Electro-hydraulic power steering reduces steering effort. The hydraulic system functions in the same way as the power steering system used on other models, however it includes an electrically driven hydraulic pump independent of the vehicle engine. The control unit and the hydraulic system pump form a single module and can only be replaced as an assembly.
The force from the EPS is controlled by the control unit as a function of the engine speed and in accordance with the three-dimensional control characteristic of the engine.
The car uses two different EPS systems.
TRW's EPS is used on small petrol and Z 17 DTL versions and is not included in the CAN bus system. For diagnostic purposes, this system has its own diagnostic line (diagnostic output 7). TRW's EPS provides two-stage, load-dependent pump speed control (2 speed). The load value is determined by the current that the electric motor consumes from the vehicle's electrical system.
EPS from ZF is used on high power petrol engines and Z 17 DTH and is connected to the HS CAN bus. The ZF EPS is speed and load dependent and uses two different control maps. Appropriate driving conditions are determined by vehicle speed as a function of steering wheel offset, and steering servo force decreases as vehicle speed increases. On vehicles with a sports chassis, the second adjustment map can be activated with the sports mode button.
The EPS control unit controls the system according to the map depending on the angular speed of rotation of the hydraulic system pump. The vehicle speed is transmitted to the EPS control unit via the CAN bus.
Angle sensor
TRW's EPS, which is not connected to the CAN bus, does not have a steering angle sensor. Instead, it uses information about the increase in steering pressure that occurs when the steering wheel is turned. Vehicles with ESP MK60 anti-lock brakes have a steering angle sensor in the CIM. On vehicles with EPS ZF, information from the CIM steering angle sensor is transmitted via the HS CAN bus to the EPS control unit. Due to the high power requirement, the hydraulic pump is not activated until the engine is running. If the alternator fails while driving, the hydraulic pump is turned off to prevent the battery from being discharged.
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