Servotronic (E38 only)
Purpose of the System
The servotronic steering system is based on a conventional power assisted steering sys-
tem. An additional electro-hydraulic solenoid and oil passageways are added to the con-
ventional worn and roller type steering box. The GM monitors the vehicle’s road speed and
regulates an electro-hydraulic solenoid. The servotronic steering system provides the fol-
lowing:
• A light steering effort at low speeds and while parking.
• Increased steering effort as road speed increases.
The "Direct Hydraulic Reaction" provided by the system allows the frictional forces between
the front wheels and road surface to be transmitted to the driver. This allows the driver to
make a better judgment in regard to the driving conditions.
This principle of direct hydraulic reaction can be used with the servotronic system because
of the spool valves that are already used with the worm and roller steering box.
Refer to ST056 Chassis Dynamics for further “mechanical” details of Servotronic.
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Central Body Electronics ZKE III
System Components
Electro-Hydraulic Solenoid: The solenoid regulates the
amount of power steering assist for the servotronic system. It
contains a needle valve that can restrict oil flow. When current
is applied, the needle valve closes against spring pressure to
restrict the oil flow.
With maximum current applied, the valve is closed. This is the
condition of the valve for slow speed driving and parking.
Without current applied, the needle valve is held open by
spring pressure. This is the condition of the solenoid during
higher driving speeds.
Electro-Hydraulic Solenoid Control: The GM monitors the road speed input and sup-
plies the electro-hydraulic solenoid with power. The road speed signal “A” is supplied from
the IKE (and K-Bus) on E38 vehicles and from the DSC Control Module on E39/E53 vehi-
cles.
The solenoid is pulse width modulated for control and varies the amount of assist based
on the road speed. The maximum assist is available while parking and driving at slow
speeds.
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Central Body Electronics ZKE III
