Principles of Operation

Principles of Operation

Electrical Power Steering System
The electrical power steering system provides power steering assist to the driver by replacing the conventional hydraulic valve system with an electric motor coupled to the steering gear. The motor is controlled by an electronic control unit that senses the steering effort through the use of a torque sensor mounted between the steering column shaft and the steering gear. Steering assist is provided in proportion to the steering input effort and vehicle speed.

The electrical power steering system requires a 12-volt, hot at all times feed for fault management. The power steering control module is activated when power is applied to the hardwired ignition/run input. After activation, the power steering control module monitors the controller area network (CAN) bus to determine if the vehicle is operating in a manner capable of supporting the electrical power steering system. The CAN bus signal sent by the powertrain control module (PCM) to the power steering control module, is used to indicate that the vehicle has been activated and power is being supplied to the DC/DC system, which provides power to the electrical power steering system. The electrical power steering system powers itself down within 3 seconds after the key is moved to the OFF position.

The vehicle speed, which is sent by the PCM over the CAN bus, provides the necessary vehicle speed information to the electrical power steering system to determine the amount or level of power assist. As vehicle speed increases, the amount of power assist provided by the system is reduced to improve and enhance road feel at the steering wheel. If the vehicle speed is missing or out of range, the power steering control module defaults to a reduced level of assist. If the vehicle speed returns to the correct in-range values, the power steering control module adjusts the steering assist level accordingly.

The power steering control module provides a periodic CAN message that indicates the operating condition of the electrical power steering system. Upon receiving an indication of a failure condition, the instrument cluster displays a text message. The instrument cluster, in addition to responding to the power steering malfunction message from the electrical power steering system, also displays the failure message if the instrument cluster does not receive a CAN message for one second or more.