Power Steering - System Operation and Component Description

amount of assist increases to ease vehicle maneuvering. Compensation is made to reduce the effect of pull or drift experienced when driving on roads with a high degree of camber. Compensation is also made for the impact of wheel imbalance on steering feel, up to a predetermined threshold.

The steering torque sensor senses the torque at the steering wheel. It is integrated into the PSCM and works by measuring the relative rotation between an input and output shaft which are connected by a torsion bar. The steering 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 Diagnostic Trouble Codes (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-CAN2 to the GWM which relays the message to the IPC over the HS-CAN3.

Failure Modes

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

The EPAS enters a reduced steering assist mode to protect the internal components of the EPAS when a non-critical safety concern is detected by the PSCM, concerns such as a loss of the vehicle speed message, low battery voltage, high battery voltage or an over-temperature condition are considered non-critical safety concerns. This reduced steering assist mode gives the steering operation a heavier than normal feel.

The EPAS enters a manual steering mode (no electrical steering assistance is provided) when a concern considered to be a critical safety concern is detected. In manual steering mode, the vehicle has mechanical steering only which gives the steering operation a heavy feel.

Selectable Steering

The selectable steering button on the FCIM allows the driver to select 1 of 3 steering modes. These steering modes alter the feel and response of the EPAS system. After a selection is made, a soft feedback bump may be felt in the steering wheel. Pressing the switch causes the FCIM to send a message to the IPC and the PSCM. The IPC displays the current mode in the message center and the PSCM modifies EPAS operation according to the selected steering mode.

Normal : Default system setting. The system defaults to standard mode if the battery is disconnected. Sport : Slightly higher effort required for steering with more road feel through the steering wheel. Comfort : Slightly less effort required for steering with less road feel through the steering wheel.

If the vehicle is equipped with the selectable drive mode feature, the steering feel changes when a new drive mode is selected. This automatic selection can be overridden by using the selectable steering button. For additional information on the selectable drive mode feature, Refer to: Anti-Lock Brake System (ABS) and Stability Control - System Operation and Component Description (206-09 Anti-Lock Brake System (ABS) and Stability Control, Description and Operation).

Component Description

EPAS Steering Gear

The EPAS gear is an assembly consisting of a PSCM, a motor, and a steering torque sensor, all of which are serviced as an assembly. The inner and outer tie rods and the steering gear bellows boots are available separately for service.

The steering torque sensor is mounted near the input shaft of the EPAS gear and is used by the PSCM to determine how much force is being used to turn the steering wheel. The EPAS gear has one inner tie rod located at each end of the gear assembly and is available separately for service. The EPAS gear has one outer tie rod located at each end of the gear assembly and is available separately for service. The EPAS gear has one bellows boot located at each side of the EPAS gear assembly. Each boot is held in place with 2 boot clamps. The boots and clamps are available for service.

PSCM

The PSCM is the ECU for the EPAS system. The module monitors all sensor inputs and HS-CAN2 messages