Self-Study Programme 259 EPHS.pdf

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Service.
Self-Study Programme 259
EPHS – Electrically Powered
Hydraulic Steering
Design and Function
EPHS – Electrically Powered
Hydraulic Steering
259_033
The EPHS = Electrically Powered Hydraulic
Steering is familiar from the Lupo FSI.
This system is supplied by TRW and by KOYO.
The new steering system offers a wide range of
advantages while at the same time retaining the
excellent steering properties of the conventional
hydraulic power-assisted steering.
Advantages of the EPHS:
Improvement in comfort,
easy to operate when parking,
but steering firm at high speeds
(safety factor).
Fuel savings,
as the energy consumption is based on demand
– irrespective of the operating state of the
internal combustion engine.
NEW
Important
Note
The Self-Study Programme describes the
design and function of new developments!
The contents are not updated.
Please always refer to the relevant
service-literature for up-to-date inspection,
adjustment and repair instructions.
2
At a Glance
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
System Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Design and Function . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Function Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Test Your Knowledge . . . . . . . . . . . . . . . . . . . . . . . . . .24
Subject Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26
3
Introduction
The pressure required for the steering servo
assistance, is produced by a hydraulic pump.
This pump is driven directly by the vehicle
engine, as in the conventional, familiar power-
assisted steering system.
In other words, part of the engine power output
is required for driving the pump.
At the moment at which the greatest steering
servo assistance is required – mainly when
parking – engine speed is at its lowest.
The pump capacity is designed for such a case.
The faster the steering rate, the higher is the
pump speed and thus the volume flow.
Pump capacity which is not required at higher
engine speeds, is dumped via a bypass.
In the new steering system, although the
hydraulics likewise assist human steering effort,
the hydraulic pump – a gear pump – is driven in
this case by an electric motor and is mechani-
cally independent of the vehicle engine.
The hydraulic control is of identical design.
A new feature is the steering servo assistance
based on the steering angle rate and vehicle
speed.
A steering angle sensor is additionally integrated
for this purpose in the housing of the rotary disc
valve (refer to Fig. below), which transmits the
steering angle rate to the electronic control.
The steering angle information flows along a
sensor cable directly to the control unit.
In addition, the vehicle speed is detected in the
control unit during the analysis.
This information is supplied over the CAN data-
bus.
Power steering sensor G250
Steering gea
Reservoir
Power steering
control unit J500
Gear pump with pump motor
259_028
4
The steering system fitted to Volkswagen models
is supplied by TRW and by KOYO.
The operating principle of both steering systems
is identical. The steering systems differ in how
they determine the steering angle rate. This is
already recognizable from the external shape of
the steering angle sensors.
In order to be able to see the power steering
sensor in the vehicle, it is necessary to raise the
vehicle and to turn the steering to full right lock
(see arrow in Fig. on right for direction of
viewing).
259_043
The middle illustration opposite shows a TRW
steering fitted in the vehicle. It is recognizable
from the power steering sensor which is flat and
wide in design (more detailed description from
page 13 on).
259_002
The lower illustration shows a KOYO steering
installed in the vehicle. Its power steering sensor
is cylindrical in shape (more detailed description
from page 15 on).
Certain components of the two steering
systems differ from each other and cannot
be interchanged. This applies both to
electrical/electronic, as well as to purely
mechanical components, such as track
rods and track rod ends.
259_001
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