FORD
Ford’s Variable-Assist Power Steering (VAPS) which was first used in 1988 on the Lincoln Continental, and in 1989 on the Ford T-Bird and Mercury Cougar, uses a stepper motor actuator valve to vary steering assist. The valve in the older Lincoln applications is mounted on the steering gear while one on the T-Bird replaces the outlet valve on the power steering pump. The VAPS control module receives two sensor inputs: A vehicle speed signal from a transmission-mounted sensor, and an optical steering wheel rotation sensor in the steering column.
At speeds up to 5 mph, the control module’s output to the actuator valve is zero milliamps (mA) and the actuator valve is wide open allowing full assist as required.
As the vehicle’s speed increases above 5 mph, the module starts to close the actuator valve to reduce power assist in direct proportion to speed. At 20 mph, module output to the actuator valve is 225 mA and the actuator is partly closed to reduce steering assist 20 to 25 percent. At 80 mph, the module’s output reaches its maximum of 550 mA closing the actuator valve to its minimum orifice opening. Assist is now reduced 50 percent from its maximum level.
As the speed drops back below 80 mph, the actuator valve starts to open again to gradually increase power assist in proportion to speed.
The steering wheel rotation sensor also modifies the amount of power assist depending on how quickly the driver is turning the wheels. A sharp sudden turn will cut power to the actuator valve allowing full assist.
On Ford’s later “Electronic Variable Orifice” or EVO system (1991 models & up), a small stepper motor is also used to open a variable orifice valve on the steering rack.
http://www.aa1car.com/library/2004/bf10434.htm
FORDFord’s Variable-Assist Power Steering (VAPS) which was first used in 1988 on the Lincoln Continental, and in 1989 on the Ford T-Bird and Mercury Cougar, uses a stepper motor actuator valve to vary steering assist. The valve in the older Lincoln applications is mounted on the steering gear while one on the T-Bird replaces the outlet valve on the power steering pump. The VAPS control module receives two sensor inputs: A vehicle speed signal from a transmission-mounted sensor, and an optical steering wheel rotation sensor in the steering column.
At speeds up to 5 mph, the control module’s output to the actuator valve is zero milliamps (mA) and the actuator valve is wide open allowing full assist as required.
As the vehicle’s speed increases above 5 mph, the module starts to close the actuator valve to reduce power assist in direct proportion to speed. At 20 mph, module output to the actuator valve is 225 mA and the actuator is partly closed to reduce steering assist 20 to 25 percent. At 80 mph, the module’s output reaches its maximum of 550 mA closing the actuator valve to its minimum orifice opening. Assist is now reduced 50 percent from its maximum level.
As the speed drops back below 80 mph, the actuator valve starts to open again to gradually increase power assist in proportion to speed.
The steering wheel rotation sensor also modifies the amount of power assist depending on how quickly the driver is turning the wheels. A sharp sudden turn will cut power to the actuator valve allowing full assist.
On Ford’s later “Electronic Variable Orifice” or EVO system (1991 models & up), a small stepper motor is also used to open a variable orifice valve on the steering rack.
http://www.aa1car.com/library/2004/bf10434.htm
