Download Automotive Engineering Higher Vehicle Management Systems 9049

Automotive Engineering
Vehicle Management Systems
Higher
9049
.
Autumn 2001
HIGHER STILL
Automotive
Engineering
Vehicle Management Systems
Higher
Support Materials
This publication may be reproduced in whole or in part for educational purposes provided that no
profit is derived from the reproduction and that, if reproduced in part, the source is acknowledged.
First published 2001
Learning + Teaching Scotland
Northern College
Gardyne Road
Broughty Ferry
Dundee
DD5 INY
Tel. 01382 443 600
TECHNICAL SECTION OF UNIT SUPPORT DOCUMENT
Vehicle Management Unit
Lecturer/Teacher support material
This section of the support document is designed to provide the subject specialist with
material which supports the unit delivery. This includes the following detail:
•
Unit co-ordination
•
Unit Outcomes
•
Support Material Outcome 1
Lesson Plan
Analogue and Digital Signals
Sensor representation
Sensor signal production
•
Support Material Outcome 3
Signal Processing
Analogue to Digital Conversion
The ECU
Handling Data
•
Support Material Outcome 2
Actuators
Magnetism
Linear actuators
Rotary actuators
Stepper motors/solenoids
•
Support Material Outcome 4
System investigation
System operation
Components
Selected system operation
System testing
Investigation
•
Publications
The outcomes are not in a linear progression in this support pack, rather the
outcomes are in the sequence expected in the normal course of teaching and
learning.
Automotive Engineering Vehicle Management - Higher
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UNIT CO-ORDINATION
It can be determined from the unit outcomes and performance criteria that there are
four broad-themed areas to be covered as part of this 40-hour unit: sensors, actuators,
electronic control and system investigation. Prior to any in-depth consideration, it may
be beneficial to firstly consider a simplified approach to the concepts of sensing
physical conditions, processing the sensor information and finally adjusting or
maintaining a system operation by the use of an actuator. In addition, it may also at
this stage be beneficial to make reference to the range of systems found on modern
vehicles which employ such electronic controls in order that the candidate can judge
the scale of the importance of such systems.
This support document is concerned with the content and delivery of the Vehicle
Management Systems Unit, however where the intention is to deliver this unit as part
of the Higher Automotive Engineering course, then it would be a useful exercise for
the lecturer/teacher to familiarise themselves with the additional knowledge
requirements of the course approach. One example of such an approach which
provides cross unit integration and at the same time improves time management of
unit/course delivery can be found by utilising the operation and investigation of AntiLocking Braking systems within this unit. Integration is achieved due to ABS being an
extended knowledge aspect of the Secondary Chassis System unit where that unit is
delivered as part of the course.
VEHICLE MANAGEMENT SYSTEMS
The Outcomes:
Outcome 1
Explain the relationship and operation of sensors employed in vehicle systems with
reference to the physical conditions they measure.
Performance Criteria:
a. A range of sensors used in vehicle management systems is accurately identified.
b. Output signals from a range of sensors used in vehicle management systems are clearly
and accurately drawn.
c. An explanation of how signals are produced by a range of sensors used in vehicle
management systems are explained clearly, correctly and comprehensively.
Outcome 2
Identify and explain the function and operation of actuators employed in modern vehicle
applications.
Performance Criteria
a. The functions of a range of actuators employed in the adjustment of settings of vehicle
management systems are clearly and accurately explained.
b. The operations of a range of actuators employed in the adjustment of settings of vehicle
management systems are clearly and accurately explained.
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Outcome 3
Demonstrate an understanding of the signal processing undertaken by the electronic control
unit.
Performance Criteria
a. The difference between analogue and digital signals is clearly and correctly explained.
b. The way data is stored, compared and processed within the electronic control unit is
correctly identified.
c. Information from a three-dimensional map is read accurately.
d. A block diagram of the main components employed within the electronic control unit
showing the communication links between them is clearly and accurately set down.
e. The inputs and outputs to and from the electronic control unit for a given system are
clearly and accurately identified.
Outcome 4
Use diagnostic equipment to record readings from a chosen electronic vehicle system in
accordance with manufacturers’ procedures.
Performance Criteria
a. An understanding of sensors and actuators employed in a named system and how they
effect system operation is clearly and correctly demonstrated.
b. Diagnostic equipment is used accurately and effectively to record readings from system
sensors.
c. An oscilloscope is used to view signals generated by named sensors accurately and
effectively.
d. Diagnostic equipment is used to record readings transmitted by the ECU to system
actuators accurately and effectively.
