Steering, Basic Function - GF46.35-P-9900A

Model all (CAR) 

Overview 

This document contains information on:

• General 
• Function requirements 
• Electric power steering, general 
• Power steering 
• Steer Assist 
• Reducing holding torque 
• Limp-home mode 

General 

Steering makes it possible to turn the front wheels with different steer angles. An actuator motor provides the assistance force required for the servo effect and transfers it via an angled worm gear or a belt drive to the steering gear.

Function requirements 

• ignition ON (circuit 15 ON)
• Engine running

Electric power steering, general 

The electric power steering consists of the following components:

• Rack-and-pinion steering
• Torque sensor
• Actuator motor
• Electrical power steering control unit

Electric power steering performs continuous, speed-dependent regulation of the power steering system.

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 and the diagnostics connection by means of XENTRY Diagnostics.

On vehicles with the Direct-Steer system, a rack and pinion steering gear with variable gear ratio is used.

The Direct-Steer system has the following features:

• Agile steering behavior, particularly in urban traffic, when maneuvering or on twisty roads. This has been achieved by the design of the toothed rack and as a result of the associated sharp 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

Power steering 

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

• Current steering torque, from the torque sensor
• Current wheel speeds, determined via 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 ESP® control unit
• Steering wheel angle and steering speed from the steering wheel angle sensor in the steering column tube module control unit
• Current engine speed > 400 RPM from the combustion engine control unit

The electrical power steering control unit uses this information to calculate the assistance to be set from the stored characteristics map and actuates the electric power steering actuator motor.

The direction of rotation of the actuator motor depends on the movement direction of the steering wheel.

The actuator motor transfers the electric power steering to the rack-and-pinion steering and thus assists the driver's steering torque.

When the engine and ignition are switched off and the vehicle is stationary, the power steering system is not available. 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. If the vehicle is maneuvered at a speed of at least 1.75 km/h when the engine and ignition are switched off, the power steering system is active.

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 electrical power steering control unit and the ESP® 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 driving condition is recorded by the ESP® control unit. If necessary, the message "Steer Assist torque request" is transmitted to the electric power steering control unit, which then actuates the actuator motor.

When parking, the parking system control unit (with CODE 220 (PARKTRONIC) or with CODE 235 (Active Parking Assist with PARKTRONIC)) transmits the following signals 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 transmitted by the electrical power steering control unit to the parking system control unit (with CODE 220 (PARKTRONIC) or with CODE 235 (Active Parking Assist with PARKTRONIC)):

• Current status of the electrical power steering control unit

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 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 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 ESP® control unit.

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

Limp-home mode 

When the electrical power steering is switched on (via circuit 15 ON), the electric power steering control unit carries out a self-test. In the event of a detected fault, electric power steering does not switch on. The vehicle can be steered by hand with increased force.

During operating status, the electrical power steering control unit checks the signals from the torque sensor. If these signals are outside of a defined upper or lower limit, the support of the steering is switched off by the electrical power steering control unit. The vehicle can be steered by hand with increased force.

The electric power steering control unit transmits the request for a warning message to the instrument cluster.

The multifunction display of the instrument cluster then shows the warning message "Power steering defective! Service Required".

	Further basic functions
	Mechanical steering components, basic function		GF46.00-P-1000B
	Steering column adjustment, basic function	Model 167, 177 with code 275 (Driver's seat, electrically adjustable with memory function)	GF46.15-P-1000B
	Components
	Electric power steering control unit, basic function		GF46.35-P-9890A
	Parking system control unit, basic function	Model all (CAR) with code 235 (Active Parking Assist)	GF54.65-P-9890A
	Function schematic
	Function schematic for electric power steering	Model 118, 167, 177, 247	PE46.35-P-2500A