Power Steering System Description and Operation TRW
The Electric Hydraulic Power Steering (EHPS) system reduces the amount of effort needed to steer the vehicle. The system uses the power steering control module (PSCM), power steering motor, discrete battery voltage supply circuit to perform the system functions. The PSCM is serviced separately from the steering column assembly. However, the motor rotational sensor is integrated within the power steering motor. The torque sensor and power steering motor are serviced as part of the steering column assembly. The PSCM also monitors vehicle speed from the engine control module. The At low speeds more assist is provided for easy turning during parking maneuvers. At higher speeds less assist is provided for improved road feel and directional stability.
The Electric Hydraulic Power Steering uses hydraulic oil pressure produced by an electric hydraulic pump. A control unit decides the pump speed and hydraulic pressure dependent of vehicle speed. ECU and pump are one unit which can not be divided and therefore must be replaced completely in case of failure of one of the components.
The TRW version without Bus connection does not use a steering angle sensor but calculates the steering angle by checking pressure increase in the steering gear when the steering wheel is being turned. The electric pump will only run when the engine is running, to prevent the battery from discharging.
The TRW version is used on the smaller engine versions and the Z17( lighter weight front section) and is not incorporated into the CAN BUS system. For diagnostic purposes this system has its own diagnostics line (diagnostics plug PIN 7). The TRW EHPS system features a two stage load dependent rotational speed control. the amount of load is determined from the current which the electric motor draws from the vehicle electrical system.
Vehicles equipped with ABS MK60 electronic stability program (ESP) have a steering angle sensor incorporated into the column integration module (CIM).
The physical characteristics of the TRW power steering system that are different than the ZF system are:
| • | Round fluid reservoir. |
| • | Wiring harness connected to electrohydraulic supply unit and cannot be separated. |
| • | Hydraulic lines attached individually to electrohydraulic supply unit by means of union nut or hose clamp. |
Torque Sensor
The PSCM uses a combination of the vehicle speed, and calculated system temperature inputs to determine the amount of assist needed. As the steering wheel is turned and torsional twist is applied to the steering column shaft, the torque sensor's input and output shaft sinusoidal voltage signals are processed by the PSCM to detect and calculate the steering torque. Additionally, the difference between the angle of the steering column output and input shafts detected by the torque sensor and the sinusoidal voltage signals of the motor rotational sensor are both processed by the PSCM to detect and calculate the steering wheel angle.
EPS Motor
The PSCM responds to the change in the sinusoidal voltage signals of the torque and motor rotational sensors by commanding current to the power steering motor. The motor is attached to the base of the steering column housing and assists steering through a worm and reduction gear attached to the steering column shaft. A DC/DC converter is applied to the battery voltage input within the PSCM to boost the pulse width modulated (PWM) motor drive circuit. The motor is a brushless, 3-phase motor with a rated maximum phase current of 35 amps-rms.
Power Steering Control Module (PSCM)
The PSCM uses a combination of commanded motor voltage and current levels to calculate an estimated power steering system temperature. Neither the PSCM nor the power steering motor are designed to handle around 60 amps continuously and will enter into overload protection mode if the system is exposed to exclusive static steering conditions. If the steering wheel is turned to its maximum rotation point and held at this position for an extended period of time, the PSCM will reduce the amount of current commanded to the power steering motor, which reduces the amount of steering assist as well as system temperature. The PSCM has the ability to detect malfunctions within the power steering system. Any malfunction detected will cause the SERVICE POWER STEERING message to be displayed on the driver information center (DIC).
