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Student Workbook

LV39 Steering Systems (3)

kap all phase 2 & 3 6/11/03 11:37 am Page 31

Student Workbook for Technical Certificates in

Light Vehicle Maintenance and Repair

MODULE LV39 STEERING SYSTEMS (3)

Contents

Page Page

…

Introduction: 3 Electronic Power Steering (EPS): 15

Objectives 3 Review of construction 15

Principle of operation 16

Hydraulic Type Power Steering: 3 Troubleshooting 18

Review of construction 3 Steering geometry 18

Hydraulic pump 4 ‘Ackerman’ steering 19

Exercise 1 4 Steering axis inclination (SAI) 20

Control valve 6 Camber angle 22

Exercise 2 6 Caster angle 23

Hydraulic piston 7 Toe angle 24

Principle of operation 8 Summary 25

Speed sensitive steering 9 Progress check 1 26

Troubleshooting 10

Diagnostic procedures 11 Rear wheel steering 27

Exercise 3 12

Exercise 4 13 Active rear wheel steering – 4WS 27

A road speed sensitive system 14 Progress check 2 29

Introduction

This is the third study module for power steering In this module we will focus

on the troubleshooting techniques that you will need to master in order to

resolve problems with power steering issues We will also introduce you to electronic power steering systems and how to troubleshoot these increasingly common systems We will also study how to assess the steering system for damage and alignment and review the steering geometry related to the

steering system Finally, we will introduce four wheel steering system (4WS) and review the basic principles used in these systems

Objectives

After studying this module, you will be able to:

• explain the common faults associated with power steering systems

• explain the steering geometry associated with steering systems

• describe the operating principles of a four wheel steering system (4WS)

Hydraulic Type Power Steering

The hydraulic type power steering system is currently the most common type used today, although the electronic type is quickly catching up and is likely to become the favoured choice in the years to come

Hydraulic type steering systems require three parts to function:

• hydraulic pump (pressure source)

• control valve

• hydraulic piston

Review of Construction

The detailed construction of the pump will vary from manufacturer to

manufacturer but they will all follow the same basic layout

to reduce the flow rate of the fluid in proportion to engine speed

The design shown uses the engine as its energy source The vane pump is driven by a belt from the engine For some vehicles, this can be difficult to

achieve, for example mid engine vehicles An alternative to the engine driven pump is the electro hydraulic pump studied in Phase 2 Steering Systems

LV29 Here we show this type of pump but again the basic function is the

same as before That is to provide a constant flow rate of fluid to the control valve A key difference between these two designs is the methods of flow

control The electro hydraulic pump does not use a flow control valve

Instead, the flow rate to the control valve is regulated by the speed of the

pump The speed of the pump is controlled by a steering ECU

Control valve

The control valve is installed on the steering gear shaft on the steering gear assembly Its function is to distribute the regulated flow of fluid from the pump

to the hydraulic piston The position of the control is determined by the input

of the driver Three types have been used depending on the construction of the steering gear assembly

Exercise 2

Using your knowledge from your study of Phase 2 Steering Systems LV29,

can you identify the control valve as shown and identify the components

Hydraulic piston

The hydraulic piston is integrated with the steering rack The piston chamber

is formed by the steering rack casing Hydraulic force is applied to both sides

of the piston As the control valve moves pressure is increased on one side and decreased on the other The balance of the pressure forces the piston to move and therefore the steering rack moves

Principle of operation

The operating principle of the hydraulic power steering is based on controlling the direction of fluid flow into and out off the hydraulic piston The control

valve is the key component that determines the passage of fluid

The control valve is constructed of two parts, one attached to the steering

column, the other attached to the pinion gear The shape of the two halves of the control valve create fluid passages which allow fluid to flow from the pump

to each side of the piston and back to the reservoir Between them is a

torsion bar

When there is no input from the driver, the two parts of the control valve are forced to neutral position by the torsion bar The fluid passages in the control valve are equal and as a result the pressure on each side of the piston is

equal When the driver turns the steering wheel the torsion bar allows the

outer section of the control to rotate relative to the inner section attached to the pinion In this condition the passages inside the control are no longer

equal One side of the hydraulic piston is opened to the reservoir and the

other is opened to the pump supply The pressure difference between each side of the hydraulic piston causes the rack to move

