on each side of the piston. When load is applied at the steering wheel the two halves of the pinion valve are displaced making the restrictions unequal. The differential pressure then directs the rack piston in favour of the higher pressure. As the turning load is removed the pressures equalize and the steering returns to the straight ahead position, aided by suspension geometry.
The displacement of the halves of the pinion valve (rotor and sleeve) is controlled by the elastic deformation of the torsion bar which is concentric with the pinion and valve.
The rotary valve within the pinion housing on the 'Servotronic' system is fitted with a 'Hydraulic Reaction Piston' (HRP). The HRP moves axially, relative to the rotor, on ball bearing travellers (acting as a face cam) and is connected to the sleeve by a four bearing helical screw. Pressure applied either side of the HRP is translated into a rotational force which resists torsion bar/valve displacement thus increasing steering effort. The amount of pressure applied to the HRP is controlled by: a) the differential pressure on the rack piston, proportional to vehicle cornering force and b) the position of the transducer valve controlled by the SCM from information gathered on road speed. Road speed data is processed from the left hand rear wheel speed sensor.