Principles of Operation

Principles of Operation

Power Steering
The power steering system uses a vane-type pump to move the fluid from the reservoir to the rack-and-pinion steering gear and through the rest of the steering hydraulic system. The power steering pump is mounted to the engine and driven by the engine accessory drive belt. Power steering fluid is pulled into the pump from the reservoir. The power steering fluid is trapped between the pump vanes and moved to the high-pressure side of the pump creating a flow of fluid. The restriction of this flow by the steering gear creates the pressure that provides the steering assist. A combined pressure relief/flow valve is built into the pump to control the maximum pressure and flow provided to the steering system. This action prevents damage to the system and provides the correct level of assist during all engine speeds. While under pressure, the power steering fluid flows through the high-pressure power steering line to the steering gear. The fluid exits the gear and flows through the return line, cooler and finally to the reservoir. The reservoir slows the fluid, allows air to escape and filters the fluid before returning it to the pump.

Fuel Efficient Power Steering (FEPS)
The fuel efficient power steering (FEPS) system reduces vehicle fuel consumption by reducing the amount of power steering assist provided at higher engine speeds, when less assist is needed. The FEPS system is controlled by the steering effort control module (SECM), FEPS solenoid/actuator and the steering wheel rotation sensor. The steering wheel rotation sensor is mounted to the steering column. As the steering wheel is turned, the sensor generates a turning rate signal. On vehicles equipped with interactive vehicle dynamics (IVD) the rotation sensor is hard-wired to the ABS module. The ABS module sends the turning rate signal to the SECM over the high-speed controller area network (HS-CAN) bus. On vehicles not equipped with IVD, the rotation sensor is hard-wired to the SECM so the module receives the turning rate signal directly from the sensor. The SECM also receives vehicle speed information from the PCM over the HS-CAN bus. The SECM uses both of these signals to determine how much assist is needed. At slower speeds, full power steering assist is provided. As vehicle speed increases, the SECM sends a signal to the FEPS solenoid/actuator, which is mounted to the power steering pump. The FEPS solenoid/actuator is then activated to reduce the flow of power steering fluid through the power steering system. Power steering pump effort is reduced, which, in turn, reduces the pumps demands on the engine. With less load on the engine, the vehicle has greater fuel efficiency.

Steering Effort Control Module (SECM)
The SECM monitors vehicle speed and steering wheel rate to determine the correct amount of fluid flow provided to the system. When the driver is turning the steering wheel slowly, the module sends current to the solenoid/actuator on the power steering pump to reduce fluid flow from the pump. This reduced fluid flow decreases the back pressure in the power steering system, which reduces the power needed to drive the pump. When the driver turns the steering wheel quickly, the module reduces the current to the solenoid/actuator, which increases the pump flow to provide acceptable steering efforts. If the SECM detects a problem, the default mode is full flow (0 current).

The vehicle speed is provided to the SECM by the ABS module over the HS-CAN bus.

Steering Wheel Rotation Sensor
The steering wheel rotation sensor measures the rate of rotation of the steering wheel by monitoring the steering wheel rotation ring as it passes through the sensor gap. On vehicles not equipped with interactive vehicle dynamics (IVD), the steering wheel rotation sensor is wired directly to the SECM and sends the information about how far the steering wheel is being turned directly to the module. On vehicles equipped with IVD, the steering wheel rotation sensor is hard wired to the ABS module instead of the SECM and the ABS module sends the information to the SECM over the HS-CAN bus.

The rotation sensor receives power from the battery junction box (BJB) and is grounded through the SECM (on vehicles without IVD) or through the ABS module (on vehicles with IVD).