P46.35-2084-79
A1 | Instrument cluster | L6/4 | Right rear axle rpm sensor | N68 | Electrical power steering control unit | ||
A1p13 | Multifunction display | N3/9 | CDI control unit (with diesel engine) | N73 | Electronic ignition switch control unit | ||
A91b1 | Electric power steering torque sensor | N3/10 | ME-SFI [ME] control unit (with gasoline engine) | N80 | Steering column module control unit | ||
A91m1 | Electric power steering actuator motor | N10/1 | Front SAM control unit with fuse and relay module | X11/4 | Diagnostic connector | ||
L6/1 | Left front axle rpm sensor | N30/4 | Electronic Stability Program control unit | CAN B | Interior CAN | ||
L6/2 | Right front axle rpm sensor | N49 | Steering angle sensor | CAN D | Diagnostic CAN | ||
L6/3 | Left rear axle rpm sensor | N62 | Parking system control unit (with CODE 230 (Exclusive parking assist)) | CAN E | Chassis CAN |
1 | Rack-and-pinion steering gear |
2 | Belt drive housing |
A91b1 | Electric power steering torque sensor |
A91m1 | Electric power steering actuator motor |
N68 | Electrical power steering control unit |
P46.35-2050-05
Function requirements, general
Circuit 15 ON
The "Circuit 15 ON" status is sent by the electronic ignition lock control unit via the chassis CAN to the electrical power steering control unit. The engine rpm is sent to the electrical power steering control unit over the chassis CAN by the ME-SFI [ME] control unit (with gasoline engine) or the CDI control unit (with diesel engine).
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 stepless, speed-dependent regulation of the power steering.
The advantages compared to hydraulic power steering are:
- Enhanced steering feel
- Fuel saving
– Hydraulic fluid not required
- Compact design
– Speed-dependent steering power assistance
- Steering return is assisted
- Diagnosis capability
The ES comprises the following subfunctions:
Power steering function sequence
Steer Assist function sequence
Function sequence for reducing holding torque
Function sequence for limp-home mode
Display on instrument cluster function sequence
Power steering function sequence
The electrical power steering control unit requires the following information to calculate the steering power assistance required:
- Current steering moment, from electric power steering torque sensor
– The current wheel speeds calculated by the left front axle rpm sensor, right front axle rpm sensor, left rear axle rpm sensor and right rear axle rpm sensor and sent from the Electronic Stability Program control unit via the chassis CAN
- Steering angle and steering speed, over chassis CAN from steering angle sensor in the steering column tube module control unit
– Current engine speed > 400 rpm, via chassis CAN from ME-SFI [ME] control unit (with gasoline engine) or CDI control unit (with diesel engine)
The electric power steering control unit uses this information to calculate the steering force support to be applied from the stored characteristics map 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 steering force assistance is transmitted from the electric power steering actuator motor to the rack-and-pinion steering gear, thereby supporting the steering moment applied by the driver.
When the vehicle is at a standstill, no steering force support is available if the engine and ignition are switched off. If the vehicle is parked using the ECO start/stop function, a limited degree of steering power assistance is available at a standstill. The steering power assistance is deactivated after the ignition is switched off. Steering assistance is active if the vehicle is maneuvered when the engine is off and the ignition is switched on.
Steer Assist function sequence
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 when braking on alternating 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 chassis CAN to the electrical power steering control unit, which actuates the electric power steering actuator motor accordingly.
During a parking procedure, the parking system control unit (with CODE 230 (Exclusive parking assist)) 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 process
– Requested toothed rack position
The following signals are sent by the electrical power steering control unit to the parking system control unit (with CODE 230 (Exclusive parking assist)):
– Current status of the electrical power steering control unit
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 any faulty compensation (e.g. highway corner with large radius), the electrical power steering control unit receives the rotational speed (vehicle speed about the vertical axis) and the vehicle's lateral acceleration over chassis CAN from the Electronic Stability Program control unit.
The reduction of the vehicle's holding torque depends on the 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 steering force assistance 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 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 on instrument cluster function sequence
The electrical power steering control unit control unit transmits the request for a warning message via chassis CAN to the instrument cluster.
The instrument cluster's multifunction display then shows the warning message "Power steering defective! Service Required!" is then displayed in the instrument cluster.
Referenced documents
- gf4635p9997ce Overview of system components of electric power steering (ES)