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LECTURE SUPPORT MATERIAL
Outcome 1
Lesson plan (sensors) Outcome 1
On conclusion of this outcome the student will be expected to have a knowledge of a
range of sensors, what physical conditions they measure and how the measurement is
turned into a signal. The approach shown here is only one suggested plan to the
delivery of the subject matter, although this outcome is concerned with sensors it may
prove useful to relate the sensors to discrete vehicle systems.
•
•
Introduce electronic systems
Introduce the concept of a system
A system can be defined as an ordered arrangement of physical objects. The system
can be arranged into three basic elements: input, processor and output.
The input signal is usually the cause of a change in the system and the output action
which occurs due to the input signal is referred to as the effect. The response of the
system to the input signal is known as the process where the input is processed to
provide a determined and desired output. The information provided by the sensor can
either in the form of an analogue or digital signal.
Analogue and digital signals
The temperature sensor/ thermistor employed within an engine system provides an
output of information which is delivered by means of an analogue signal. There are
three basic types of electrical signal: variable or linear, pulse or trigger and fixed level
or digital. The majority of electrical signals are continuous but variable quantities. A
speedometer of a vehicle which has a needle that sweeps across a fixed scale provides
a continuous signal that can change by small amounts, such a signal is referred to as an
analogue signal.
Pulse or trigger signals are employed mostly in timing or switching circuits, where a
voltage pulse or sudden rise is the initiation signal for a set of operations to commence.
Digital quantities are expressed in fixed levels as whole numbers, so when applied to an
instrument, the read-out can only alter when the value has changed by a set amount. A
digital watch is an example of this type of measurement. If the unit of time used is the
minute then the read-out on the watch face will only change each time a minute passes,
a second will not alter the face reading. On modern vehicles employing programmed
electronic control units digital language is employed within the ECU and therefore
analogue signals produced from sensors must be converted into digital signals.
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Sensors
The majority of modern vehicles employ sensors to measure the following physical
conditions:
• movement/speed
• pressure
• position and level
• flow
• temperature
• gas composition
• vibration/knock
A number of different sensors can be utilised to measure each of the above quantities.
Sensor representation
Sensors should be introduced at this stage in terms of what they measure, signals
produced, method of measurement, use in systems and graphical representation.
Employing the table below the candidates attention should be drawn to the fact that, in
some cases the same sensing principles can be employed to sense a number of physical
quantities, e.g.: the Hall sensor can be utilised to measure, speed, position and absolute
pressure. In addition a number of different sensing media can be employed to sense the
same physical quantities.
It would also be useful to draw the candidates attention to the fact that in some cases a
combination of different sensors may be required in order to provide accurate data
relative to the physical quantity to be measured, e.g.: In some engine management
systems an air flow meter combined with an air temperature sensor is employed in
order that air mass can be determined.
The data in the table below is not intended to represent all the various types of sensors
to be found as part of a vehicle management system, only the more common sensors
are included. It is not the intention of the vehicle management unit or this pack to
cover all sensor types. Where a particular centre utilises certain vehicle management
systems as part of their teaching hardware, then the addition of other sensor types may
be beneficial.
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ABS wheel speed sensor
Engine speed sensor
Crank angle sensor
Engine speed sensor
Manifold absolute pressure
sensor
Speedometer/vehicle speed
Engine speed/ distributor
Movement/Speed Electromagnetic Induction
Hall voltage
Light sensing/photo cell
Movement/speed
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Application/System
Principle of Measurement
Condition
Measured
Sensors/Quick Reference
Signal Generated
Four
Two
6
Sensor
Representation
diagram
page:
One
TDC Sensor
Throttle position sensor
and rate of throttle
movement
Ignition trigger/distributor
Electromagnetic induction
Potential divider
(potentiometer)
Hall
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Position
v
0
Position
Two
Three
7
Hot wire
Photo cell
Potential divider
(potentiometer)
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Flow
Air flow meter/fuel system
Fluid flow/ fuel
Air flow meter/fuel system
Air-flow increase-----Temperature decrease-----
0
I
Five
Four
Three
8
Thermistor (air)
Thermistor (NTC sensor)
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Temperature
Engine air intake
temperature
