Steering, Basic Function - GF46.35-S-9900A
Model All (Light transporter)
Overview
This document contains information on:
- General
- Function requirements
- Function
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 a worm gear or a belt drive to the steering gear.
Function requirements
- Ignition switched ON (circuit 15)
- Engine in operation
Function
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.
The advantages compared to hydraulic power steering are:
- Improved steering feel
- Fuel saving
- Hydraulic oil not required
- Compact design
- Speed-dependent power assistance
- Steering return is assisted.
- Diagnosis capability
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 is 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 steering-wheel turns lock to lock 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. They are provided by the ESP® control unit. They are detected by the left and right front axle rotational speed sensors as well as the left and right rear axle rotational speed sensors.
- 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.
No power steering is available on stationary vehicles with the engine and ignition switched off. With the ECO start/stop function, reduced power steering is available at standstill if the engine was switched off using 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 Steer Assist 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 electrical power steering control unit, which then actuates the actuator motor.
When parking, the parking system control unit sends 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 procedure.
- Requested toothed rack position
The following signals are sent by the electrical power steering control unit to the parking system control unit:
- 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. in the case of 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) as well as 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 electric power steering is switched on (via circuit 15 ON), the electrical power steering control unit carries out a self-test. If a fault is detected, the electric power steering does not switch on. The vehicle can be steered by hand with increased force.
During driving operation, 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 electrical power steering control unit transmits the request for a warning message to the instrument cluster.
The instrument cluster display then shows the warning message "Power steering defective! Service Required!"
| Wiring diagrams | |||
| Overview of wiring diagrams for steering | Model 420 | PE46.00-S-1100VRA | |
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