Electric power steering (ES), function - GF46.35-P-0001FQ

MODEL 218 up to model year 2014 

Function requirements, general 

• Circuit 15 ON
• Engine speed > 400 rpm
• Current wheel speed > 0/min

The "Circuit 15 ON" status is sent by the electronic ignition lock control unit via the chassis CAN to the electrical power steering control unit. The engine rpm is sent to the electrical power steering control unit over the chassis CAN by the ME-SFI [ME] control unit (with gasoline engine) or the 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 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, over chassis CAN from steering angle sensor in the steering column tube module control unit
• Current engine speed, over chassis CAN from the ME-SFI [ME] 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 counter steering 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 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 (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 to the instrument cluster.

The warning message "Power Steering Defective! Service Required!" is then displayed in the instrument cluster.

	Electrical function schematic for electric power steering		PE46.35-P-2050-97XAA
	Overview of system components of electric power steering (ES)		GF46.35-P-9997FQ