Engine coolant
temperature
Six
Six
9
Combustion/engine knock
sensor
Vibration/Knock
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Piezo cell
Exhaust gas oxygen sensor
Gas Composition Catalyst
Eight
Seven
10
Hall
Piezo cell
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Pressure
MAP sensor
Engine manifold pressure
(manifold absolute
pressure)
MAP sensor
0
mm Hg
0
Pressure-------------------
V
V
Two
Eight
11
Reed switch (magnetic)
Capacitor
Variable resistance
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Level
Oil level (dipstick switch)
Coolant/washers
Brake fluid level
Fluid level/ washers
Fluid level
coolant/washers/fuel
Headlamp leveling
0
V
0
C
0
R
t
Fluid level
Movement---------
NINE
Ten
Eleven
12
SENSORS
Sensor Representation Diagrams
Due to copyright constraints we have been unable to publish data sheets relevant to
vehicle management sensors systems. These data books/sheets are readily available in
all centres. Please refer to the most recent editions of the following publications;
Electromagnetic Inductive Sensors
Bosch, R. Technical Training Book: Electronic Injection Systems, Potential
Divider (potentiometer)
Bosch, R. Technical Training Book: Electronic Injection Systems, Lambda
(oxygen) Sensor
Bosch, R. Technical Training Book: Electronic Injection Systems, Piezo Cell
(strain gauge)
Bosch, R. Technical Training Book: Electronic Injection Systems,
Toyota Vehicle Electronics Training Pamphlet 35042E,
Toyota Switch Type Sensors
Bosch, R. Technical Training Book: Electronic Injection Systems,
Bosch, R. Technical Training Book : Electronic Injection Systems.
Variable resistor type sensor
Mellard, Automotive Electronic Systems, Heinemann
Thornes, S. Automotive Electronics, Hillier. Photo Cell (photo diode, LED,
photo transistors)
Toyota Vehicle Electronics Training Pamphlet 35042E, Toyota Thermistor
(NTC/PTC)
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Sensor Signal Production
Following on from the process of familiarisation of sensor types, the systems in which
they can be found and the physical conditions they measure, the next stage is involved
with understanding how the sensors convey these physical quantities into signals.
The sensors to be considered must include the sensing of the following physical
conditions:
• Movement/speed
• Pressure
• Position/level
• Flow
• Temperature
• Gas composition
• Vibration/knock
Ideally an explanation of the principles of sensor operation together with a detailed
account of the sensor operation and function linked to some form of practical
investigation would provide the most appropriate learning platform. The centre can
chose a group of sensors which cover each of the physical conditions to be measured
and at the same time cover all the common types of sensor. The following provides
one example of this approach:
Physical Condition Measured
Speed/movement (position can also be
included)
Position
Temperature
Sensor Type
Electromagnetic
Hall
Thermistor (NTC)
Gas Composition
Exhaust Gas Oxygen (Lambda)
Vibration/knock (pressure can also be
included)
Piezo Cell (Wheatstone bridge)
Flow (position can also be included)
Level (position)
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Potentiometer
Switch (Reed)
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Sensor Explanation and Investigation
Movement (electromagnetic) Speed/Position
When considering this type of sensor the principles of electromagnetic induction
should be considered together with the support of basic experiments employing
permanent magnets.
As part of the sensor explanation the following points should be included:
• Uses of electromagnetic inductive type sensors: crankshaft, camshaft, ABS wheel
sensor, vehicle speed sensor and with alterations to the teeth arrangement on the
reluctor ring as a position sensor for, fuel and ignition timing. (the respective
signals are illustrated in the sensor quick reference table)
• Induced voltage is proportional to speed
• Signal frequency in direct proportion to rotational speed of reluctor
• Varying reluctance produces signal
• Improved signal amplitude with ‘DC’ exited inductive sensor
• Generated output signal with reference to reluctor position as illustrated in
Thornes, Automotive Electronics, Hillier
Investigation
To enhance the learning experience the following candidate investigations may be
incorporated in support of the sensor operating explanation.
•
Use of inductive coils and magnets to demonstrate electromagnetic induction, a
Galvanometer can be employed to indicate induced voltage where permanent
magnets and low inductive speeds are evident.
•
Employ permanent magnetic pick-up type ignition distributor (amplifier
disconnected) connect an oscilloscope between the two output cables, crank
engine (or drive model) an output signal similar to that shown below should be
viewed.
Distributor
• Employ an ABS wheel speed sensor and oscilloscope. Connect the oscilloscope
between the sensor output cables and rotate road wheel (view signal amplitude
and frequency relative to wheel speed).
•
Employ engine TDC or other position type inductive sensor. As before connect
the oscilloscope between the sensor output cables, highlight difference in
generated signal from those previously viewed. Prompt candidates as to the likely
effect of reluctor teeth being bent. Illustrated in Autodata.