In summary, to operate the power assistance the driver has to move one half

of the control valve relative to the other The torsion bar determines the force required by the driver to operate the power assistance and the amount of

power assistance is determined by the difference between the two parts of the control valve

Speed sensitive steering

As you may recall there are two types of speed sensitive steering used:

• engine speed

• road speed

Engine speed sensitive steering is the simplest form The flow control valve in the pump is designed to reduce the flow rate of the fluid when the engine

speed is above certain value The assumption is that the vehicle will be

travelling at higher speed when the engine speed is higher

The road speed sensitive type is more sophisticated and will use an ECU to modify the flow rate in direct proportion to the road speed In either case the advantage of reducing the level of assistance when the vehicle is travelling at higher road speeds is that the vehicle stability is improved and a high level of assistance is maintained at parking speeds

Troubleshooting

To understand the troubleshooting methods that can be used for hydraulic

power steering systems we will first consider what type of faults could occur The likely faults fall into the following categories:

• assistance level too high

• assistance level too low

• noise

Before considering the possible causes within the power steering systems

what other factors must we consider as possible causes of steering defects?

Always consider the basic condition of the vehicle when dealing with any fault with a steering system

Let us now consider the possible defects within the power steering system

and how to diagnose them effectively

Assistance level too high – this is probably the least likely problem that you

will encounter Power steering systems are designed to fail in a way that

produces low assistance because of the danger of high-speed instability when the assistance level is too high However, it can happen so let us consider the possible causes

Consider which components and the type of failure that could lead to a high level of assistance Consider both engine and road speed sensitive systems

Assistance level too low – this is the safer failure condition although an

unexpected loss of assistance can still be dangerous As before consider the possible causes of this type of failure for both engine and road speed

sensitive systems

Noise – this is a very general type of problem but in the context of power

steering systems is usually the result of cavitations in the hydraulic fluid An example of cavitations is the noise heard when the power steering is held on full lock The noise heard is fluid cavitations in the pressure relief valve The trouble with this type of problem is identifying the root cause Sound is readily transmitted through a fluid, which means that the cavitation noise often

appears to come from the hydraulic pipes However, the pipes are rarely the root cause What are the likely causes of cavitation noise?

Diagnostic procedures

The primary diagnostic test available for hydraulic type power steering is the pressure test This usually involves fitting a pressure gauge and an isolation valve into the supply line of the pump The diagram shows the two stages of the test procedure The first stage is to measure the pressure with the

isolation valve open Measure the pressure at low and high engine speed and

at full lock Next, check the pressure with the isolation valve closed The

table below shows a series of results The values are examples for the

purpose of this exercise Always check the manufacturers’ specification for specific values

Exercise 3

Use your knowledge to complete the results table

Isolation Valve Open

1000

rpm

3000 rpm

1000rpm

& full-lock

Isolation Valve Closed

Failed Component and type of

Effective analysis of the pressure test results will allow you to identify the

majority of hydraulic problems that could occur in the hydraulic power steering system

One cause of cavitation noise in the hydraulic fluid is air trapped in the

system The air mixes with the hydraulic fluid forming small bubbles, as the bubbles pass through the control valve and pump they compress and noise is generated To test for air in the hydraulic fluid monitor the fluid level when the engine is running Turn the engine off and check how much the fluid level in the reservoir increases The level should rise slightly, up to 5mm depending

on the size of the reservoir and the volume of fluid in the system If the fluid level increase is more than 5mm then this indicates that air is present in the system

Exercise 4

Why does the air have this effect on the fluid level?

