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 using a torque sensor mounted internally to the steering column assembly. 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) to determine if the vehicle is operating in a manner capable of supporting the EPAS system.

The PSCM receives vehicle speed from the PCM over the HS-CAN and steering shaft speed, direction and position from the steering shaft sensor to determine the amount or level of power steering assist provided by the EPAS system.

The level of steering assist is primarily based on vehicle speed. As vehicle speed increases, the amount of assist decreases to improve and enhance road feel at the steering wheel. As vehicle speed decreases the amount of assist increases to ease vehicle maneuvering. If the vehicle speed is missing or out of range, the PSCM defaults to a safe level of assist for all driving conditions. If the vehicle speed returns to the correct in-range values, the PSCM adjusts the level of assist accordingly. Steering wheel/shaft speed, torque and direction information allows the PSCM to determine how much assist is needed to turn the wheels right and left.

The PSCM continually monitors and adjusts steering efforts based on the sensor inputs to enhance the feel of the steering. Compensation is made to reduce the effect of pull or drift that may be experienced when driving on roads with a high degree of camber. Active nibble control is also employed by the PSCM to reduce the vibrating movement felt at the steering wheel that can be caused by driving on rough surfaces or out of balance wheels.

The PSCM is self-monitoring and is capable of setting and storing DTCs. Depending on the DTC set, the control module enters into a safe mode of manual steering. In addition, the PSCM may send a request to the Instrument Panel Cluster (IPC) module over the HS-CAN to display a message in the message center, part of the Front Controls Interface Module (FCIM) alerting the driver of a potential EPAS concern.

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 present 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, to protect the electronic components from overheating.

The PSCM is an integral part of the EPAS column assembly and cannot be serviced separately.

Power Steering Control Module (PSCM) Power Down

The internal circuitry of the PSCM enables the PSCM to remain powered for 90 seconds after the ignition state has been switched to OFF. When carrying out diagnostic procedures that require DTCs to be cleared and a new ignition cycle to be started, make sure the ignition state remains OFF for a minimum of 90 seconds before switching to ON. This allows the PSCM to shut down completely and a new ignition cycle to be entered.

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 serviced separately.

Steering Sensor

NOTICE: The steering sensor contains a clockspring that will be damaged if the steering column is not handled correctly. To prevent damage to the steering sensor, use the service bolt (HB1) to lock the steering column in the straight ahead position before disconnecting the steering column from the steering gear. Damage caused to the steering sensor by failing to lock the steering when disconnecting the column from the steering gear leads to a complete failure of the EPAS system and requires a new EPAS column to be installed.

The steering sensor provides the PSCM with information on the direction, speed and amount of force being applied to the steering wheel. The steering sensor is a split sensor that monitors the steering inputs from the driver and the output side of the EPAS assembly. The PSCM uses the inputs from the sensor to determine the amount of steering assist required and adjusts steering efforts based on the sensor inputs. 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.