GF46.25-P-0002LEA Electrohydraulic Power Steering, Function
GF46.25-P-0002LEA Electrohydraulic power steering, function
- as of model year 2010
Function requirements, general
^ Circuit 61 ON
System in general
The steering wheel's rotary motion is converted into horizontal movement through the steering gear of the rack-and-pinion steering.
The steering gear of the rack-and-pinion steering has a variable gear ratio. The gear ratio increases continuously from the center and reaches its maximum value at a steering wheel angle of = 90 degrees. The hydraulic steering assistance is controlled by the rack-and-pinion steering steering-gear itself. The oil pressure required for steering assist is generated by the electrohydraulic power steering's (A91/1) electrically-driven power steering pump. This power steering pump is regulated by the electrohydraulic power steering depending on requirements (vehicle at standstill, vehicle speed) to match a specified characteristic.
To cool the power steering fluid an oil cooler in the form of a coiled pipe is installed immediately upstream of the A/C system's condenser.
To recognize the vehicle status at any given time, the electrohydraulic power steering reads in the following data:
- The current vehicle speed from the instrument cluster (A1)
- The on-board electrical system status from the front SAM control unit with fuse and relay module (N10/1)
- The circuit status from the EIS [EZS] control unit (N73)
- The steering angle and steering angle rate of change from the steering column tube module (N80)
The electrohydraulic power steering communicates with the central gateway control unit (N93) for system diagnosis.
The system is made up of the following component functions:
^ Function sequence when driving straight ahead
^ Function sequence on steering lock
Function sequence when driving straight ahead
When driving straight ahead (steering wheel in center position) the rotary valve of the rotary slide shift valve is in the neutral position.
The control grooves of the rotary valve and the pilot bushing are interdependent to the extent that the oil flow, as supplied by the power steering pump, is routed straight back over the return flow to the reservoir. The working cylinders are therefore unpressurized.
The stabilizer bar is pinned on one side to the rotary valve and on the other side to the drive pinion.
Function sequence on steering lock
If the steering wheel is turned, then the drive pinion is supported on the toothed rack. The rotary valve is twisted against the pilot bushing surrounding it via the stabilizer bar and this changes the position of the control grooves in relation to one another. The pressure oil flow now reaches the right working cylinder (left wheel lock) or the left working cylinder (right wheel lock) and causes the axial movement of the gear rack to the right or left.
The oil from the unpressurized working cylinder is displaced. It flows back into the power steering pump reservoir via the open non-return grooves of the pilot bushing.
Steering force support is regulated according to the vehicle speed. Regulation is conducted through the electrohydraulic power steering, which in turn controls the variable-ratio steering assist.
A differentiation is made here between the following conditions:
^ Function with vehicle speed v = 0 km/h
^ Function with vehicle speed 0 km/h < v ≤ 100 km/h
^ Function with vehicle speed v > 100 km/h
Function with vehicle speed v = 0 km/h
Steering must be light because the steering torque that must be applied by the driver is now at its highest. The electrohydraulic power steering regulates the power steering pump to achieve maximum oil delivery rate. Due to the strong torsion of the torsion bar, the maximum oil quantity is pumped into the respective working cylinders until a balance of forces is attained.
Function with vehicle speed 0 km/h < v ≤ 100 km/h
When the vehicle is in motion, the rolling of the wheels reduces the steering force that must be applied. This means that the higher the rolling speed of the wheels, the less steering force will be required. The electrohydraulic power steering therefore selects a performance map for actuating the power steering pump to match vehicle speed.
Function with vehicle speed v > 100 km/h
At speeds over v = 100 km/h the rolling speed of the wheels is so high that no additional steering assistance is required. The electrohydraulic power steering reduces the output of the power steering pump to a minimum.