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Position Sensor (Hall)
Employing the diagrams and accompanying explanation provided within the Sensor
Representation sheet 2, emphasise the following points:
• Unlike the permanent magnet inductive type sensor the Hall sensor requires a
supply current
•
The generated ‘Hall Voltage’ is in direct proportion to the magnetic flux strength
and the current flowing through the Hall cell
•
Proximity of magnetic flux to Hall pick-up relative to the signal generated, as
illustrated in Bosch, Technical Training Book : Electronic Injection Systems
•
As shown in Bosch, Technical Training Book : Electronic Injection Systems
position B represents vane or screen in position illustrated in Hall generator
diagram. Voltage drops to ‘A’ when the vane is between the Hall IC and the
magnet)
•
The Hall sensor can detect zero motion
•
A square waveform is generated
Investigation
By employing a ‘Hall type’ ignition distributor in conjunction with an oscilloscope and
a voltmeter the following investigations can be performed:
Oscilloscope
The switching voltage of a ‘Hall’ effect pick-up can be displayed using an oscilloscope
by connecting the probe to the distributor ‘0’terminal (usually the centre terminal of
the set of three), but keeping the distributor connected to the amplifier. The
oscilloscope earth lead should be connected to a suitable earth point on the engine or
distributor body. When the engine is cranked by the starter motor a square wave
should be evident indicating a voltage between 4 to 8 volts as indicated in the diagram
shown in Autodata.
If the engine speed is increased to around 3000 rpm the wave pattern should be as
illustrated at ‘B’ within the above mentioned diagram.
Voltmeter
The ‘Hall’ effect pick-up can also be tested as using a voltmeter as follows:
Refer to the diagrams illustrated in Autodata:
Connect the meter negative lead to a suitable earth point, connect the voltmeter
positive (+) to the disconnected wiring plug ‘0’ (diagram 1), switch on the ignition and
check the voltage. A reading of between 4 to 8 volts should be indicated, this is the
switching voltage. Transfer the positive (+) meter lead to the (+) plus terminal of the
distributor and check the voltage (diagram 2) this should read between 10 to 12 volts,
this is the supply voltage.
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Temperature/thermistor (NTC)
Most electrical conductors have a positive temperature co-efficient, this means that the
hotter the conductor the higher its electrical resistance. The thermistor or NTC sensor
has a negative temperature co-efficient, its resistance reduces in direct proportion to a
reduction in its temperature. This provides a well-defined relationship between
temperature and electrical resistance variance, as indicated in the table below.
Temperature/Centigrade
20
40
60
80
90
•
Resistance (Ohms)
38000
1600
7500
3800
3000
Emphasise the importance of temperature measurement for both coolant and air
for efficient engine operation.
Investigation
The candidates can perform a basic test of the negative temperature co-efficient sensor
relative to electrical resistance by, gradually increasing the temperature of the
thermistor whilst monitoring the temperature rise with a thermometer and taking
readings from an ohm meter.
Gas Composition/Exhaust Gas Oxygen Sensor/Lambda
The candidates should be provided with knowledge of the construction of this sensor
with some detail as to how levels of oxygen in the atmosphere are compared to oxygen
levels within the exhaust gas. Reference should be made to the application of this type
of sensor within an engine management system and its use as a feedback sensor for
monitoring and fine tuning air/fuel ratios.
An explanation should be provided for the candidate which includes:
• An exhaust gas oxygen sensor indirectly measures the air/fuel ratio by sensing the
amount of oxygen in the exhaust gas relative to atmospheric oxygen levels
• The zirconia oxide sensor uses zirconia oxide sandwiched between two platinum
electrodes. One electrode is exposed to exhaust gas and the other is exposed to
normal air for reference.
• Because the exhaust contains fewer oxygen ions than air, the ‘air’ electrode
becomes negative with respect to the ‘exhaust’ electrode. The voltage developed
across the electrodes depends upon the number of oxygen ions present in the
air/fuel mixture.
• An ideal exhaust gas sensor would have an abrupt, rapid and significant change in
output voltage as the mixture passes through ‘stiochiometry’. The output voltage
would not change as exhaust gas temperature changes.
•
Exhaust gas oxygen sensors are not used for ‘feedback’ control when the
temperature of the exhaust gas is below 300o Centigrade.
