Electric Power Steering (ES), Function - GF46.35-P-0001RF

Model 205, 253 

Courtesy of MERCEDES-BENZ USA

Function requirements, general 

• Status of circuit 15 ON
• Vehicle speed > 1.75 km/h or
• "Drivetrain operational" signal set or
• Engine speed > 400 RPM (except model 253.99)

The electronic ignition lock control unit sends the status of circuit 15 via the chassis FlexRay to the electrical power steering control unit. The Electronic Stability Program control unit sends the wheel speeds over the suspension FlexRay to the electrical power steering control unit. The powertrain control unit sends the "Drivetrain operational" signal over the suspension FlexRay to the electrical power steering control unit. The CDI control unit (diesel engine) or the ME-SFI [ME] control unit (gasoline engine) sends the engine speed over the engine CAN, powertrain control unit and the suspension FlexRay to the electrical power steering control unit.

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 continuously variable, speed-dependent regulation of the power steering.

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 is performed through the electronic ignition lock control unit, diagnostics CAN and the diagnostics connection with the aid of XENTRY Diagnostics.

The ES comprises the following subfunctions:

• Power steering 
• Steer Assist 
• Reducing holding torque 
• Limp-home mode 
• Display in instrument cluster 

Power steering 

The electrical power steering control unit requires the following information to calculate the steering power assistance required:

• Current steering torque, recorded by electric power steering torque sensor.
• Current wheel speeds, recorded by the following sensors:
  • Left front axle RPM sensor
  • Right front axle RPM sensor
  • Left rear axle RPM sensor
  • Right rear axle RPM sensor

  The Electronic Stability Program control unit reads in the signals of the RPM sensors and sends them over the suspension FlexRay to the electrical power steering control unit.

• Steering wheel angle and steering speed, recorded by the steering wheel angle sensor.

  The steering column tube module control unit reads in the signals of the steering wheel angle sensor and sends this over the suspension FlexRay to the electrical power steering control unit.

• Current engine speed > 400 RPM (except model 253.99).

  The CDI control unit (diesel engine) or the ME-SFI [ME] control unit (gasoline engine) sends the engine speed over the engine CAN, powertrain control unit and the suspension FlexRay to the electrical power steering control unit.

The electrical power steering control unit uses this information and the stored characteristics map to calculate the support to be configured for the electric power steering. It then actuates 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 assistance provided by 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.

Model 205, 253 (except 253.99)

With the ECO start/stop function, reduced steering force assistance 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. If the vehicle is maneuvered when the engine and the ignition are switched off, the power steering system is activated.

Steer Assist 

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 FlexRay to the electrical power steering control unit, which actuates the electric power steering actuator motor accordingly.

Parking system (model 205 up to model year 2019 with code 235 (Parking Pilot with PARKTRONIC), model 205 as of model year 2019 with code P44 (Parking Package) or code P47 (Parking Pilot with 360° camera), model 253 (except 253.99) with code 235 (Parking Pilot with PARKTRONIC), model 253 99 with code P47 (Parking Pilot with 360° camera)) 

When parking, the parking system control unit sends the following signals over the chassis FlexRay to the electrical power steering control unit:

• Current status of parking system
• Vehicle stop signal, at which the vehicle stops at a maneuver point during the parking process
• Front wheel angle request

The electrical power steering control unit actuates the electric power steering actuator motor accordingly to implement the parking procedure. It also sends the current status of the power steering system over the suspension FlexRay to the parking system control unit.

Reducing holding torque 

If a holding torque is applied by the driver when driving the vehicle straight ahead, e.g. for crosswind or on a roadway 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 avoid any faulty compensation (e.g. freeway corner with large radius), the vehicle turn rate (vehicle speed around the vertical axis) and the lateral acceleration are evaluated. The Supplemental Restraint System control unit sends these signals over the vehicle dynamics CAN, Electronic Stability Program control unit and the suspension FlexRay to the electrical power steering control unit.

The reduction of the vehicle's holding torque depends on the vehicle speed.

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 power steering is not switched on. The vehicle can be steered with increased force of the hand.

During operating status, 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 in instrument cluster 

The electrical power steering control unit sends the request to issue a warning message over the suspension FlexRay, electronic ignition lock control unit and the user interface CAN to the instrument cluster. The warning message "Power Steering Defective! Service Required!" is then displayed.

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