The electronic variable orifice (EVO) power steering pump is a conventional power steering pump with a steering assist control solenoid mounted to the outlet port. When driving conditions require less hydraulic fluid, a flow control valve restricts the flow of fluid.

The steering assist control solenoid is mounted on the power steering pump. The steering assist control solenoid 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 PWM signal to the steering assist control solenoid. Higher current gradually moves the valve further and further into the orifice. This action restricts the passage of fluid flow through the orifice and reduces power assist in the steering gear. For diagnosis of the steering system, refer to Variable Effort Steering Diagnosis in Power Steering System. Because the EVO/Passlock module sends a variable signal, the steering assist control solenoid provides variable levels of power assist. The EVO system therefore consistently adjusts the power steering assist for changing vehicle speeds and driving conditions. For repair procedures, refer to Power Steering Solenoid Actuator and Discharge Fitting Replacement . For diagnosis, refer to Variable Effort Steering System Check .

The EVO/Passlock module is mounted behind the right side of the steering column to the IP carrier. The EVO/Passlock module uses the vehicle speed sensor and the steering handwheel speed sensor input in order to regulate electrical current to the steering assist control solenoid.

The EVO/Passlock module detects faults with the following items:

Upon detection of a fault, the EVO/Passlock module disables the EVO system and the vehicle operates with maximum power assist until the driver turns the ignition to the OFF position. When the vehicle is restarted, the EVO/Passlock module resets and enables the EVO operation until the EVO/Passlock module detects another fault. The EVO/Passlock module has fault code memory and can pinpoint specific faults. For repair procedures, refer to Theft Deterrent Module Replacement . For diagnosis, refer to Variable Effort Steering System Check .

The steering handwheel speed sensor is mounted in the lower bearing of the steering column. The steering handwheel speed sensor detects abrupt or evasive steering maneuvers and signals the EVO/Passlock module reduce steering effort. The faster the driver turns the steering wheel, the quicker the signal is generated to the HWSS. For repair procedures, refer to Steering Wheel Position Sensor or Steering Shaft Lower Bearing Replacement . For diagnosis, refer to Variable Effort Steering System Check .

The vehicle speed sensor (VSS) consists of a permanent magnet generator on the transmission housing or transfer case. For diesel engines only, the generator sends an electrical signal to the VSS buffer. For gas engines only, the generator sends an electrical signal to the VCM. The VSS buffer or the VCM then calculates the vehicle speed and notifies the EVO/Passlock module. For repair procedures, refer to Vehicle Speed Sensor Replacement in Automatic Transmission - 4L60E or Vehicle Speed Sensor Replacement in Automatic Transmission - 4L80E. For diagnosis, refer to Variable Effort Steering System Check .