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Investigation
The only practical investigation of how this sensor operates may be gained from
connecting a voltmeter across the sensor with a thermometer monitoring the exhaust
gas temperature and adjusting the air/fuel ratio of the engine between ‘rich’ and
‘weak’. As the oxygen content of the exhaust reduces (rich mixture) a rapid rise in
voltage output should occur. As the oxygen level increases (weak mixture) the
candidate should note a rapid reduction in voltage. A steady voltage reading should be
viewed when Lambda =1.
Vibration/knock sensor/Piezo strain gauge (pressure sensor)
Both the physical conditions of vibration and pressure can be monitored by employing
a Piezo cell or strain gauge which incorporates Piezo resistors in a wheatstone bridge
formation. The method of transforming the physical condition being measured to the
application of mechanical stress applied to the Piezo resistors should be discussed.
The common factors applied to the strain gauge as a knock or pressure sensor should
be explained to include:
• Piezo electric devices produce voltage when the ceramic crystals are subjected to
mechanical stress
• Flexing of diaphragm relative to voltage division across the wheatstone bridge
• Why/where pressure is monitored
• Why/where vibration or knock is monitored.
The piezoresistors in the MAP sensor are formed into a Wheatstone bridge formation.
The resistors are made from a material in which the resistance changes, with a change
in the twisting or bending force, due to stress in the material, thus providing a variable
resistance relative to the mechanical stress produced in the flexible diaphragm. The
wheatstone bridge is supplied with a constant voltage, and a current of approximately
1 mA. When there is no strain on the diaphragm all four resistances are equal, the
bridge is balanced and the output from the bridge is zero. As the stress of applied
pressure (vibration for knock sensor) causes the diaphragm to deflect, the resulting
strain on the piezoresistors causes their resistance to change in value, in such a way
that r1 and r4 increase in value proportional to pressure (resonance for knock sensor)
because they are being compressed and at the same time r2 and r3 decrease in value by
the same amount as these resistors are being elongated. This unbalances the bridge
and a voltage difference is created across the bridge output terminals A/B. This output
voltage is in proportion to the flexing of the diaphragm.
Knock Sensor
Knock sensors are normally located between the middle cylinders screwed into the
cylinder block, they are generally tightened to a specific torque in order that the strain
gauge diaphragm is pre-loaded and will act like a spring when the correct resonant
frequency is applied.
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Investigation (knock sensor)
The candidate investigation of the knock sensor operation will require reference to
manufacturers test requirements and thus will vary between manufacturers. Four
possible tests are fairly common between manufacturers, these are:
• Engine off, connect voltmeter between sensor terminals, tap the sensor securing
bolt, voltage should read approximately 0.1 Volts
• Connect voltmeter between sensor terminals, run engine up to 2000 rpm,
Voltmeter should read approximately 0.1 Volts
• Connect voltmeter between sensor terminals, note reading at idle, open throttle
quickly, note voltmeter reading, a voltage rise should be evident
• Employ an Ohm-meter to check resistance with manufacturers specification
Air Flow Meter Potentiometer (also as position sensor)
The principle of operation of a potentiometer as a measuring device should initially be
considered as a voltage divider with experiments incorporating the use of voltmeters,
followed by reference to the practical use of the potentiometer in Auto Engineering, to
include its use as a:
• Throttle position sensor
• Air flow sensor
• Level sensor.
The potentiometer is usually employed to measure either linear or rotary motion. If a
potentiometer were to be used to measure throttle position, then it would probably be
attached at one end to the throttle spindle. As the throttle is opened, the rotation of
the throttle spindle moves the wiper, which moves along either a wire wound resistor
or more likely a track of thick film ceramic-metal mixture. The computer sends a
reference voltage to one terminal of the potentiometer, point A. If the wiper is
positioned near point A (Wide Open Throttle), there will be a low voltage drop
between points A and B (low resistance) and a high voltage drop will exist between
points B and C. When the wiper is positioned near point C (idle), there is a high
voltage drop between points A and B and a low drop between points B and C. The
computer monitors the voltage drop between points B and C and interprets a low
voltage, 0.5 for example for idle position. A high voltage, around 4.5 Volts will be
interpreted as full or wide-open throttle. Voltages between these positions will be
determined as mid throttle settings.
Air Flow Meter
This flow meter senses the air flow rate by measuring the angle of deflection of a flap
or vane. The mechanical movement of the vane is translated into an analogue signal by
the potentiometer. The sensing vane or flap, placed in the air intake is spring loaded to
oppose the force of the incoming air flow. Flap pulsations, caused by the irregular air
flow are dampened by a small chamber and compensating air flap. The flap moves a
slider over a thick film potentiometer which provides the voltage signal. Using a thick
film resistor minimises the effect of temperature changes and by varying the resistance
of each resistor segment of the potentiometer, a linear relationship can be obtained
between the sensors output voltage and the fuel to be injected.
