Operation

Operation

Hydraulic Circuit Operation
The 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.





Steering In Neutral Position
When the engine is started, the PAS pump draws fluid from the reservoir down the low pressure suction line. The fluid passes through the pump and is delivered, 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 the open return fluid control edges and the return fluid control grooves of the valve rotor. The fluid then passes via connecting bores to the return fluid chamber end to the reservoir via an external hose.
Simultaneously, the radial grooves of the valve body and their associated pipes provide a connection between the left and right power assist cylinders.





Steering in Right Turn Position
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
When the vehicle is maneuvered into and out of a parking space (or other similar manoeuvre), the Servotronic function of the BCU uses road speed data from the ABS ECU to determine the vehicle speed, which in this case will be slow or stationary. The BCU microprocessor analyzes 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 BCU microprocessor analyzes the road speed signals from the ABS ECU and reduces the amount of control current supplied to the Servotronic valve. The Servotronic valve reacts to the control current and opens the valve by an amount appropriate to the road speed. This allows a controlled fluid supply from the feed fluid radial groove to the reaction chamber. The orifice prevents the loss of large amounts of fluid to the return fluid chamber. The higher fluid pressure acting on the reaction piston causes compression of the balls which are located between the reaction piston and the centering piece which in turn is securely connected to the valve sleeve. When driving in a straight line, this has a positive effect on the exact centralizing of the steering valve. When the steering valve is actuated, the balls, with a higher load on them, provide additional torsional resistance to rotation of the valve rotor. This mode of Servotronic assistance requires an established higher steering wheel torque until the hydraulic assistance required in the left or right power assist cylinder is determined.

At high driving speeds, i.e. on a motorway, the Servotronic valve is fully open due to a very low or nonexistent control current supplied from the BCU. This enables maximum pressure supply from the feed fluid radial groove to the reaction piston. When the steering wheel is turned, the reaction pressure increases appropriate to the existing operating pressure and pressurizes the piston in the reaction chamber. As soon as the reaction pressure reaches its upper limit, the fluid is discharged to the return fluid chamber via the cut-off valve and prevents further increases in reaction pressure. This maintains the input torque applied through the steering wheel and provides the driver with an improved 'road feel' allowing precise steering and directional stability.

Steering Lock ECU Operation
The steering lock ECU operates in conjunction with the immobilization ECU and the ignition switch.

Detection of Unlock Request
When a 'valid' key is inserted in the ignition switch, the immobilization ECU transmits a 'valid key in lock' message on the K Bus. The steering lock ECU receives this message and starts the unlocking process of operating the steering lock motor to disengage the locking bolt.

The immobilization ECU goes into sleep mode 60 seconds after the vehicle is unlocked and a key is not inserted in the ignition switch. When the key is subsequently inserted in the ignition switch and rotated, the rotation is sensed by the Hall rotation sensor in the switch. Further rotation is prevented by the ignition switch lock solenoid actuator. The rotation sensor transmits a signal which is received by the steering lock ECU. This ECU sends a 'request immobilization ECU status' message on the K bus which wakes the immobilization ECU. When the immobilization ECU confirms that a valid key is inserted, the ignition switch lock disengages and the steering lock ECU starts the unlocking process.

If the K bus is inoperative, the unlocking operation is initiated by the rotation sensor signal. This is indicated to the driver through a delayed operation of the solenoid actuator preventing the key from being turned initially. The solenoid actuator is operated on a delay which will initially prevent the key from being turned. The solenoid actuator will unlock and allow the key to turn on the second attempt. A defect in the K bus is detected if no response from the immobilization ECU is received after a 2.4 seconds period after the 'request immobilization ECU status' is sent.

Unlocking of Steering Lock
If the bakery voltage is 9V or less, the steering lock will be inoperative.

When a valid key is inserted and confirmed, the steering lock ECU activates the steering lock motor in the unlock direction. Movement from the locked to the unlocked position is detected by the Hall sensor in the motor and sensed by the steering lock ECU.

When the Hall sensor detects that the unlocked position has been reached, the ECU brakes the motor. The safety release solenoid actuator is deenergized and its spring loaded pin drops into a recess in the locking bolt. This forms a mechanical lock securing the locking bolt in the unlocked position preventing the steering lock from being applied when the vehicle is being operated. A Hall sensor in the safety release solenoid actuator informs the steering lock ECU of this condition. Once the unlocking procedure is successfully performed, the steering lock ECU deenergizes the steering lock motor.

Once the steering is successfully unlocked, the ignition switch lock disengages allowing the key to be turned to positions II and III.

Detection of Locking Request
When the ignition key is turned to position 0, the ignition switch lock solenoid actuator remains active (unlocked) for 16 minutes. This function allows immediate restarting of the vehicle as long as the key is not removed from the ignition switch. When the key is removed, the immobilization ECU senses the removal and transmits a 'no valid key in lock' message on the K bus. The steering lock ECU checks that the ignition switch position II or III outputs and the rotation sensor are not active and initiates the steering locking process. This check ensures that the steering is not locked when the vehicle may be being driven.

If the K bus is inoperative, locking of the steering column is prevented by the steering lock ECU. This is indicated to the driver when attempting to start the vehicle through a delayed operation of the ignition switch lock solenoid actuator preventing the key from being turned initially. The solenoid actuator is operated on a delay which will initially prevent the key from being turned. The solenoid actuator will unlock and allow the key to turn on the second attempt. A defect in the K bus is detected if no response from the immobilization ECU is received after a 2.4 seconds period after the 'request immobilization ECU status' is sent.

Locking of Steering Lock
The battery voltage must be greater than 10V and the vehicle must be stationary.

The ABS ECU transmits a vehicle speed signal on the CAN which is received by the steering lock ECU. The steering lock ECU uses the signal to determine when the vehicle has stopped.

When the key is removed and the steering lock ECU receives the 'no valid key in lock' message from the immobilization ECU, the steering lock motor is activated. The safety release solenoid actuator in the steering column is energized, withdrawing the pin from the locking bolt. The Hall sensor in the solenoid actuator informs the steering lock ECU that the lock is released.

The steering lock ECU then operates the steering lock motor in the lock direction. Movement from the unlocked to the locked position is detected by the Hall sensor in the motor and sensed by the steering lock ECU. The safety release solenoid actuator is deenergized and rests on the locking bolt.