For 1990-2009 cars

The power rack and pinion steering gear has a rotary control valve. This valve directs the hydraulic fluid coming from the hydraulic pump to one side or the other side of the rack piston. The integral rack piston is attached to the rack. The rack piston converts the hydraulic pressure to a linear force that moves the rack to the left or to the right. The force is transmitted through the inner and the outer tie rods to the steering knuckles.

If hydraulic assist is not available then manual control is still possible. Manual control requires more steering effort. Without hydraulic assist, the movement of the steering wheel is transferred to the pinion. The pinion's movement is then transferred through the pinion teeth. The pinion teeth mesh with the teeth on the rack. This causes the rack to move.