Principles of Operation

Electronic Power Assist Steering (EPAS) System -   The electronic power assist steering (EPAS) system provides power steering assist to the driver by replacing the conventional hydraulic valve system with an electric motor coupled to the steering shaft. The motor is controlled by the power steering control (PSC) module 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 EPAS system requires a 12-volt, hot at all times feed for system operation. The PSC module is activated when power is applied to the hardwired ignition/run input. After activation, the power steering control module monitors the high speed communications area network (HS-CAN) bus to determine if the vehicle is operating in a manner capable of supporting the electronic power steering system.

The vehicle speed, which is sent by the PCM over the HS-CAN bus, provides the necessary vehicle speed information to the EPAS 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.

Power Steering Control Module -   The power steering control module manages the EPAS system. The power steering control module receives vehicle speed information from the PCM via the HS-CAN bus, the module also receives information from the steering shaft torque sensor to determine the amount of assist that is needed. At lower speeds, the control module will provide greater assist. At higher speeds, the control module will provide less assist. The power steering control module is part of the steering column and is not serviceable separately.

The power steering control module is self-monitoring and has the capability of setting and storing DTCs. Depending on the DTC set, the control module will enter into a "safe mode" and will provide a set amount of assist to the EPAS system.

Power Steering Motor -   The EPAS system uses a 12-volt brushless reversible motor to control the steering effort. The motor is connected to the steering shaft through a worm gear and a reduction gear on the steering column. The motor is part of the steering column assembly and is not serviceable separately.

Steering Shaft Torque Sensor -   The steering shaft torque sensor provides the power steering control module with information on the direction and amount of steering assist required. The sensor is a 5-volt dual analog sensor with an output voltage range of 0.25 volt to 4.75 volts. When the steering wheel is turned to the left, the left signal increases while the right signal decreases, likewise when the steering wheel is turned to the right, the right signal increases while the left signal decreases. The control module recognizes the change in the signals and applies steering assist as required. The torque sensor is part of the steering column assembly and is not serviceable separately.

Steering Wheel Position Sensor -   The steering wheel position sensor (also called the Electronic Stability Program [ESP] sensor) monitors the position of the steering wheel by measuring voltage differences in position signals. The steering wheel position sensor informs the power steering control module whether the steering wheel is being turned left or right and how far it is being turned. The control module uses the HS-CAN bus to transmit this information to other control modules. The position sensor is part of the steering column assembly and is not serviceable separately from the module.