Electric power steering (ES), function

P46.35-2007-06

A91 Electromechanical controller unit (ES)

A91b1 Electric power steering (ES) torque sensor

A91m1 Electric power steering (ES) actuator motor

N68 Electric power steering (ES) control unit

Basic function

Modern power steering requires continuously-variable speed-dependent closed-loop control of the steering servo. This is achieved with electric power steering (ES).

Other advantages of ES:

 Improved steering feel

 Fuel saving

 No steering oil required

 Compact design

 Speed-dependent power assistance

 Steering return is assisted

 Diagnosable

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ES is based on a conventional rack and pinion steering gear. The electric power steering (ES) actuator motor (A91m1) supports the driver's steering torque via a worm gear at the drive pinion of the rack-and-pinion steering, when this is necessary. The direction of rotation of the electric power steering (ES) actuator motor (A91m1) depends on the direction of movement of the steering wheel.

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Function requirements:

 Circuit 15 ON

 Engine speed >300 rpm

The electric power steering (ES) control unit (N68) requires the following information to calculate the steering power assistance required:

 The currently used steering torque, from the electrical power steering (ES) torque sensor (A91b1)

 The current wheel speeds, over the Controller Area Network bus class C (engine compartment) (CAN-C) from the ESP and BAS control unit (N47-5)

 The steering angle and steering speed from the steering angle sensor (N49) in the steering column module (N80) via the CAN C

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 The current engine speed from the ME-SFI [ME] control unit (N3/10) or CDI control unit (N3/9) via the CAN C

 The current rotor position of the electric power steering (ES) actuator motor (A91m1) from the motor position sensor in the electromechanical controller unit (ES) (A91)

The electric power steering (ES) control unit (N68) uses the information to calculate the steering force support to be applied from the stored performance map and activates the electric power steering (ES) actuator motor (A91m1) accordingly.

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Steering assist function as of 1.12.05

The steering assist function is intended to help the driver achieve the ideal steering ability when in a critical situations. The steering assist function is realized by means of the electrical power steering (ES) control unit (N68) and additional software in the ESP and BAS control unit (N47-5). The following functions are performed here:

 Driver support or stabilization through countersteering of a vehicle with oversteer

 Driver support when braking on one-sided road surfaces

The ESP and BAS control unit (N47-5) forwards the "Steering assist torque request" message over CAN-C to the electrical power steering (ES) control unit (N68).