Servotronic controls the power-assisted steering dependent on speed by means of the Servotronic solenoid valve (electro-hydraulic converter). To achieve this, the hydraulic oil flow of the power-assisted steering is throttled at different levels by the Servotronic solenoid valve.
Servotronic solenoid valve
The Servotronic solenoid valve is an electro-hydraulic converter, located in the hydraulic oil circuit of the power-assisted steering. The solenoid valve has a variable shutter ( 0 - 0.7 mm) that varies the throttling of the oil flow depending on activation. The shutter diameter is varied by a speed-dependent flow characteristic curve
Control unit
The Servotronic function is integrated in the Chassis Integration Module (CIM). The CIM control unit regulates the Servotronic solenoid valve. The different characteristic curves for the speed-dependent activation of the Servotronic solenoid valve are stored in codable form in a nonvolatile memory.
The Dynamic Stability Control (DSC) sends the speed signal V_DSC to the CIM control unit via K-CAN SYSTEM.
The Electronic Damping Control (EDCK) sends the Status_dampingprogramm signal to the CIM control unit via the K-CAN SYSTEM.
The system regulates the power-assisted steering on a speed-dependent basis. The Servotronic solenoid valve variably throttles the hydraulic oil flow of the power-assisted steering. The degree of throttling depends on the current activating the solenoid valve. The driver can select either of two characteristic curves (comfort or sport characteristics), which variably influence the power-assisted steering depending on the speed. The characteristic curves specify the current for the solenoid valve and/or the specified pressure depending on the speed. The characteristic curves are defined by 16 reference points, whereby the speed and current and/or pressure are coded for each reference point. If the Electronic Damping Control (EDCK) is not fitted, only the comfort characteristic curve is used. The Servotronic only activates the Servotronic solenoid valve when terminal 15 is on and the engine is running. As long as there is no speed signal at engine start-up, the maximum possible steering power assistance is set. When the correct travelling speed is received, the specified value is taken from the electrical characteristic curve.
If any of the following faults occur, the Servotronic solenoid valve is no longer supplied with current. The power-assisted steering is reduced to a minimum:
In the case of the fault SERV1 (short circuit to positive), the full system voltage is applied to the Servotronic solenoid valve. The solenoid valve is fully activated. The power-assisted steering is at its maximum.
CC message ”Servotronic failure”
Supplementary information: ”Servotronic failure! Steering is difficult to move. Have fault checked by your BMW Service as soon as possible.”
Cause: Servotronic without function. The Servotronic solenoid valve is not supplied with current; minimum power-assisted steering.
CC message ”Servotronic fault Drive with moderation.”
Supplementary information: ”Steering always smooth. Have the fault checked by the nearest BMW Service.”
Cause: Servotronic fault. The Servotronic solenoid valve is supplied with current continuously; maximum power-assisted steering.