Description

Power Steering

NOTE:
A = Hardwired; D = High speed CAN (controller area network) bus.









For the EPS (electric power steering) system to provide assistance the engine must be running.
If a fault occurs which affects control of the electric motor, the power steering control module disables the electric motor. The driver no longer benefits from power assisted steering, but incorrect control of the electric motor is prevented and the vehicle remains fully steerable, although with greater physical effort.

VARIABLE POWER ASSISTANCE
At low speeds, when tire resistance to steering inputs is highest, more assistance is applied to reduce the steering wheel torque to comfortable levels. At higher speeds, when tire resistance to steering inputs is less, less assistance is required.

PULL DRIFT COMPENSATION
This feature aids the driver by compensating for the accumulated effects of various factors that contribute to the pull/ drift of the vehicle. The feature reduces the steering wheel torque offset that the driver feels while driving in a straight line, by applying a counter torque. The vehicle will drift less if the driver's hands are taken off the steering wheel, and steering effort in general is reduced.

ASSISTANCE REDUCTION IF SYSTEM OVERHEATS OR VOLTAGE IS TOO HIGH OR LOW
This feature ensures that the driver's steering requirements continue to be supported during various extreme conditions including:
- Extreme voltage fluctuations (over and under voltage).
- Extreme high temperature.
- Steering system vibration optimization, detected for example when steering on a low friction surface.
A temperature sensor is installed in the power steering control module for overload protection of the electric motor. The power steering control module reduces the level of power assistance if the temperature of the electric motor is too high, to reduce the amount of heat generated.
Temperature overload can occur at high ambient temperatures combined with high levels of steering activity, especially when stationary. A temperature overload can also occur if an attempt is made to repeatedly turn the front wheels against a solid object such as a curb, which the power steering control module detects by comparing the control signals to the electric motor with the motion of the motor. Reducing power assistance in this situation protects the power steering components against excessive mechanical stresses and signals to the driver that there is a solid object preventing the wheels turning.
The reduction of power assistance begins at a temperature of approximately 110 °C (230 °F) and reduces to zero at a temperature of 125 °C (257 °F). At a given level of reduction in the power assistance, the power steering control module records a DTC (diagnostic trouble code) and signals the instrument cluster to produce a warning that assistance has been reduced.

ASSISTANCE CHANGES DURING ECO STOP/START FUNCTIONS
This feature actively controls the driver's steering torque feedback during the engine Eco stopping and starting activities. It integrates the main steering assistance feature with hybrid, micro hybrid or enhanced starter motor powertrain events, ensuring smooth, progressive, yet timely decreases and increases in power assistance. It is also designed to remove unwanted steering wheel movement during these events, which are not directly instigated by the driver.

SOFT END STOPS
The end stop feature reduces the level of power assistance shortly before the mechanical end stops are reached. Although the driver will perceive this as increased steering resistance, it makes turning the wheels to full lock much smoother overall. In addition, this feature reduces the stresses on the mechanical and electrical components of the steering system.

PARK ASSIST SYSTEM (WHERE FITTED)
When the park assist system is active, steering trajectory requests from the parking aid module are processed by the power steering control module and applied to the electric motor. This feature assists the driver, during parallel parking maneuvers, by selectively applying turning forces to the steering system to automatically steer the vehicle.

STEERING GEAR









The steering gear is installed on the front subframe and secured with two bolts. A yoke connects the input shaft of the steering gear to the bottom of the steering column. A rubber seal assembly is installed on the input shaft where it passes through the lower firewall. Ball jointed tie-rods connect the steering rack to the knuckles of the front suspension. The ends of the steering rack are protected by corrugated rubber boots secured with cable ties. A heat shield is attached to the top of the steering gear to reflect radiant heat from the exhaust system.
The steering rack is installed in a steering gear housing that also contains the pinion shaft and the torque sensor. A torsion bar connects the pinion shaft to the input shaft. The steering gear housing also incorporates a mounting for the electric motor, and two threaded bosses for the steering gear securing bolts. A reduction gear housing, on the end of the steering gear housing, contains a recirculating ball nut installed on a threaded section of the steering rack. A toothed belt transmits drive from the electric motor to the recirculating ball nut to provide the power assistance. The ratio of electric motor revolutions to steering wheel revolutions is approximately 20:1.
A water drain valve, consisting of a spring and plunger, is installed in the mounting for the electric motor. The drain valve is held closed by a salt tablet. If the salt tablet is exposed to water from the inside of the rack, it dissolves, which allows the spring to move the plunger to the open position to drain the water. A visual inspection of the water drain valve should be made at regular intervals. If the valve is open the steering gear must be replaced.

