Servotronic Operation When the vehicle is manoeuvred into and out of a parking space (or other similar manoeuvre, the Servotronic function of the BCU uses road speed data from the ABS ECU to determine the vehicle speed, which in this case will be slow or stationary. The BCU microprocessor analyses the signals and outputs an appropriate control current to the Servotronic transducer valve. The Servotronic valve closes and prevents fluid flowing from the feed fluid radial groove to the reaction chamber. An orifice also ensures that there is return pressure in the reaction chamber. This condition eliminates any 'reaction' ensuring that the steering is very light to operate, reducing the effort required to turn the steering wheel. As the vehicle is driven and the road speed increases, the BCU microprocessor analyses the road speed signals from the ABS ECU and reduces the amount of control current supplied to the Servotronic valve. The Servotronic valve reacts to the control current and opens the valve by an amount appropriate to the road speed. This allows a controlled fluid supply from the feed fluid radial groove to the reaction chamber. The orifice prevents the loss of large amounts of fluid to the return fluid chamber. The higher fluid pressure acting on the reaction piston causes compression of the balls which are located between the reaction piston and the centering piece which in turn is securely connected to the valve sleeve. When driving in a straight line, this has a positive effect on the exact centralising of the steering valve. When the steering valve is actuated, the balls, with a higher load on them, provide additional torsional resistance to rotation of the valve rotor. This mode of Servotronic assistance requires an established higher steering wheel torque until the hydraulic assistance required in the left or right power assist cylinder is determined. At high driving speeds, i.e. on a motorway, the Servotronic valve is fully open due to a very low or nonexistent control current supplied from the BCU. This enables maximum pressure supply from the feed fluid radial groove to the reaction piston. When the steering wheel is turned, the reaction pressure increases appropriate to the existing operating pressure and pressurises the piston in the reaction chamber. As soon as the reaction pressure reaches its upper limit, the fluid is discharged to the return fluid chamber via the cutoff valve and prevents further increases in reaction pressure. This maintains the input torque applied through the steering wheel and provides the driver with an improved 'road feel' allowing precise steering and directional stability.