Steering Functions
Steering Column
Principle of Operation
The electrically adjustable steering column used in the various models (if equipped) is similar in components, mechanical linkages and electrical operation.
The steering column adjusting switch is mounted on the left side of the steering column and provides for four directions of column movement. The steering column switch inputs are processed by the seat module.
Motor Drives:
Forward Back (Telescope)
A flexible drive shaft operates a screw drive which causes a forward/back movement of the steering column.
Up/Down
A flexible drive shaft moves an offcenter lever causing up/down steering movement.
The motors for tilt and length also employ ripple counter modules for motor control and memory recall.
The control electronics for the steering column are integrated into the seat module.
The output signals are sent from the seat module to the steering column ripple counter modules for motor activation.
System Components: Inputs - Processing - Outputs
Entry/Exit Aid
The Steering Column Memory provides the feature of raising the steering column to ease in exiting the vehicle and return it to the previous position after the vehicle is entered.
Raising the Steering Column:
- The ignition is switched "off".
- The ignition is switched to KL R and the driver's door is opened.
- The ignition is switched to KL 15, driver's door open with the handbrake applied.
Lowering the Steering Column:
- The ignition is switched to KL 15.
- The Handbrake is released or the door is closed.
The steering column recall movement can be interrupted at any time by tapping the adjusting lever.
Servotronic (E38 only)
Purpose of the System
The servotronic steering system is based on a conventional power assisted steering system. An additional electro-hydraulic solenoid and oil passageways are added to the conventional 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 following:
- 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.
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 supplies 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 vehicles.
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.
Principle of Operation
Control logic (example E38) includes:
- Servotronic control electronics active with KL R being switched "ON" ensure no delay in operation if engine is started and vehicle is immediately driven.
- Plausibility check for speed signal - the control electronics monitor both the Speed signal "A" from the IKE and the vehicle speed signal on the K-Bus.
- The ability to detect both acceleration and deceleration from the two speed signals - the speed signal from the IKE is updated every two seconds.
- The servotronic assist is reduced gradually when the vehicle is under acceleration.
- The servotronic assist is adopted to the lower direct reading during decel or braking.
Electric/electronic failures with the servotronic system will result in the following:
- Power/electronic failure of the control module or solenoid - steering assist the same as high speed driving (increased effort).
- Vehicle speed signal missing - control module retains the assist mode in effect when the speed signal was lost.
- Speed signal implausible - steering assist the same as high speed driving (increased effort).
The GM also provides the diagnostic "gateway" to the Servotronic status and Component Activation via the DISplus/MoDiC.