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service self-study programme 225 the electro-mechanical power steering system design and function

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the electro-mechanical power steering system assists the steering movement performed by the driver by means of an electric motor this motor in turn drives a worm gear the speed-dependent steering system conveys a direct steering feel without any annoying feedback from the road to the driver this self-study programme explains this new technology and the differences compared with conventional hydraulic power steering systems the electro-mechanical power steering system is currently fitted to the lupo 3l tdi the secret of the electro-mechanical power steering system 225_097 new important note the self-study programme explains the design and function of new developments the contents of this ssp are not updated please always refer to the relevant service literature for all inspection adjustment and repair instructions 2

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table of contents introduction 4 the steering mechanism 8 system overview 12 the electronics of the steering system 13 functional description 18 function diagram 21 service 22 test your knowledge .23 3

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introduction the component parts of the steering column the main components of the new power steering system are the steering column switch the steering column tube the worm gear steering position sender and steering moment sender the electro-mechanical power steering motor the steering column electronics control unit and the universal joint shaft to the mechanical steering gear steering column switch steering column tube worm gear steering column electronics control unit electro-mechanical power steering motor universal joint shaft to mechanical steering gear 225_002 4

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what you need to know about the function of the electro-mechanical power steering system 1 the system offers the driver power-assisted steering in dependence on actual driving conditions 225_060 225_061 2 the steering movement of the driver is transmitted to the worm gear and steering gear via the steering shaft and an intermediate spindle 3 self-alignment of the steering wheel in the straight-ahead position is assisted by the electro-mechanical power steering system 225_062 4 the system conveys the feeling of contact with the road to the driver 225_063 5 the system monitors the input and output signals as well as operation of the component parts of the steering system 225_066 6 the safety steering column is adjustable for height and incorporates the proven crash concept pioneered in the lupo 225_065 7 it provides anti-theft protection through a lock on the steering shaft 225_064 5

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introduction overview steering column switch and steering-wheel lock universal joint shaft steering column tube with height adjustment control unit with sensors electric motor and worm gear steering gear 225_038 the entire electro-mechanical power steering system is integrated in a compact unit this unit comprises all component parts of the steering gear e.g control unit electric motor and the sensors required for control hence there is no longer any need for complex wiring arrangements.

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a electro-mechanical power steering system and a hydraulic system have totally different modes of operation by comparison the specifications emphasise the difference hydraulics weight power consumption urban cycle motorway cycle additional consumption compared to mechanical steering gear in litres per 100 km 16.3 kg electro-mechanical system 11.3 kg 400 w 800 1000 w 0.1l 25 w 10 w 0.01 referred to 44 kw sdi engine referred to lupo 3l with 1.2l tdi engine the hydraulic power steering system universal joint shaft in the hydraulic power steering system the system components after the universal joint shaft are engaged in the steering operation whereby steering is assisted by oil pressure 225_040 the electro-mechanical power steering system universal joint shaft in the electro-mechanical power steering system steering is assisted in front of the universal joint shaft in this system the assisting moment is generated by an electric motor 225_039

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the steering mechanism the steering column and its component parts the key component parts of the electro/mechanical power steering system are steering shaft steering column tube with height adjustment intermediate spindle torsion bar sensor housing with steering moment sender and steering position sender electric motor and coupling worm drive with worm and gear gearbox housing steering column electronics control unit and universal joint shaft sensor housing with electric motor steering moment sender and steering position sender coupling intermediate spindle torsion bar worm drive with worm and gear steering shaft steering column tube with height adjustment 225_017 universal joint shaft steering column electrics control unit

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the torsion bar assembly the central component part of the electro mechanical power steering system is the torsion bar it is made from tempered steel which allows the bar to rotate about its longitudinal axis the torsion bar mechanically connects the intermediate spindle to the worm drive shaft 225_044 intermediate spindle torsion bar worm drive shaft with gear wheel 225_043 ball bearing function through this connection the intermediate spindle and the worm drive shaft are able to counterrotate about a narrow angle this narrow angle is enough for the system to detect the start of a steering operation mechanical connection 225_026 225_025 twist of the torsion bar at the start of a steering wheel movement

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steering mechanism the worm gear is located in an aluminium gearcase where the electric motor is also mounted a worm on the motor shaft meshes with the gear on the steering shaft the gear ratio is 22:1 the gear body and the worm are made of metal the gear ring is manufactured from plastic to reduce mechanical noise 225_020 electric motor housing rubber coupling 225_018 worm gear shaft with gear wheel worm

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the height adjustment the height adjustment mechanism is securely connected to the steering column tube adjustment travel is 39 mm 225_003 the universal joint shaft the two forks of the drive shaft are connected to a short telescopic arm this telescopic arm allows length compensation of the height adjustment mechanism and affords the occupants protection in a head-on collision telescopic arm 225_009 225_023 if the steering wheel is adjusted upwards the telescopic arm is compressed i.e made shorter this reduces the distance between steering wheel and steering gear 225_021 225_024 if the steering wheel is adjusted downwards the distance between steering wheel and steering gear is increased and the telescopic arm is extended 225_022

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system overview steering column electronics control unit j527 sensors actuators steering position sender g268 and steering moment sender g269 electro-mechanical power steering motor v187 road speed signal from control unit with display unit in the dash panel insert j285 electro/mechanical power steering warning lamp k161 in the control unit with display unit in the dash panel insert j285 225_041 3-phase ac alternator d terminal 61

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the electronics of the steering system the sensor housing steering position sender g268 and steering moment sender g269 are located in a housing the sensor housing is mounted on the worm gear shaft above the gear the sensor housing is connected to the control unit via a 6-pin connector torsion bar sender housing worm gear shaft 225_010 steering position sender g268 is connected to the worm gear shaft it registers the steering wheel lock and/or the current position the steering steering moment sender g269 225_012 is connected to the torsion bar it registers a rotation angle of the torsion bar in relation to the intermediate spindle the control unit calculates a torque from this signal if the calculated torque exceeds a value of 0.01 nm the control unit assumes that a steering assistance is required 225_011

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the electronics of the steering system electrical circuit g268 g269 the two sensors are connected to the control unit by three separate wires effects of failure if the steering moment sender fails the system is deactivated if the steering position sender fails the »active self-alignment« function is deactivated in both cases the fault indicator lamp comes on j527 225_046 sensor design inner ring the two sensors are sliding contact potentiometers the steering position sender registers the steer angle of the steering system through rotation of the inner ring in relation to the housing lower section the steering moment sender registers the rotation of the torsion bar the housing has an inner ring this ring is mounted on the worm drive shaft with a clamping ring and can rotate relative to the housing two pairs of potentiometers scan the brushes of the inner conductor on the pcb in the housing this part is the steering position sender the other conductors transfer the signal from the steering moment sender potentiometer brushes housing lower section 225_010 potentiometer conductors 225_013 clamping ring

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the steering moment sender is mounted in the inner ring the inner ring is a plastic ring with two pairs of potentiometer brushes these brushes scan the four conductors in the inner ring the sender ring is connected to the housing cover it fits exactly on the head of the torsion bar when the torsion bar rotates the cover also rotates relative to the inner ring this movement is detected by the potentiometer brushes and transferred to the control unit as a signal along the printed conductors in the base of housing housing cover potentiometer brushes sender ring open inner ring 225_015 225_014 printed conductor

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