Electric Motor





The electric motor is a brushless DC (direct current) motor. The electronics that convert the DC power supply into phase voltages are contained in the power steering control module.
Motor Position Sensors
Motor position is detected by two Hall effect sensors in the power steering control module, adjacent to rotating magnets on the electric motor shaft. The two sensors have different resolutions, which allows the power steering control module to monitor both fast and slow movement effectively. From the position of the electric motor, the power steering control module can deduce the position of the road wheels and the steering angle.
After first calibrating the straight-ahead position using the steering angle sensor, the power steering control module subsequently uses the motor position sensor inputs for control of the power steering, because of their greater accuracy.

Reduction Gear









The recirculating ball nut in the reduction gear housing is held in a bearing race, allowing the nut to rotate but preventing it from moving axially. As the electric motor rotates the nut, the balls are driven along the channels in the nut and the steering rack, and produce an axial force on the steering rack. A channel in the nut, and mechanisms at either end of the bearing race, recirculate the balls from one side of the nut to the other.

Torque Sensor









The torque sensor provides the power steering control module with inputs of the steering torque applied by the driver. The torque sensor is installed at the junction between the input shaft and the pinion shaft, inside the steering gear housing.
Torque applied to the input shaft is transferred to the pinion shaft through the torsion bar, which twists under load. The torque sensor calculates the amount of twist in the torsion bar using two Hall effect sensors to monitor the angular position of the input shaft in relation to the pinion shaft, then transmits the result in a digital signal to the power steering control module.

Power Steering Control Module









The power steering control module is located on the end of the electric motor, under a hermetically sealed cover. A diaphragm in the cover, made from a waterproof breathable fabric, maintains the interior of the cover at ambient pressure, avoiding the need for an open vent.
As well as the control circuitry, the power steering control module also contains the power electronics for operating the electric motor.
Power assistance is provided when the following are present:
- An ignition feed from the ignition relay in the CJB (central junction box).
- The power mode CAN (controller area network) signal is set to engine running.
Electrical power for operation of the electric motor is provided by a permanent feed from the EJB (engine junction box). For 100% nominal power assistance, supply voltage must be between 10.5 and 18 V. If supply voltage is outside of these limits, power assistance is either reduced or discontinued.
The power steering control module receives hardwired digital inputs from the torque sensor and high speed CAN (controller area network) bus inputs of:
- Steering angle, from the steering angle sensor.
- Engine speed, from the ECM (engine control module).
- Vehicle speed and yaw rate, from the ABS (anti-lock brake system) module.
- Automatic steering requests, from the parking aid module (if park assist system fitted).
- Reverse gear signal from the CJB (central junction box) (manual transmission only).
- Vehicle speed from the TCM (transmission control module) (automatic transmission only).
- Terrain mode request from the Terrain Response switchpack and control module.
- Power mode from the CJB (central junction box).
- Eco stop/start system stop request from the CJB (central junction box).
The power steering control module uses the input data and software algorithms to control electric motor operation. If required, the power steering control module can also transmit outputs, on the high speed CAN (controller area network) bus, of:
- Warning status, to the instrument cluster.
- Steering status, to the parking aid module (if park assist system fitted) and Terrain Response switchpack and control module.
- Eco stop/start system inhibit requests to the ECM (engine control module).
- Tie-rod load forces.

REDUCED ASSISTANCE WARNING INDICATION
If there is a significant reduction in power assistance, the amber general warning indicator illuminates and a warning message is displayed in the message center. When the warning message is first displayed, the instrument cluster produces a single alert chime. For additional information, refer to Information and Message Center Locations
If the steering gear is replaced, the warning will be activated until the appropriate configuration of the EPS (electric power steering) has been completed.