Principles of Operation

Variable Assist Power Steering (VAPS)
The VAPS system controls the level of power assistance available to the driver based on vehicle speed. Below 3.2 km/h (2 mph), full power steering assist is provided to lessen steering efforts and increase maneuverability. Between 3.2 km/h (2 mph) and 191 km/h (119 mph), steering assist will decrease gradually at a calibrated rate to raise steering efforts for increased directional stability and greater road feel. Above 191 km/h (119 mph), steering assist is constant.
The Front Electronic Module (FEM) outputs a Pulse-Width-Modulated (PWM) current to the control valve actuator. The control valve actuator controls the hydraulic valve that determines the amount of hydraulic assist provided to the steering gear. The amount of assistance provided varies with the control valve actuator current, which is based on vehicle speed according to a look-up table internal to the FEM. The FEM pulse-width modulates the control valve actuator current to provide the desired amount of assistance. The amount of hydraulic steering assistance provided by the VAPS subsystem is proportional to the average control valve current.
The vehicle speed is provided to the FEM through the Standard Corporate Protocol (SCP) network from the Anti-Lock Brake System (ABS).
The engine rpm is provided to the FEM through the SCP network from the Powertrain Control Module (PCM). The FEM uses the engine rpm to determine if the engine is running. The FEM will only output control valve actuator current when the engine is running to minimize noise, vibration, or harshness when the engine is off. The FEM will assume the engine is running only if the engine rpm is greater than 100 rpm.
Upon engine start, the FEM will provide a 200 millisecond full-on pulse followed by a 150 millisecond full-off pulse to the control valve actuator. Normal VAPS system operation will begin after this cleaning stroke is complete. In an event that the rpm signal is invalid from the PCM (less than 100 rpm), and the vehicle speed is less than 1.6 km/h (1 mph), the VAPS system will not carry out the cleaning stroke or apply current to the control valve actuator. When the engine speed exceeds 100 rpm, the system carries out the cleaning stroke and begins applying calibrated current to the actuator.