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Investigation
All potentiometer type sensors should have a smooth incremental increase in voltage
output reading with respect to the movement of the potentiometer mechanism. The
following investigations are based upon checking the operation of an air flow meter
and a throttle position sensor following general test procedures.
Throttle position Potentiometer
Checks:
• Connect analogue voltmeter between potentiometer output terminals
• Turn ignition on take note of reading throttle closed, reading for a 5 volt supply
would be in the region of 0.1volts
• Open throttle to full throttle position reading should be in the region of 5.0volts
• Gradually move throttle from closed to fully open position, readings should either
rise with movement or move up in incremental stages depending upon the type of
thick film track used (segments or continuous)
Air Flow Meter
Checks:
The operation of the potentiometer can use either a voltmeter or an oscilloscope.
Oscilloscope Trace
An oscilloscope can also be connected to the potentiometer. The low voltage test lead
of the oscilloscope is connected to the potentiometer (on a Bosch LE-Jetronic system
this would be terminal 7), and make an earth connection to the engine with the
oscilloscope ground lead. As the air flap is operated manually (ignition on) checks
should be made for any evidence of ‘hash’ or irregular voltage readings in the signal.
The diagram above indicates the type of signal to be expected on the oscilloscope.
Alternately the voltmeter may be employed similar to its application on the throttle
position check. The following is an example of such a check performed on a Bosch
Motronic 1.7 fuel system.
Checks:
With voltmeter connected between pins 2 and earth and engine running:
• At idle reading should be: 0.1 - 0.3 Volts
• Engine running at 3000rpm reading should be 5.0 Volts
• Voltage change should be viewed as a smooth incremental progression.
Switch type sensor/position/level (reed switch type)
The reed switch is a magnetically sensitive switch which can be utilised as either a
simple position switch or when combined with other reed switches as a level sensor.
The reed switch consists of two or more contacts mounted within a glass vial to
exclude contaminates. The vial is evacuated of air or filled with an inert gas to reduce
damage by arcing. When used as a position or proximity sensor the switch is operated
by a permanent magnet. Exposing the reeds of the switch to a magnetic flux
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causes each reed to take up the polarity of the magnetic pole nearest to it. Since the
reed polarities are opposite, they will be attracted together this will close the switch
and allow the flow of current to create the sensor pulse.
Investigation
The signal generated by this type of sensor is digital in format in that the circuit is
either closed or open, On or off, which produces a square wave form, this is the case
for all switch type sensors. This basic signal can be viewed by connecting a voltmeter
across the switch contacts and viewing the meter reading as the switch position
changes from ‘on’ to ‘off’.
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Outcome 3 Signal Processing
Following on from the investigation of the common sensors to be found on modern
vehicles, which produce either analogue or digital signals, the next section considers
the natural progression of this signal information by considering the processing of the
included data. As stated in ‘Introduce Analogue and Digital Signals’ it should be
reinforced that digital signals have only two levels ‘high’ or ‘low’, 1 or 0. This is
referred to as a digital or binary system and the ‘signal state’ can only alter between
these two levels when the value of the voltage changes by a predetermined amount.
The analogue signal is continuously variable and shows every alteration in voltage no
matter how small the incremental or decremental change.
Binary and denary numbering
The numbers in every day use are based in denary notation; this uses the ten digits
between 0 and 9. Binary notation is based on two digits 0 and 1with every digit
representing a ‘power of 2’.
Hence the binary number of 1111 = 15 and 0001 = 8 with 1011 = 13
Logic circuits
Binary notation allows any denary number or analogue signal to be represented as a
series of zero’s and ones. Electrical circuits which respond to signals based on these
two digits are called binary or logic circuits. Such circuits respond to two voltage
levels or logic levels, commonly these are:
• logic 1 which in analogue or denary terms is a voltage between 2.4 and 5 volts and
• logic 0 which in analogue or denary terms is a voltage between 0 and 0.8 volts.
Electronic processors such as those employed as the control function of modern
vehicle electronic circuits operate on this digital or logic language. The analogue
signals must therefore be converted into digital before the processor can read the
information carried by the signal. Such analogue signals are converted into digital by
an analogue to digital converter (an ‘A to D’ converter).
When the analogue voltage exceeds 2.4 volts the digital signal switches to the ‘high
state’ or 1, and remains at this state until the analogue voltage drops below 0.8 volts
the ‘low state’ or logic 0.