A road speed sensitive system

Road speed sensitive hydraulic steering systems introduce an additional

element to consider and test Road speed sensitive steering systems utilize a solenoid controlled by an ECU to alter the flow rate to the control valve The ECU will control the solenoid using a duty ratio type signal Invariably the

solenoid will be arranged in the hydraulic circuit so that if the electronic

system fails then the steering will be heavy

For example, the arrangement shown is designed so that increasing the duty ratio will close the valve and therefore increase the level of assistance If the duty ratio is reduced or the electrical signal fails, the valve will be held open and the assistance force will be reduced The value of duty ratio will be

determined by the ECU based on the road speed signal

To troubleshoot this type of system first establish if it is functioning The

simplest method to check the function of the system is to disconnect the

solenoid If the system is working then the steering will become heavier If the system is not working then the steering effort will be the same

The next stage is to confirm the input and output signals Shown are the two signals The most reliable method for measuring these types of signals is to use an oscilloscope

Electronic Power Steering (EPS)

EPS systems have become increasing popular with manufacturers They are more compact and can be easily adapted to many different vehicles by

adapting the control software

Review of construction

The EPS system uses an electric motor acting on the steering column The steering column is in two halves linked with a torsion bar The torque sensor measures the relative position of the two sections of steering column The

example shown uses optical type sensors An LED and phototransistor are

arranged either side of a shadow plate fixed to the steering column The

outputs from the phototransistors are processed by the sensor circuit to

produce a turning torque signal The ECU calculates the required assistance level and controls the motor It also provides a diagnostic system

Turning Torque Signal

Principle of operation

The EPS system provides the following control functions:

• road speed sensitive power assistance

• assisted steering return

Both of these functions are the result of the motor voltage control by the ECU The ECU determines the voltage supply to the motor based on the following data:

• force applied to the steering wheel by the driver

• vehicle speed

• steering angle position

• speed of steering angle change

Torque sensor measures the input force applied by the driver When the

driver turns the steering wheel the torsion bar between the two halves of the steering column will twist The amount of twist is proportional to the force

applied The output signal of the two phototransistors, will move out of phase The size of the phase change will be proportional to the twist in the torsion bar and therefore to the force applied by the driver

The torque sensor will also measure the steering angle position and the speed

of angle change The steering angle position is calculated by counting the

number of pulses from the phototransistor The speed of steering angle

change is proportional to the frequency of the signal from the phototransistor

The torque sensor circuit measures these three parameters and then outputs

a digital signal to the EPS ECU

The EPS ECU combines this data with the road speed to calculate the

direction and speed of the motor

Troubleshooting

Troubleshooting EPS systems is actually much simpler than you may think Invariably the motor, gear housing and torque sensor will be integrated with the steering column as a single assembly Diagnosis of a system defect

becomes a simple matter of confirming that the input signals, power source and ground are correct at the ECU Then confirm that the ECU connections between the torque sensor and the motor are correct The operation of the system will also be supported by diagnostic codes from the ECU allowing you

to identify the trouble area very easily

Steering Geometry

Correct steering geometry is essential to maintaining safe and consistent

handling characteristic Faulty steering geometry can at best lead to increased tyre wear and at worst dangerous handling Diagnosing steering geometry

faults is an important skill for any vehicle technician and a basic

understanding of the geometry angles and how to check them will ensure that you can successfully diagnose and repair this type of problem

must be arranged to turn the inside wheel through a larger angle than the

outside wheel The ‘Ackerman’ steering geometry provides a simple solution

to this problem Shown is the ‘Ackerman’ geometry First, draw two lines

from the front swivel joints so that they intersect the centre line of the vehicle between the rear wheels The track rod ends are then located along these

lines This geometry results in the inside wheel turning through a larger angle than the outside wheel, allowing the vehicle to travel around a curve without scrubbing on of the tyres

The ‘Ackerman’ principle defines the relationship between the connection

points of the top and bottom swivels and the track rod end connection point Damage in this area will lead to increased tyre wear usually in the form of

edge wear To diagnose this type of damage check the toe change between the inside and outside wheel as the steering angle increases This is often

called the toe out on turns