MODEL | 218 |
as of model year 2014 |
A1 | Instrument cluster |
A91b1 | Electric power steering torque sensor |
A91m1 | Electric power steering actuator motor |
CAN D | Diagnostic CAN |
CAN E1 | Chassis CAN 1 |
CAN E2 | Chassis CAN 2 |
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 [ME] control unit (with gasoline engine) |
N10/1 | Front SAM control unit with fuse and relay module |
N30/4 | Electronic Stability Program control unit (except CODE 233 (DISTRONIC PLUS), except CODE 990 (AMG vehicles)) |
N30/7 | Premium Electronic Stability Program control unit (with CODE 233 (DISTRONIC PLUS) or with CODE 990 (AMG vehicles)) |
N49 | Steering 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 |
X11/4 | Diagnostic connector |
P46.35-2090-76
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
Engine speed > 400 rpm
Current wheel speed > 0/min
The status of circuit 15 ON is sent by the electronic ignition lock control unit via chassis CAN 1 and chassis CAN 2 with the front SAM control unit with fuse and relay module interface to the electrical power steering control unit. The engine speed is sent to the electrical power steering control unit via chassis CAN 1 and chassis CAN 2 with the front SAM control unit with fuse and relay module interface by the ME-SFI control unit (with gasoline engine) or CDI control unit (with diesel engine).
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
Display on instrument cluster function sequence
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, detected via left front axle rpm sensor, right front axle rpm sensor, left rear axle rpm sensor and right rear axle rpm sensor and sent via chassis CAN 1 and chassis CAN 2 with front SAM control unit with fuse and relay module interface by Electronic Stability Program control unit (except CODE 233 (DISTRONIC PLUS), except CODE 990 (AMG vehicles)) or Premium Electronic Stability Program control unit (with CODE 233 (DISTRONIC PLUS) or with CODE 990 (AMG vehicles))
– Steering angle and steering speed, via chassis CAN 1 and chassis CAN 2 with front SAM control unit with fuse and relay module interface from steering angle sensor in steering column tube module control unit
– Current engine speed, via chassis CAN 1 and chassis CAN 2 from ME-SFI control unit (with gasoline engine) or CDI control unit (with diesel engine)
The electric power steering control unit uses this information to calculate the steering force support to be applied from the stored characteristics map and activates 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.
On vehicles with and without CODE B03 (ECO start/stop function): When the vehicle is at a standstill, no steering power assistance is available if the engine and ignition are switched off. On vehicles with CODE B03 (ECO start/stop function): When the vehicle is at a standstill, reduced steering power assistance is available if the engine was switched off via the ECO start/stop function. After the ignition is switched off, steering power assistance is also deactivated on vehicles with ECO start/stop function. 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.
The Steer Assist function is realized with the aid of the electric power steering control unit and with the Electronic Stability Program control unit (except CODE 233 (DISTRONIC PLUS), except CODE 990 (AMG vehicles)) or the Premium Electronic Stability Program control unit (with CODE 233 (DISTRONIC PLUS) or with CODE 990 (AMG vehicles)).
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 acquired by the Electronic Stability Program control unit (except CODE 233 (DISTRONIC PLUS), except CODE 990 (AMG vehicles)) or by the Premium Electronic Stability Program control unit (with CODE 233 (DISTRONIC PLUS) or with CODE 990 (AMG vehicles)).
If necessary, the "steering assistance torque request" message is sent over the chassis CAN 1 and chassis CAN 2 with the front SAM control unit with fuse and relay module interface to the electrical power steering control unit, which actuates the electric power steering actuator motor.
During a parking procedure, the parking system control unit (with CODE 220 (PARKTRONIC) with CODE 235 (Active Parking Assist)) 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 2 to the instrument cluster.
The warning message "Power Steering Defective! Service Required!" is then displayed in the instrument cluster.
Referenced documents
- gf4635p9997fq Overview of system components of electric power steering (ES)