Electric steering, function

P46.35-2088-06

1 Vehicle speed

2 Steering moment

3 Request, steering wheel reset

4 Request, steering power assistance

5 Steering angle, steering speed

6 Engine speed

B23 Power steering torque sensor

M19 Power steering motor

N3/9 CDI control unit
(with engine 660.9)

N3/10 ME-SFI [ME] control unit
(with engine 132.9)

N47-5 ESP control unit

N49 Steering angle sensor

N68 Steering assist control unit

N127 Drivetrain control unit (for model 451.390/490)

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CAN C Engine compartment CAN

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Function requirements, general

 Circuit 15 ON

 Engine speed > 500 rpm

 No overvoltage or undervoltage

Electric steering, general

The electric steering is based on a conventional rack and pinion steering gear which is assisted by the power steering motor. The steering power assistance takes place steplessly and speed-dependent.

Electric steering functions

 Power steering assistance

 Steering reset assistance

 "active center feeling" function

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The steering assist control unit requires the following information to calculate the steering power assistance:

 Steering torque from power steering torque sensor

 Vehicle speed over engine compartment-CAN from ESP control unit

 Steering angle and steering speed over engine compartment CAN from steering angle sensor

 Engine rpm over engine compartment-CAN from ME-SFI [ME] control unit, CDI control unit or the drivetrain control unit

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Steering power assistance

By turning the steering wheel a torque on the control rod is produced which is recorded by the power steering torque sensor and forwarded to the power steering control unit. By means of the steering torque determined, the steering angle and vehicle speed the assistance torque is established by the steering assist control unit and the power steering motor actuated accordingly. This supports the driver steering torque over a worm gear on the drive pinion of the rack-and-pinion steering.

 The steering power assistance is at a maximum at low speeds and when the vehicle is stationary and decreases as the vehicle speed increases.

Once the engine is switched off (except model 451.390/490) or the vehicle brought to a standstill, a gradual deactivation of the steering power assistance takes place after approx. 3 seconds.

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Steering reset assistance

The steering reset is introduced due to the shaft kinematics
(after-run). As a result the power steering motor is excited passively and operates as a generator. On the basis of this signal the power steering control unit detects the assistance requirement and assists the return force by the corresponding energizing of the power steering motor.

"active center feeling" function

When the steering wheel center position is detected and in the absence of steering movements, the power steering motor is energized by the power steering control unit so that an adequate torque is produced in order to hold the steering in the center position.

 As a result the driver has an improved "feel" for the center position of the steering.

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Limp-home function

The steering assist control unit permanently checks all input signals for plausibility. In the case of a faulty component the steering power assistance is switched off and a fault is stored in the steering assist control unit. The steering then continues to function as a conventional rack-and-pinion steering.

 With code (494) USA version the steering power assistance shutoff is indicated on the electric steering indicator lamp (A1e18) in the instrument cluster (A1).