Electric Power Steering (ES), Function - GF46.35-P-0001KJ
MODEL 453.0/3/4
Function requirements, general
- Circuit 15 ON
Circuit 15 is activated via the ignition lock (S2) and forwarded via the vehicle interior fuse and relay module (F2) to the electrical power steering control unit (N68).
Electric power steering (ES), general
The electric power steering (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 continuous, speed-proportional power steering control. The advantages compared to hydraulic power steering are:
- Improved steering feel
- Fuel saving
- Hydraulic fluid not required
- Compact design
- Speed proportional steering
- Steering return is assisted
- Diagnosis capability
System diagnosis takes place via the center SAM control unit (N10/10) and the diagnostic connector (X11/4) using XENTRY Diagnostics. For function feedback, the power steering warning lamp is provided in the instrument cluster; this lamp lights up briefly after the ignition (circuit 15) is switched on, and then goes out again provided that no faults are stored in the fault memory of the electrical power steering control unit.
The ES comprises the following subfunctions:
- Function sequence for steering force assistance
- Function sequence for reducing holding torque
- Function sequence for limp-home mode
- Display on instrument cluster function sequence
Function sequence for steering force assistance
The electrical power steering control unit requires the following information to calculate the steering force assistance required:
- Current steering torque, from electric power steering torque sensor (A91b1)
- Current wheel speeds determined via 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 Electronic Stability Program control unit (N30/4) via interior CAN
- Steering wheel angle and steering rate, via chassis CAN from steering wheel angle sensor (N49) and put on interior CAN by Electronic Stability Program control unit
- Current engine speed > 400 rpm, determined via crankshaft position sensor and made available via interior CAN by ME-SFI control unit (N3/10) (except model 453.091/391/491)
- "Drivetrain operational" signal via interior CAN by electric drive control unit (N127) (only model 453.091/391/491)
Using this information, the electric power steering control unit calculates from the stored characteristics map the torque to be set in order to provide steering force assistance, and activates 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 torque for boosting the steering force 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.
When the vehicle is at a standstill, power steering is not available if the internal combustion engine (except model 453.091/391/491) or the drivetrain (only model 453.091/391/491) and the ignition are switched off. With code B03 (ECO start/stop function), reduced steering force assistance is available if the internal combustion engine was switched off via the ECO start/stop function. The steering power is deactivated after the ignition is switched off. Active steering force assistance is provided if the vehicle is maneuvered with the internal combustion engine switched off and the ignition switched on.
Function sequence for 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 road surface 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 vehicle's holding torque is reduced in relation to the current vehicle speed.
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 power steering 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 limits, the power steering is switched off. The vehicle can be steered with increased force of the hand.
Display on instrument cluster function sequence
If the ES is faulty or fails resulting in a loss of steering force assistance, the electrical power steering control unit sends the request to switch on the power steering warning lamp (except model 453.091/391/491) or the request to output a warning message via the interior CAN to the instrument cluster.
The power steering warning lamp is then actuated by the instrument cluster in order to inform the driver. On vehicles with code K36 (Instrument cluster with color 8.9 cm (3.5") display), the warning message "Power steering fault, see owner's manual" or "Electronics fault, see owner's manual" (depending on the type of fault that has occurred) is additionally displayed on the multifunction display of the instrument cluster. An acoustic warning is also output via the instrument cluster at the same time as the warning message is displayed. On model 453.091/391/491 with code K36 (Instrument cluster with 8.9 cm (3.5") color display), only one warning message is issued.
| Electrical function schematic for electric power steering | PE46.35-P-2050-97BBA | ||
| Overview of system components of electric power steering (ES) | GF46.35-P-9997KJ |