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 system requires a 12-volt, hot at all times feed for system operation. The PSCM is activated when power is applied to the hardwired ignition/run input. After activation, the PSCM 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 system.

The vehicle speed, which is sent by the PCM over the HS-CAN bus, provides the necessary 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 PSCM defaults to a reduced level of assist. If the vehicle speed returns to the correct in-range values, the PSCM adjusts the steering assist level accordingly.

Low battery and/or charging system voltage or a charging system DTC being present may cause the PSCM 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 to allow the safe, continued operation of the vehicle in the event of an EPAS concern. A DTC may or may not be preset when the PSCM is operating in limited operation mode. Additionally, overheating the steering system may cause the PSCM 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 manages the EPAS system. The PSCM 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 PSCM is part of the steering column and is not serviceable separately.

The PSCM 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 Sensor

The steering sensor provides the PSCM 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.