Electric power steering (ES), function

P46.35-2136-79

Function requirements, general

 Terminal 15 ON

 Engine running or drive train operational

 The electronic ignition lock control unit sends the status of circuit 15 via the chassis FlexRay to the electrical power steering control unit.
The CDI control unit or the ME-SFI control unit sends the "engine running" signal or the "drivetrain operational" signal via the drive train CAN, powertrain control unit and chassis FlexRay to the electrical power steering control unit.

Electric power steering, general

The electric power steering consists of the rack-and-pinion steering gear, the electric power steering torque sensor, the electric power steering actuator motor and the electrical power steering control unit. Electric power steering performs continuous, speed-dependent regulation of the assisting force.

The electric power steering has the following advantages compared with the hydraulic power steering system:

 Compact design

 No hydraulic fluid required

 Speed-dependent power assistance

 Steering return is assisted

 Improved steering feel

 Fuel saving

 Diagnosis capability

 Limp-home mode
When the electric power steering is switched on the electrical power steering control unit runs a self-test. If a fault is detected, the electric power steering is not activated. The vehicle can be steered with increased force of the hand.
During driving operation, the electric power steering control unit checks that the electric power steering torque sensor is functioning correctly. In the event of any malfunctions, the electric power steering control unit switches off the electric power steering.

 Model 213
as of model year 2018
with code 22P (Lane tracking package)
Model 213
as of model year 2018
with code 23P (Driving Assistance package)
Model 213
as of model year 2018
with code P20 (Driving Assistance package Plus)
If the vehicle leaves the lane unintentionally, the driver is warned through vibrations on the steering wheel. The vibrations are generated by the electric power steering actuator motor. The vibrations are transferred through the steering column to the steering wheel.
Vehicles with code 22P (Lane Tracking Package)
The mono multifunction camera sends the request to issue the warning and vibration time over the peripherals CAN, electronic ignition lock control unit and the suspension FlexRay to the electric power steering control unit. The electrical power steering control unit then actuates the electric power steering actuator motor accordingly.
Vehicles with code 23P (Driving Assistance Package) or code P20 (Driving Assistance Plus Package)
The stereo multifunction camera sends the request to issue the warning and the vibration time over the suspension FlexRay to the electric power steering control unit. The electrical power steering control unit then actuates the electric power steering actuator motor accordingly.

The electric power steering function includes the following partial functions:

 Power steering

 Steer Assist

Power steering

To calculate the required amount of assistance, the electrical power steering control unit evaluates, among other things, the following input factors:

 Steering moment

The electric power steering torque sensor records the steering torque applied by the driver. The electrical power steering control unit reads in the signals from the electric power steering torque sensor directly.

 Wheel speeds

The Electronic Stability Program control unit sends the wheel speed signals via the chassis FlexRay to the electrical power steering control unit.

 Steering wheel angle and steering speed

The steering wheel angle and the steering speed are detected by the steering wheel angle sensor. The steering column tube module control unit reads in the signals from the steering wheel angle sensor directly and sends information about the steering wheel angle and the steering speed via the chassis FlexRay to the electrical power steering control unit.

 Engine speed

The CDI control unit or the ME-SFI control unit sends the engine speed via the drive train CAN, powertrain control unit and chassis FlexRay to the electrical power steering control unit.

The electric power steering control unit uses this information to calculate the required electric power steering assistance and actuates the electric power steering actuator motor accordingly. The direction of rotation of the electric power steering actuator motor depends on the direction of movement of the steering wheel. The assistance provided by the electric power steering is transmitted by the electric power steering actuator motor to the rack-and-pinion steering gear.

If a component part in the electric power steering is damaged, the electrical power steering control unit sends the request to issue a warning message over the suspension FlexRay, electronic ignition lock control unit and the user interface CAN to the instrument cluster. The warning message "Power Steering Defective! Service Required!" is then displayed.

 If the engine has been switched off and the ignition is switched off (vehicle standstill), the electric power steering is not available for use. If the engine has been switched off via the ECO start/stop function, reduced assistance is available. If the vehicle is maneuvered with the engine switched off and the ignition switched on, the electrical power steering control unit activates the electric power steering.

Steer Assist

Steer Assist supports the driver in difficult driving situations. The electrical power steering control unit calculates the amount of assistance required. The Electronic Stability Program control unit assesses the driving situation. The acceleration sensor in the supplemental restraint system control unit assesses the natural dynamics of the vehicle and sends corresponding information (turn rate, lateral acceleration) via the vehicle dynamics CAN to the Electronic Stability Program control unit. In critical driving situations, the Electronic Stability Program control unit sends corresponding requests via the chassis FlexRay to the electrical power steering control unit. The electrical power steering control unit then actuates the electric power steering actuator motor accordingly.

The following functions are performed here:

 Driver support through countersteering for a vehicle with oversteer

 Driver support for braking on varying road surfaces

The electrical power steering control unit detects when a holding torque needs to be applied by the driver when driving the vehicle straight ahead, e.g. with a crosswind or on a road surface inclined on one side. The required holding torque is partially compensated by the electric power steering.

To rule out incorrect compensation (e.g. highway bend with a large radius), the supplemental restraint system control unit sends information about the natural dynamics of the vehicle via the vehicle dynamics CAN, Electronic Stability Program control unit and chassis FlexRay to the electrical power steering control unit. Reduction of the vehicle's holding torque depends on the speed at which the vehicle is being driven.

 Model 213
with code 235 (Parking Pilot with PARKTRONIC)
When entering or exiting a parking space with assistance, the parking system control unit sends corresponding steering requests over the suspension FlexRay to the electrical power steering control unit.