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.