Electric Power Assist Steering
Many late model vehicles have EPAS Electronic Power Assist Steering systems. They use an electric motor instead of a pump and hydraulic pressure to provide power steering assist. The EPAS control module uses inputs from steering and speed sensors to adjust the direction and amount of assist.
The motor may be located on the column or the rack and pinion gear. An electromechanical unit is completely electric and no hydraulic fluid is needed. An electrohydraulic unit has a motor that drives a pump. This is because the belt driven pump associated with typical hydraulic power steering system is omitted. The alternator provides the voltage required for operation instead of the crankshaft. This is economical offering greater fuel mileage.
Motor operation is controlled by a control module that may be dedicated or combined into a single unit with the suspension module. It receives input from the steering wheel sensor indicating the speed and effort required to turn the wheel. This and vehicle speed sensor input are used to control the motor.
Voltage is increased during parking maneuvers and slow speed operation and decreased, reducing assist and providing a tighter feel approaching highway speeds. While test driving a vehicle with EPAS, the steering should feel progressively tighter as vehicle speed increases.