Variable Assist Power Steering
Variable assist power steering systems are used on a variety of vehicles to provide maximum assist while parking and less assist and a firmer feel at highway speeds. There are different designs and names for these systems. It is typically accomplished by controlling and varying the pressure to the steering gear or rack.
The vehicle's 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 it into the traction and braking control units. As vehicle speed increases, the control unit restricts fluid flow to the steering gear system. This action reduces the chance of a driver oversteering from a perceived threat in the road and gives the vehicle better stability.
When these systems fail, there steering will feel jerky or loose at highway speeds. It's because the system's normal state is in the open position, allowing full system pressure. When the system is not functioning correctly, it is not restricting fluid flow and pressure as vehicle speed increases. The system's valve, located in the pump or gear assembly, can become restricted, resulting in steering difficulty at lower speeds.