The electronic variable orifice (EVO) power steering pump (1) is a conventional power steering pump with a steering assist control solenoid (2) mounted to the outlet port. A flow control valve directs surplus pressurized fluid to the pump reservoir when the driving conditions require less hydraulic fluid.

The EVO vehicles have a special pressure hose that includes an in-line check valve. The valve reduces the "kick" that can occur at high vehicle speeds if the vehicle comes across irregular road surfaces. Railroad tracks or pot holes are examples of irregular road surfaces. The check valve allows fluid flow from the power steering pump to the steering gear. The check valve prevents fluid flow from the gear to the pump.

The steering assist control solenoid (2,3) contains a solenoid operated valve. Discharged fluid passes through an orifice in the actuator. The EVO/Passlock™ module increases the electrical current to the fluid flow actuator as the vehicle speed increases. The higher current gradually moves the valve farther and farther into the orifice. This action restricts the passage of the fluid flow through the orifice. The restricted passage reduces the power assist in the steering gear. The steering pump flow ranges from 3.5 gallons per minute at idle to 0.7 gallons per minute when driving down the road. For the diagnosis of the power steering system, refer to Diagnosis of Power Steering System in Power Steering System.

The current through the fluid flow actuator is approximately 100-550 milliamps when the vehicle is stationary. At highway speeds the EVO/Passlock™ module receives a higher frequency signal from the vehicle speed sensor. The EVO/Passlock™ module increases the fluid flow actuator current to 600-850 milliamps at highway speeds.

The steering assist control solenoid (2, 3) provides variable levels of power assist because the EVO/Passlock™ module sends a variable signal. Therefore, the EVO system consistently adjusts the power steering assist for the changing vehicle speeds and the driving conditions. For repair procedures, refer to Power Steering Solenoid Actuator and Discharge Fitting Replacement . For diagnosis of the power steering system, refer to Diagnosis of Power Steering System in Power Steering System.

The EVO/Passlock™ module is mounted behind the instrument panel and to the right of the radio. The EVO/Passlock™ module uses the following components in order to supply the electrical current to the steering assist control solenoid:

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

When the EVO/Passlock™ module detects a fault, the module disables the EVO system. The vehicle will operate with maximum power assist until the driver turns the ignition to the OFF position. When the vehicle is restarted, the EVO/Passlock™ module will reset and enable the EVO operation until the EVO/Passlock™ module detects another fault. For repair procedures, refer to Elec Variable Orifice Passlock Module Replacement . For electrical diagnosis, refer to Diagnostic System Check.

The sensor (8) is mounted in the lower bearing of the steering column. The sensor (8) detects abrupt or evasive steering maneuvers. The sensor (8) signals the EVO/Passlock™ module in order to reduce the steering effort. The faster the driver turns the steering wheel, the quicker the signal generates from the steering handwheel speed sensor (8) . If the steering handwheel speed sensor (8) needs to be replaced, remove the steering column from the vehicle. For repair procedures, refer to Steering Wheel Position Sensor or Steering Shaft Lower Bearing Replacement . For electrical diagnosis, refer to Diagnostic System Check.

The VSS consists of a permanent magnet generator on the transmission housing or the transfer case. The generator sends an electrical to the vehicle control module (VCM). The VCM calculates the vehicle speed. The VCM notifies the EVO/Passlock™ module of the vehicle speed. For repair procedures, refer to Vehicle Speed Sensor Replacement in Automatic Transmission. For electrical diagnosis, refer to Diagnostic System Check.