The Microcomputer (electronic control unit ECU)
The ECU receives a range of signals from the various sensors which are monitoring the
dynamics of the system and provide the input data. After comparing these input
signals with a stored programme of instructions, the control element of the ECU, the
microprocessor performs a required duty which can produce an output signal to the
actuators in order that a change to the system can be achieved.
A microcomputer such as the ECU employed on vehicles stores information on a
temporary basis within the microprocessor registers. Information is transferred
between the processor and the memory or input/output sections by means of one or
more sets of multiple wires; each set being referred to as a ‘bus’. An example of this
can be seen in Mellard, Automotive Electronic Systems, Heinemann
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ECU Internal Components and Lines of Communication
Input Stage
Pulse Shaper
Removes ‘hash’ and smoothes the input signal.
Analogue to digital converter
Represents constantly variable signals in binary code in order that the data can be read
by the processor.
Multiplexer
The multiplexer receives all the inputs as they are produced by the various sensors
from around the system and holds the information within each signal until the
processor requests the information.
Microprocessor CPU (Central Processing Unit)
The CPU has the ability to store instructions temporarily while it is processing other
data. Information waiting to be processed is held in temporary storage units known as
registers. The data received by the CPU is firstly separated and then stored in the
appropriate register until required by the control unit. Illustrated in Thornes, S.
Automotive Electronics, Hillier
The programme counter is the main or referencing register which locates the various
pieces of data stored in the different registers when it is required to be processed. The
unit within the CPU which performs all arithmetic calculation is known as the ALU
(Arithmetic and Logic unit), all data supplied to the CPU requiring addition or
subtraction (comparison) is performed by this unit. As these arithmetic processes are
being performed the ALU stores binary data temporarily within the accumulator as
other calculations are being performed. The command unit that directs the processing
operation is performed by the control unit. This unit arranges the movement of data
between the various sections of the ECU and provides the appropriate control signals
to activate the parts that actually process the data.
Memory
The memory function of the ECU must provide the CPU with the predetermined
instructions written by the manufacturer which determine the operations of the CPU
relative to the input data received from the sensors. A memory consists of a number of
separate cells which store the ‘bits’ of binary information so that they can be read by
the processor when it is required. To enable the ECU to operate in a set way, it must
be given a description of the tasks it has to carry out. This description is known as the
software and it is conveyed to the CPU by the ‘program’. The program is a list of
instructions held within the memory unit that informs the CPU how to process the
software. The memory unit is separated into two parts; one part stores the program
for the CPU and the other holds the information data either as an input to the CPU or
as an output from the ECU.
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Read Only Memory (ROM)
The ‘read only’ memory is part of the ECU hardware in that the contents of this
memory hold the operating parameter instructions which the CPU employs when
determining an output with respect to input data. This memory cannot be written to by
the CPU and is retained even when the power to the ECU is interrupted.
Random Access Memory (RAM)
Many modern vehicles require the ECU to include additional memory space in order
that the larger number of pieces of information computed can be stored temporarily
and updated as system dynamics alter. This is additional memory to that found within
the CPU. The RAM will be lost if the power supply to the ECU is disrupted, some
ECU’s include internal battery backup to protect the stored data. Such data may relate
to service interval information or recorded faults found within the systems being
monitored by the ECU.
Output Stage
The output stage or power stage of the ECU will increase the voltage levels to those
required to operate vehicle sensors and due to the majority of such sensors requiring an
analogue signal the output stage will also convert the digital /binary data into analogue
via a digital to analogue converter.
The ECU internal communication paths
Movement of data between the various parts of the ECU are prompted by the control
clock, this clock produces a timed pulse which oscillates at a high frequency. When
this voltage pulse is applied to two parts of the ECU simultaneously then this allows
information to flow between these parts. Other parts of the ECU will remain inactive
until they receive a pulsed signal. This system of data control and movement allows
the various ECU sections to be interconnected by a series of parallel wires called a
‘bus’. This can be seen in Thornes, S. Automotive Electronics, Hillier
The Bus
The communication pathways known as the ‘bus’ can allow data to flow either in one
direction only or as a two way communication link. Within the ECU the bus is divided
into three sections which are referred to in relation to the signals they carry, namely:
The Data Bus, Address Bus and the Control Bus.
The Data Bus
The Data Bus normally has eight wires that carry data in both directions between the
CPU the RAM, ROM and the input/output stages.
