Power Steering System Description and Operation (W/ Electro-Hydraulic Steering)

Power Steering System Description and Operation (w/ Electro-Hydraulic Steering)
The Electro-Hydraulic Power Steering (EHPS) module controls the power steering motor which has the function of providing hydraulic power to the brake booster and the steering gear. A secondary function includes the ability to improve fuel economy by operating on a demand basis and the ability to provide speed-dependent variable-effort steering.

The EHPS module controls the EHPS powerpack, an integrated assembly consisting of the following components:
^ The electric motor
^ The hydraulic pump
^ The fluid reservoir
^ The reservoir cap
^ The reservoir baffles, if required
^ The fluid level sensor, if required
^ The electronic controller
^ The electrical connectors

EHPS Module
The Elector-Hydraulic Power Steering (EHPS) module shall interface with the vehicle electrical subsystem to provide connections for the +36-volt (nominal) power supply, for signal inputs necessary to control the EHPS module output and for serial communications. If the 125 A EHPS fuse or the +36-volt circuit supplying the EHPS module is open, the EHPS system will not operate and only communication codes will be set by modules which communicate with the EHPS module. The Powertrain Control Module (PCM) shall act as the gateway to translate Controller Area Network (CAN) messages into class 2 messages when required for diagnostic purposes.

The EHPS module shall receive the following messages from the CAN bus:
^ Vehicle speed (direct signal in km/h)
^ Service disconnect status
^ PRNDL (shift lever) position
^ Torque Converter Clutch (TCC)/Cruise Dump signal (gives zero-adjust brake switch position)

The EHPS module shall output the following messages to the CAN bus:
^ Brake pedal rate, position, in-range rationality, and out-of-range diagnosis
^ EHPS system status
^ Diagnostic messages to Driver Information Center (DIC) via Hybrid Control Module (HCM)
^ Diagnostic information requested by service technicians via Tech 2 link (class 2 via PCM)
^ Steering wheel sensor diagnostic message (in-range, out-or-range failure)

The EHPS module receives several hardwire signals. Digital steering wheel speed signals from the steering wheel sensor mounted on the steering column and an analog brake pedal position signal from the brake-pedal mounted Brake Pedal Position (BPP) sensor.

The steering wheel speed sensor outputs 3 digital signals characterizing the steering wheel position. The digital output consist of 3 open collector output data lines referred to as phase A, phase B, and Index. The normal operating range of the digital signals is plus or minus 720 degrees of steering wheel rotation. Phase A and phase B are 90 degrees out of phase to provide quadrature pulsed data corresponding to steering shaft rotational displacement and direction. The signal is resolved to within 1 mechanical degree of resolution. The index output references a steering wheel mechanical position of 0° plus or minus 10 degrees (steering wheel centered) and is repeated every 360 degrees of steering wheel rotation.

The BPP sensor outputs an analog signal, referenced to 5 volts, which may increase or decrease monotonically with brake pedal depression. The sensor analog output shall have a specified electrical output over a mechanical range of 32 degrees (rotation of the brake pedal pivot). Out-of-range values will be provided outside of the 32 degrees range. The electrical range of the BPP sensor motion is -55 degrees to +25 degrees. The mechanical range of the BPP sensor is -70 degrees to +40 degrees.

The EHPS module also receives ignition key position signals. These signals are the ignition signal and the accessory signal. The EHPS module receives an ignition input (ignition 1) on the 24-way signal connector. Ignition signal logic is defined as follows:
^ When the ignition switch is in the RUN or CRANK positions, then Ignition 1 = high, greater than 6 volts for more the 50 milliseconds.
^ When the ignition switch is in the LOCK/OFF or ACCY positions, then Ignition 1 = low, less than 3 volts for more than 0.5 milliseconds.
The EHPS module receives an input from the ignition 0 circuit indicating when the key is in the ACCY position. This input has 2 functions, to provide an independent wake-up signal in the event of loss of the ignition input, and to activate the EHPS module when the key remains in the ACCY position. The ACCY signal logic is defined as follow:
^ When the ignition switch is in the ACC or RUN positions, then Ignition 0 = high, greater than 6 volts for more than 50 milliseconds.
^ When the ignition switch is in the LOCK/OFF or CRANK positions, the Ignition 0 = low, less than 3 volts for more than 5 milliseconds.

The EHPS module will function normally when the voltage at the EHPS connector is in the range of 34 - 50 volts. If the supply voltage at the EHPS connector drops below 18 volts for more than 10 ms, the EHPS module shall go into the Standby mode (power stage turned OFF). Once the supply voltage rises above 22 volts again, the power stage shall be turned back ON without a key cycle being required. If the supply voltage drops below 12 volts, the EHPS module drivers shall shut down and a key cycle will be required to reset. The EHPS module shall withstand voltage up to a transient voltage of 58 volts for 400 milliseconds. If the supply voltage exceeds 55 volts, the EHPS module drivers shall shut down and a key cycle will be required to reset.

EHPS system performance may be reduced with power steering fluid temperature change. The temperature range at which full performance is achieved is approximately -20 to +105°C (-4 to +220°F). The EHPS system performance will be affected as follows:
^ At approximately less than -40°C (-40°F), the system will be disabled.
^ At approximately -40 to -29°C (-40 to -20°F), the hydraulic output power reduction less than 50 percent.
^ At approximately -29 to -20°C (-20 to -4°F), the hydraulic output power reduction less than 20 percent.
^ At approximately -20 to +105°C (-4 to +221°F), the hydraulic output power is at full performance.
^ At approximately 105 - 135°C (220 - 275°F), the hydraulic output power reduction less than 20 percent.
^ At approximately greater than 135°C (275°F), the system will be disabled.
EHPS system performance may also be reduced with voltage change. The voltage range at which full performance is achieved is approximately 34-50 volts. The EHPS system performance will be affected as follows:
^ At approximately less than 18 volts, the EHPS system will be disabled.
^ At approximately 18-33 volts, the EHPS system performance is reduced.
^ At approximately 34-50 volts, the EHPS module will be at full performance.
^ At approximately greater than 55 volts, the EHPS module will be disabled.
Whenever a replacement EHPS powerpack is installed, reprogramming is necessary. If the EHPS powerpack is not programmed, DTC C0564 will be set.