Variable Effort Steering Circuit Operation

POWER STEERING HYDRAULIC SYSTEM
The Electronic Variable Orifice (EVO) power steering pump is a conventional power steering pump with a Power Steering (PS) solenoid actuator mounted to the outlet port. When the driving conditions require less hydraulic fluid, a flow control valve restricts the flow of the fluid.





ELECTRONIC VARIABLE ORIFICE (EVO) ACTUATOR ASSEMBLY
The PS solenoid actuator, mounted in the power steering pump, contains a solenoid operated valve. Discharged fluid passes through an orifice in the solenoid. As the vehicle speed increases, the EVO/Passlock module increases the Pulse Width Modulation (PWM) signal to the PS solenoid actuator. Higher current gradually moves the valve further and further into the orifice. This action restricts the passage of the fluid flow through the orifice, reducing the power assist in the steering gear. For diagnosis of the steering system, refer to Power Steering System Test Procedure under Testing and Inspection.
Because the EVO/Passlock module sends a variable signal, the PS solenoid actuator provides various levels of the power assist. The EVO system therefore consistently adjusts the power steering assist for changing vehicle speeds and changing driving conditions. For the repair procedures, refer to PS Solenoid Actuator, Discharge Fitting Replace. For diagnosis, refer to Variable Effort Steering System Check under Testing and Inspection.





ELECTRONIC VARIABLE ORIFICE/PASSLOCK MODULE
The Electronic Variable Orifice (EVO)/Passlock module, mounted to the instrument panel carrier underneath the radio, uses the input from the Vehicle Speed Sensor (VSS) and the input from the steering handwheel speed sensor in order to regulate the electrical current to the PS solenoid actuator.
The EVO/Passlock module also detects faults with the steering wheel rotation sensor, the PS solenoid actuator, and the associated circuits. When the EVO/Passlock module detects a fault, the EVO/Passlock module disables the EVO system. Then, the vehicle operates with the maximum power assist, until the driver turns the ignition to the OFF position. When driver restarts the vehicle, the EVO/Passlock modules resets and enables the operation of the EVO, until the EVO detects another fault. The EVO/Passlock module has fault code member. The EVO/Passlock module can pinpoint specific faults. For the repair procedure, refer to Electronic Variable Orifice (EVO)/Passlock Module Replacement. For diagnosis, refer to Variable Effort Steering System Check under Testing and Inspection.

STEERING HANDWHEEL SPEED SENSOR
This sensor is mounted in the lower bearing of the steering column. The sensor detects abrupt or evasive steering maneuvers, and signals the EVO/Passlock module in order to reduce the steering effort. The faster the driver turns the steering wheel, the quicker the steering wheel rotation sensor generates the signal. For repair procedures, refer to Steering Wheel Speed Sensor Replacement. For diagnosis, refer to Variable Effort Steering System Check under Testing and Inspection.

VEHICLE SPEED SENSOR
The Vehicle Speed Sensor (VSS) consists of a permanent magnet generator on the transmission housing or transfer case. The generator sends an electrical signal to the Powertrain Control Module (PCM) on vehicles with diesel engines, or to the Vehicle Control Module (VCM) on vehicles with gasoline engines. The VCM/PCM then calculates the vehicle speed and notifies the EVO/Passlock module. For repair procedures, refer to Transmission Controls. For diagnosis, refer to Variable Effort Steering System Check under Testing and Inspection.