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 when traveling at highway speeds. This gives the steering a tighter feel at higher 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 one into the traction and braking control units. As vehicle speed increases, the control unit restricts fluid flow to the steering gear system. This reduces the chance of a driver oversteering from a perceived treat in the road and gives the vehicle greater road feel and stability.
When these systems fail, there steering will feel sloppy at highway speeds. The customer may complain the vehicle feels jerky or loose at higher speeds. This is because the system’s normal state is in the open position, allowing full system pressure. When the system is not functioning properly, it’s because it is not restricting fluid flow and pressure as vehicle speed increases. This gives the steering that loose feel as speed increases and is more noticeable at highway speeds. This valve may become restricted resulting in steering difficulty at lower speeds. The valve can be located in the pump or gear assembly.