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      06-12-2015, 06:28 AM   #20
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Drives: 2008 BMW 135i MT
Join Date: Apr 2015
Location: Melbourne, Australia

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Did some digging in WDS.

The Servotronic controls the power-assisted steering of the power steering depending on the speed by means of the Servotronic valve (electro-hydraulic converter). To do so, the flow of hydraulic oil of the power steering is throttled to varying degrees by the Servotronic solenoid valve.

Brief description of components
Servotronic valve

The Servotronic valve is an electro-hydraulic converter located in the hydraulic oil circuit of the power steering. The valve has a variable shutter that throttles the oil flow to varying degrees depending on activation. The shutter diameter is varied by a speed-dependent flow characteristic curve

Control unit

The Servotronic function is integrated in the junction box electronics (JBE). The Junction Box electronics regulate the Servotronic valve. The characteristic curve for the speed-dependent activation of the Servotronic valve is stored as codable in a non-volatile memory.

The following messages or signals, among others, are required for the Servotronic function:

Road speed from the DSC control unit via the PT-CAN.

Status of engine from the DME control unit via the PT-CAN.

Terminal status of the CAS control unit via the K-CAN.

The system regulates the power-assisted steering on a speed-dependent basis. The Servotronic valve variably throttles the flow of hydraulic oil in the power steering. The degree of throttling depends on the current used to activate the valve.
The characteristic curve for the Servotronic valve specifies the current depending on the speed.
The characteristic curve is described by reference points, whereby the speed and current are coded for each reference point.

The Servotronic only activates the Servotronic valve if 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.
Now looking at the various nodes, it looks like the S63 engine has servotronic outputs on its DME. All the other models that have servotronic appear to instead connect to the junction box module.

Looking at NCS dummy, only the newest JBBF modules have servotronic options/map curves. Would need to have one of those modules to be compatible. Seems like the people who have retrofitted the rack ended up didn't end up bothering with servotronic since it wasn't a big deal:
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