Block diagram |
A1 | Instrument cluster |
A1p13 | Multifunction display |
A91b1 | Electric power steering torque sensor |
A91m1 | Electric power steering actuator motor |
L6/1 | Left front axle rpm sensor |
L6/2 | Right front axle rpm sensor |
L6/3 | Left rear axle rpm sensor |
L6/4 | Right rear axle rpm sensor |
N3/9 | CDI control unit (with diesel engine) |
N3/10 | ME-SFI control unit (with gasoline engine) |
N30/4 | Electronic Stability Program control unit |
N49 | Steering wheel angle sensor |
N62 | Parking system control unit (with CODE 220 (PARKTRONIC) or with CODE 235 (Active Parking Assist)) |
N68 | Electrical power steering control unit |
N73 | Electronic ignition switch control unit |
N80 | Steering column module control unit |
N127 | Powertrain control unit (on electric vehicles) |
X11/4 | Diagnostic connector |
CAN D | Diagnostic CAN |
CAN E | Chassis CAN |
P46.35-2068-76
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 via the chassis CAN. The engine speed is sent to the electrical power steering control unit via the chassis CAN by the CDI control unit (with diesel engine) or by the ME-SFI control unit (with gasoline engine) or by the powertrain control unit (with electric vehicles).
Electric power steering (ES), general
The ES 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 ES performs continuous, speed-proportional power steering control.
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.
A rack and pinion steering gear with variable gear ratio is used on vehicles with CODE 213 (Direct-Steer system).
The direct-steer system has the following features:
More responsive steering behavior, especially in urban traffic, when maneuvering or on twisty roads, has been achieved by the design of the toothed rack and as a result of the associated increase in gear ratio from the center position as of approx. 6° steering angle.
Improved comfort, especially when parking, by reducing the lock-to-lock steering wheel turns to 2.5
The ES 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
Current wheel speeds determined by the left front axle rpm sensor, right front axle rpm sensor, left rear axle rpm sensor and right rear axle rpm sensor and made available by the Electronic Stability Program control unit via the chassis CAN.
Steering wheel angle and steering speed, via the chassis CAN from the steering wheel angle sensor in the steering column tube module control unit
Current engine speed > 400 rpm, via the chassis CAN from the CDI control unit (with diesel engine) or from the ME-SFI control unit (with gasoline engine) or from the powertrain control unit (with electric vehicles)
The electric power steering control unit uses this information to calculate the steering assistance 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 electric power steering is transmitted from the electric power steering actuator motor to the rack-and-pinion steering gear, thereby supporting the steering torque 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 electric power steering 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.
During a parking procedure the parking system control unit (with CODE 220 PARKTRONIC) or CODE 235 (Active Parking Assist) sends the following signals over the chassis CAN 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 220 (PARKTRONIC) or with CODE 235 (Active Parking Assist)):
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 via the chassis CAN.
The reduction of the vehicle's holding torque depends on the speed.
Function sequence for limp-home mode
When the ES is switched on (via "circuit 15 ON"), the electric power steering control unit carries out a self-test. If a fault is detected, the electric 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 ES 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 control unit transmits the request for a warning message via chassis CAN to the instrument cluster.
The instrument cluster's multifunction display then shows the warning message "Power steering defective! Service Required!" is then displayed in the instrument cluster.
Referenced documents
- gf4635p9997kn Overview of system components of electric power steering (ES)