P46.35-2085-79
Block diagram |
A1 | Instrument cluster | N3/9 | CDI control unit (with diesel engine) | N80 | Steering column module control unit | ||
A91b1 | Electric power steering torque sensor | N3/10 | ME-SFI [ME] control unit (with gasoline engine) | N127 | Drivetrain control unit | ||
A91m1 | Electric power steering actuator motor | N30/4 | Electronic Stability Program control unit | X11/4 | Diagnostic connector | ||
L6/1 | Left front axle rpm sensor | N49 | Steering wheel angle sensor | CAN C | Engine CAN | ||
L6/2 | Right front axle rpm sensor | N62 | Parking system control unit (with code 235 (Active Parking Assist with PARKTRONIC) or with code P44 (Parking package)) | CAN D | Diagnostic CAN | ||
L6/3 | Left rear axle rpm sensor | N68 | Electrical power steering control unit | CAN HMI | User interface CAN | ||
L6/4 | Right rear axle rpm sensor | N73 | Electronic ignition switch control unit | Flex E | Chassis FlexRay |
Function requirements, general
Status "Circuit 15 ON"
The "Circuit 15 ON" status is sent by the electronic ignition lock control unit to the electrical power steering control unit over the suspension FlexRay. The engine rpm is sent to the electrical power steering control unit over the chassis FlexRay by the powertrain 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. The electric power steering system performs stepless, speed-dependent regulation of the power steering system.
The advantages compared to hydraulic power steering are:
Improved steering feel
Fuel saving
Hydraulic fluid not required
Compact design
Speed-dependent power assistance
Steering return is assisted.
Diagnosis capability
System diagnosis takes place via the electronic ignition lock control unit, the diagnostic CAN and the diagnostic connector using Xentry Diagnostics.
The electric power steering comprises the following subfunctions:
Function sequence for power steering
Steer Assist function sequence
Function sequence for reducing holding torque
Function sequence for limp-home mode
Display on instrument cluster function sequence
Function sequence for power steering
The electrical power steering control unit requires the following information to calculate the power steering required:
Current steering moment, from electric power steering torque sensor
The current wheel speeds calculated by the left front axle rpm sensor, right front axle rpm sensor, left rear axle rpm sensor and right rear axle rpm sensor and provided by the Electronic Stability Program control unit over the suspension FlexRay.
Steering wheel angle and steering angle rate of change, via the chassis FlexRay from the steering wheel angle sensor in the steering column tube module control unit
Current engine speed >400 rpm, over the suspension FlexRay and engine CAN with the powertrain control unit interface from the CDI control unit (with diesel engine) or ME-SFI [ME] control unit (with gasoline engine)
The electric power steering control unit uses this information to calculate the power steering to be set from the stored characteristics map and activates 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 power steering is transmitted from the electric power steering actuator motor to the rack-and-pinion steering gear, thereby supporting the steering moment applied by the driver.
When the vehicle is at a standstill, no power steering is available if the engine and ignition are switched off. With the ECO start/stop function, reduced power steering is available when the vehicle is at a standstill if the engine was switched off via the ECO start/stop function. The steering power is deactivated after the ignition is switched off. Power steering is active if the vehicle is maneuvered when the engine is off and the ignition is switched on.
Steer Assist function sequence
The steering assistance function helps the driver to achieve an optimum steering reaction in critical driving situations. Steering assistance is provided with the aid of the electric power steering control unit and Electronic Stability Program control unit.
The following functions are performed here:
Driver support through countersteering for a vehicle with oversteer
Driver support for braking on varying road surfaces
The current vehicle status is sensed by the Electronic Stability Program control unit.
If necessary, the message "Steering assistance torque request" is transmitted via the chassis CAN to the electrical power steering control unit, which actuates the electric power steering actuator motor accordingly.
When parking, the parking system control unit (with CODE 235 (Active Parking Assist)) sends the following signals over the suspension FlexRay to the electrical power steering control unit:
Current status of parking system control unit
Vehicle stop signal, at which the vehicle stops at a maneuver point during the parking process.
Requested toothed rack position
The following signals are sent by the electrical power steering control unit to the parking system control unit (with CODE 235 (Active Parking Assist with PARKTRONIC)):
Current status of the electrical power steering control unit
Function sequence for reducing holding torque
If a holding torque is applied by the driver when driving the vehicle straight ahead, e.g. for a crosswind or on a road surface inclined on one side, the electrical power steering control unit detects it. The applied holding torque is partially compensated for by the electric power steering.
To rule out incorrect compensation (e.g. highway bend with large radius), the electrical power steering control unit receives the turn rate (vehicle speed about the vertical axis) and the vehicle's lateral acceleration from the Electronic Stability Program control unit. The reduction of the vehicle's holding torque depends on the speed.
Function sequence for limp-home mode
When the electric power steering is switched on (through circuit 15 ON) the electrical power steering control unit runs a self-diagnosis. If a fault is detected, the power steering is not switched on. The vehicle can be steered with increased force of the hand.
During driving operation, the electrical power steering control unit checks the signals from the electric power steering torque sensor. If these signals are outside the defined upper or lower limit, the electric power steering is switched off by the electrical power steering control unit. The vehicle can be steered with increased force of the hand.
Display on instrument cluster function sequence
The electrical power steering control unit sends the request for a warning message over the chassis FlexRay and user interface CAN with the electronic ignition lock control unit interface to the instrument cluster. The warning message "Power Steering Defective! Service Required!" is then displayed.
Referenced documents
- gf4635p9997lf Overview of system components of electric power steering (ES)