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
- Display in instrument cluster
General
Steering makes it possible to turn the front wheels with different steer angles. An actuator motor provides the force required for the servo effect and transfers it via a angled gear drive 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 Electronic Stability Program 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. Steer Assist takes place with the aid of the electric power steering control unit and the 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 "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)) transmits the following signals to the electric 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)):
- 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 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 Electronic Stability Program 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 with increased force of the hand.
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, electric power steering is switched off by the control unit. The vehicle can be steered with increased force of the hand.
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 177 with code 275 (Driver's seat, electrically adjustable with memory function) | GF46.15-P-1000B | |
| 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 with PARKTRONIC) | GF54.65-P-9890A |