Rotary Control Valve

WARNING: This page is about a different car, the 2004 Subaru Impreza. However, it is still accessible from the selected car via links, so may be relevant.

The rotary control valve consists of a rotor (which rotates together with the steering shaft), a pinion (which is connected to the rotor and torsion bar), and a sleeve (which rotates together with the pinion). The rotor and sleeve have grooves C and D, respectively, which form fluid passages V₁ through V₄

The pinion is in mesh with the rotor with adequate clearance, which enable the rack to be moved manually by rotating the steering shaft (fail-safe feature).

Fig 1: Identifying Components Of Gearbox Assembly - Rotary Control Valve

Courtesy of SUBARU OF AMERICA, INC.

Principle of operation
When the torsion bar is twisted by a rotational force applied to the steering wheel, the relative position between the rotor and sleeve changes. This changes the cross-sectional area of fluid passages V₁ , V₂ , V₃ and V₄ . The fluid passages are thus switched and the fluid pressure is controlled in accordance with the operation of the steering wheel.
- When no steering force is applied:
  The rotor and sleeve are held at the neutral position. Fluid passages V₁ , V₂ , V₃ and V₄ , which are formed by grooves C and D are open equally. Under this condition, the fluid from the pump returns to the reservoir tank so that neither fluid pressure builds up nor the rack piston moves in the power cylinder.
  
  Fig 2: Principle Of Operation - When No Steering Force Is Applied
  Courtesy of SUBARU OF AMERICA, INC.
When steering force is applied:
When the steering wheel is turned to the right, for example, fluid passages V₁ and V₃ are opened while fluid passages V₂ and V₄ are nearly closed.

At this point, the fluid pressure in chamber A of the power cylinder increases depending on the degree of closure of fluid passages V₂ and V₄ so that the rack piston moves to the right. The fluid in chamber B, on the other hand, is drained through fluid passage V₃ into the reservoir tank.

Fig 3: Principle Of Operation - When Steering Force Is Applied
Courtesy of SUBARU OF AMERICA, INC.
Fail-safe function
If fluid pressure fails to build up due to, for example, a broken fluid pump drive belt, the steering wheel rotating torque is transmitted from the valve rotor to the pinion through mechanical engagement between them.