EPAS System

WARNING: This page is about a different car, the 2017 Lincoln Navigator and 2017 Ford Expedition. However, it is still accessible from the selected car via links, so may be relevant.

NOTE: The EPAS steering gear may experience a symptom where the steering wheel moves slightly as the PSCM attempts to find the center position when it temporarily loses center position of the steering wheel. This is most likely caused by rotating the steering wheel excessively when the ignition is OFF. This symptom corrects itself after the vehicle is driven straight for at least 152 m (500 ft), with little to no steering wheel torque input. Once the PSCM learns the center position, the EPAS system returns to normal operation.

The PSCM controls the functions of the EPAS system and communicates with other modules over the HS-CAN.

To activate, the EPAS system requires battery voltage input and ignition voltage input at the PSCM. In addition, the system must communicate with other modules over the HS-CAN. The PSCM must also receive the power system data message from the BCM in order to obtain full operation mode.

The main input for calculating the level of EPAS assist is the torque sensor signal from the steering torque sensor located inside the EPAS gear. Vehicle speed is also taken into consideration in order to achieve the vehicle speed dependent steering assist characteristic.

The EPAS gear uses a reversible motor to apply the steering assist. The motor is connected to the rack of the steering gear by a pulley and bearing assembly and a toothed belt. The PSCM uses the motor to move the rack inside the steering gear housing.

The PSCM continually monitors and adjusts steering efforts based on the torque sensor, motor position and HS-CAN inputs to enhance the feel of the steering system. 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. Compensation is made to reduce the effect of pull or drift that can be experienced when driving on roads with a high degree of camber. The PSCM also compensates for the impact of wheel imbalance on steering feel, but only up to a predetermined threshold.

The steering torque sensor senses the torque at the steering wheel. It is hard wired to the PSCM and works by measuring the relative rotation between an input and output shaft which are connected by a torsion bar. The torque sensor sends out 2 PWM signals which allows a channel to channel cross-check and an accurate correction of the neutral point.

The PSCM is self-monitoring and is capable of setting and storing DTCs. Depending on the DTC set, the PSCM may enter a failure mode. In addition, the PSCM may send a request to the IPC to display a message in the message center, alerting the driver of a potential EPAS concern. The warning message is sent over the HS-CAN to the BCM where it is converted to a MS-CAN message and then sent on to the IPC over the MS-CAN.

EPAS Failure Modes

When a DTC is present in the PSCM, the EPAS enters 1 of 2 modes of operation.

Reduced Steering Assist Failure Mode

The EPAS system enters a reduced steering assist mode to protect the internal components of the EPAS gear when a concern is detected by the PSCM, such as, low or high battery voltage or over-temperature concerns which are not considered to be a critical safety concern. The steering assistance sets to a default level and does not change with vehicle speed. While the EPAS system operates in this mode, the steering assistance available for slow speed maneuvering is reduced, giving the steering a heavy feel. The assistance at high speed is greater than normal and can give the steering a lighter than normal feel.

Manual Steering Failure Mode

The EPAS system enters a manual steering mode (no steering assistance is provided) when a concern which is considered to be a critical safety concern is detected or when a concern which caused the EPAS system to enter reduced steering assist mode has been present for a predetermined number of key cycles. In manual steering mode, the vehicle has mechanical steering operation only, which gives steering operation a heavy feel.