Steering: Description and Operation

Electronically Controlled Power Steering System (ECPS) System Description

System Outline

This vehicle is equipped with an Electronically Controlled Power Steering System (ECPS). Depending on the vehicle speed, the ECPS control unit controls the ECPS valve. The ECPS valve changes the P/S fluid pressure of the pressure side of the power cylinder. By controlling the P/S fluid pressure electrically, it made it possible to supply stable P/S fluid pressure to the power cylinder which ensures that the steering feel is not influenced by the fluctuation of the engine's speed, and corresponds to the driving situation.






Flow control valve

This valve stabilizes the P/S fluid pressure of the power steering pump output.

ECPS valve assembly

The spool valve built in the ECPS valve assembly has a built in spool valve that is operated by the valve motor which is controlled by the ECPS control unit. The spool valve changes the P/S fluid pressure to the power cylinder. When this valve closes, the pressure to the power cylinder increases.

Valve body unit

The valve body unit has a built in rotary valve that is driven by the rotation of the steering wheel pinion shaft. This changes the fluid passage between the power steering pump side, power cylinder side, reservoir side, and the ECPS valve side.

Kickback valve

When kickback occurs from the steering rack, the kickback valve in the feed port decreases the kickback to the steering wheel.

VFV (Variable force control valve)

The valve lessens rapid P/S oil pressure change.

ECPS control unit

The ECPS control unit drives the valve motor built inside the ECPS valve assembly depending on the engine speed signal and the vehicle speed signal from the PCM. When a failure occurs, this unit stores a DTC and sends a signal to the gauge control module to turn on the ECPS indicator.






Operation when the vehicle is stopped (high assist)

When the vehicle is moving slowly (such as in a parking lot) or when stopped, the ECPS control unit turns off the valve motor causing the ECPS spool valve to close the port (A) to the reservoir. This creates high pressure at the rotary valve. When the steering wheel turns the pinion shaft, the rotary valve opens the variable ports (B and C) between the pump and the increased pressure side of the steering rack. At the same time, the rotary valve opens passages on the decreased pressure side of the steering rack allowing the P/S fluid to return to the reservoir. By this operation, the power steering system can create strong or full assist in the direction the steering wheel is turned.






Operation when the vehicle is moving (duty assist)

When the vehicle is moving, the ECPS control unit drives the valve motor speed according to the vehicle speed. The valve motor controls the ECPS spool valve which opens a variable port (A) to the reservoir varying the amount of pressure applied to the rotary valve. When the steering wheel turns the pinion shaft, the rotary valve opens the variable ports (B and C) to the increased pressure side of the steering rack. The amount of steering assist depends on the amount the variable ports are opened or closed at the ECPS spool valve and the rotary valve. Because of these variable clearances, the power steering assist is lower while driving than when the vehicle is stopped.