Variable Assist Power Steering
Variable assist power steering systems are used on a variety of vehicles to provide maximum assist to the driver while parking, while providing a lesser degree of assist while traveling at highway speeds. This gives the steering a tighter feel at higher speeds. There are different designs and names for these systems but they all perform the same function. It is typically accomplished by controlling and varying the pressure to the steering gear/rack system.
The vehicles wheel speed sensor and a steering wheel rotation sensors send the necessary signals to the control unit. Some vehicles have a dedicated unit while others incorporate one into the traction and braking control units. As vehicle speed increases the control unit will restrict fluid flow to the steering gear system. This reduces the chance of a driver over-steering from a perceived treat in the road at highway speeds and gives the vehicle greater road feel and stability. 
When these systems fail there will be higher sensitivity at highway speeds. The customer may complain the vehicle feels jerky or loose at higher speeds. This is because the systems normal state is in the open position allowing full system pressure. When the system is not functioning properly it is because it is not restricting the fluid as vehicle speed increases. This gives the steering that loose feel as speed increases and is far more noticeable at highway speeds. It is rare but possible that this valve may become restricted causing steering difficulty at lower speeds. The valve can be in the pump or gear assembly.
These systems work well and have been around for a while. They have diagnostic codes and older ones can be tested with a test light or meter. The newer system can be scanned. Check all of the connections and look for problems with the wheel speed or rotation sensor, control unit, or a possible problem with the control valve assembly.
