Speed Dependent Steering Components and Operation

POWER STEERING HYDRAULIC SYSTEM
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.

STEERING ASSIST CONTROL SOLENOID

Steering Assist Control Solenoid Location:

Electronic Variable Orifice (EVO) Actuator Assembly:

The Steering Assist Control Solenoid, mounted on 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 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, reducing power assist in the steering gear.

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 power steering assist for changing vehicle speeds and driving conditions.
ELECTRONIC VARIABLE ORIFICE (EVO) PASSLOCK MODULE

Electronic Variable Orifice (EVO)/Passlock Module:

The EVO/Passlock Module, mounted behind the right side of the steering column to the IP carrier, uses the Vehicle Speed Sensor and Steering Handwheel Speed Sensor input to regulate electrical current to the Steering Assist Control Solenoid.

The EVO/Passlock Module also detects faults with the Steering Handwheel Speed Sensor, the Steering Assist Control Solenoid, and associated circuits. Upon detecting 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 EVO operation until it detects another fault. The EVO/Passlock Module has fault code memory and can pinpoint specific faults.

STEERING HANDWHEEL SPEED (HWSS) SENSOR
This sensor is mounted in the lower bearing of the steering column. It detects abrupt or evasive steering maneuvers, and signals the EVO/Passlock Module to reduce steering effort. The faster the driver turns the steering wheel, the quicker the signal is generated by the HWSS.

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 VSS buffer (Diesel Engines Only) or to the VCM (Gas Engines Only). The VSS buffer or VCM then calculates vehicle speed and notifies the EVO/Passlock Module.