MODEL | 207.373/473 |
as of model year 2012 up to model year 2014 |
1 | Rack-and-pinion steering gear |
2 | Belt drive |
A91b1 | Electric power steering torque sensor |
A91m1 | Electric power steering actuator motor |
N68 | Electrical power steering control unit |
P46.35-2050-05
Function requirements, general
Circuit 15 ON status
Engine speed > 400 rpm
The status of circuit 15 is sent by the electronic ignition lock control unit (N73) via the chassis CAN to the electrical power steering control unit. The engine speed is sent to the electrical power steering control unit via the chassis CAN by the ME-SFI control unit (N3/10).
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 stepless, speed-dependent regulation of the power steering.
The advantages compared to hydraulic power steering are:
- Enhanced steering feel
- Fuel saving
- No steering oil required
- Compact design
- Speed-dependent power assistance
- Steering return is assisted
- Diagnosis capability
The ES comprises the following subfunctions:
Function sequence for steering assistance
Steer Assist function sequence
Function sequence for limp-home mode
Function sequence for display in instrument cluster (A1)
Function sequence for steering assistance
The electrical power steering control unit requires the following information to calculate the steering power assistance required:
- Current steering moment, from electric power steering torque sensor
– Current wheel speeds, determined by left front axle rpm sensor (L6/1), right front axle rpm sensor (L6/2), left rear axle rpm sensor (L6/3) and right rear axle rpm sensor (L6/4) and provided via chassis CAN by Electronic Stability Program control unit (N30/4) (except CODE 233 (DISTRONIC PLUS) or Premium Electronic Stability Program control unit (N30/7) (with CODE 233 (DISTRONIC PLUS))
– Steering angle and steering angle speed, via chassis CAN from steering angle sensor (N49) in steering column tube module control unit (N80)
– Current engine speed, via chassis CAN from ME-SFI control unit
The electric power steering control unit uses this information to calculate the required steering force support from the stored characteristics map and actuates the electric power steering actuator motor accordingly. The steering force assistance is transmitted from the electric power steering actuator motor to the rack-and-pinion steering gear via a belt drive, supporting the steering moment applied by the driver. The direction of rotation of the electric power steering actuator motor depends on the direction of movement of the steering wheel.
When the vehicle is at a standstill, no steering force support is available if the engine and ignition are switched off. With the ECO Start-Stop function, reduced steering force support is available when the vehicle is at a standstill if the engine was switched off via the ECO start/stop function. The steering power assistance is deactivated after the ignition is switched off. Steering assistance 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. Steer Assist is realized with the aid of the electric power steering control unit and the Electronic Stability Program control unit (except CODE 233 (DISTRONIC PLUS)) or the Premium Electronic Stability Program control unit (with CODE 233 (DISTRONIC PLUS)).
The following functions are performed here:
- Driver support or stabilization through countersteering of vehicle with oversteer
- Driver support when braking on alternating road surfaces
The current driving status is recorded by the Electronic Stability Program control unit (except CODE 233 (DISTRONIC PLUS)) or by the Premium Electronic Stability Program control unit (with CODE 233 (DISTRONIC PLUS)).
If necessary, the message "Steering assistance torque request" is transmitted via 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 (N62) (with CODE 230 (Exclusive Parking Assist) without CODE 498 (Japan version) or with CODE 220 (PARKTRONIC) with CODE 498 (Japan version)) sends the following signals to the electronic 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 steering angle
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 steering force assistance 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 warning message "Power Steering Defective! Service Required!" is then displayed in the instrument cluster.
Referenced documents
- gf4635p000102ca Electric power steering (ES), location of components
- gf4635p000103ca Electric power steering (ES), block diagram
- gf4635p9997ca Overview of system components of electric power steering (ES)