Principles Of Operation

Steering System

Principles of Operation

Electronic Power Assist Steering (EPAS)

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 Module (PSCM) 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 (Electronic Power Assist Steering) system requires a 12-volt, hot at all times feed for system operation. The PSCM (Power Steering Control Module) is activated when power is applied to the hardwired ignition/run input. After activation, the PSCM (Power Steering Control Module) monitors the High Speed Controller Area Network (HS-CAN) bus to determine if the vehicle is operating in a manner capable of supporting the EPAS (Electronic Power Assist Steering) system.

The vehicle speed, which is sent by the PCM over the HS-CAN (High Speed Controller Area Network) bus, provides the necessary information to the EPAS (Electronic Power Assist Steering) 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 PSCM (Power Steering Control Module) defaults to a reduced level of assist. If the vehicle speed returns to the correct in-range values, the PSCM (Power Steering Control Module) adjusts the steering assist level accordingly.

Low battery and/or charging system voltage or a charging system DTC being present may cause the PSCM (Power Steering Control Module) to enter into a limited operation mode resulting in uneven steering assist or a lack of steering assist. This limited operation mode feature is built into the PSCM (Power Steering Control Module) to allow the safe, continued operation of the vehicle in the event of an EPAS (Electronic Power Assist Steering) concern. A DTC may or may not be preset when the PSCM (Power Steering Control Module) is operating in limited operation mode. Additionally, overheating the steering system may cause the PSCM (Power Steering Control Module) to go into the limited operation mode, this is in order to protect the electronic components from overheating.

Power Steering Control Module (PSCM)

The PSCM (Power Steering Control Module) manages the EPAS (Electronic Power Assist Steering) system. The PSCM (Power Steering Control Module) receives vehicle speed information from the PCM via the HS-CAN (High Speed Controller Area Network) 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 PSCM (Power Steering Control Module) is part of the steering column and is not serviceable separately.

The PSCM (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 (Electronic Power Assist Steering) system.

Power Steering Motor

The EPAS (Electronic Power Assist Steering) 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 Sensor

The steering sensor provides the PSCM (Power Steering Control Module) with information on the direction, speed and amount of force being applied to the steering wheel. The module uses this information to determine the 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.

Electronic Power Assist Steering (EPAS) Column Assembly Initialization

After installing a new EPAS (Electronic Power Assist Steering) steering column assembly, the steering column assembly must be configured and the steering wheel position sensor must be calibrated. Refer to Steering Wheel Position Sensor Calibration .

Electronic Power Assist Steering (EPAS) Column Assembly Configuration

Refer to Programmable Module Installation (PMI) in Information Bus Programming and Relearning.

Active Park Assist

When the active park assist system is enabled, the Parking Aid Module (PAM) will utilize ultrasonic sensors to locate and measure a parking space. Once a parking space is identified by the system and accepted by the driver, the PAM (Parking Aid Module) will send steering command messages to the PSCM (Power Steering Control Module) over the HS-CAN (High Speed Controller Area Network) bus. The PSCM (Power Steering Control Module) will utilize steering angle information and wheel speed sensor information (wheel speed is transmitted by the ABS module over the HS-CAN (High Speed Controller Area Network) bus) to actuate the steering system as requested by the PAM (Parking Aid Module).

Any driver inputs via the steering wheel will deactivate the active park assist system. Also, the presence of certain DTCs in the PSCM (Power Steering Control Module), PAM (Parking Aid Module) and/or ABS module will disable the system or keep the system from being activated. Refer to Active Parking Assist for additional information on the active park assist system.