Electronic Power Assist Steering (EPAS)

WARNING: This page is about a different car, the 2010 Mercury Mariner and 2010 Ford Escape. However, it is still accessible from the selected car via links, so may be relevant.

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.