The Address Bus
The Address Bus caries the data address in the form of binary code from the CPU to
the memory. The signal carried by this bus identifies the actual place in the memory
where a given item of information is stored. This bus normally has sixteen wires and
allows information to travel in one direction only.
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The Control Bus
The signals carried within this bus control the functions of the ECU; they select the
units required and determine the direction of data movement at a given time. The
terms reading and writing apply to the direction of data movement. Reading refers to
the fact that data is being passed to the CPU for action, writing refers to the fact that
data is being delivered from the CPU for storage and future action.
Handling Data (an example)
When digital information in the form of 0’s and 1’s is received by the CPU this digital
code must be compared with data stored in memory, the CPU recognises the data by
the combination of 0’s and 1’s and by the input pin from which the information
emanates. The CPU by applying a clock pulse through the address bus to the RAM
identifies the place in the memory where the appropriate data can be accessed. This
information is held in a series of ‘look up tables’ which are in the form of binary code.
The codes are fed back to the CPU one line at a time and compared by the ALU. This
comparison is undertaken by entering both the codes from the RAM and the sensor
input code into a logic gate where the comparison takes place. For example by feeding
each code held in the appropriate look up table in the RAM bit by bit into the logic
gate in parallel to a bit by bit input of the sensor code a comparison can be achieved.
For example by employing a logic gate known as an ‘Exclusive OR’, the two codes can
be compared. The Exclusive OR will provide an output of logic ‘1’ if either of the
inputs are logic ‘1’, where each bit of each code is the same then all the outputs will be
logic ‘0’. When all the outputs are 0 then the CPU identifies a code match and sends a
clock pulse via the data bus to the output stage. The information contained in the
message sent through the data bus will determine to which output terminal a voltage
should be applied and the duration of the applied voltage.
The matching of the codes determines the engine warm up period in minutes and
therefore the output signal could be sent to either a cold start/running actuator to
provide fuel enrichment or to provide an increased voltage pulse to the main fuel
injectors thereby increasing the fuel sprayed.
This type of information contained in the ECU RAM can be represented in the form of
three dimensional tables which indicate variances for such data as ignition advance
given engine speed load and temperature, or for fuel injection duration given engine
speed, temperature and manifold pressure. The diagram in Bosch, R. Technical
Training Book: Electronic Injection Systems represents a 3D graphical
representation of such data for ignition advance.
The ignition data illustrated in Bosch, R. Technical Training Book: Electronic
Injection Systems the from of a three dimensional optimum spark advance map is
obtained from information stored within ‘Lookup tables’ in the ECU ROM/RAM.
The ignition point can be determined by reading the ‘x’ and ‘y’ co-ordinates for engine
load and speed and then projecting a vertical line upwards. The point of ignition
advance can be determined by the point at which this vertical line breaks the surface of
the map.
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The diagram referred to previously provides a simple representation of an engine
management system where the parameters such as ignition point and the quantity of
fuel to be injected are controlled by the microcomputer (ECU Electronic control unit).
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The inputs (sensors)
Engine Speed
Teeth of the induction- type pulse generator on the flywheel ring gear provide pulses
relative to engine speed.
Crank Angle
Induction type reference mark for ignition point.
Air Quantity
The angular position of the air flow sensor flap with potentiometer provides the base
information from which injection period is determined by the ECU. This information
combined with the sensor information provided by the throttle position switch (load
range) and the temperature data provided by the engine temperature thermistor refines
the period of injection.
Throttle position switch (load range)
In addition to its use described above this switch provides engine load information to
determine ignition point variance (this information could also have been provided by a
MAP sensor)
Outputs
Injection
The ECU controls the injector (solenoid) by applying a voltage to the injectors for a
period of time determined by the sensor inputs compared to memory data. The longer
a voltage is applied to the injector the greater the quantity of fuel injected.
Ignition
The ECU calculates an ignition angle using the input quantities of speed, load and
variances such as temperature. In addition the ECU determines dwell angle. Both
quantities are employed in determining the ignition duration signal which is equal to the
dwell period. The ignition signal from the ECU controls the flow of current through
the ignition coil.
Fuel Pump
The ECU controls the switching of the fuel delivery pump via a relay circuit.
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Outcome 2 Actuators
Initially actuators will be considered it terms of their operational categories followed
by a detailed description of their operation and a description of their function within a
number of vehicle systems.
The actuator responds to the output delivered from the ECU in order that the system
of which it is part can be varied according to predetermined system parameters. The
electronic control unit applies the required voltage to the actuator for a period of time
which determines the operating period of the actuator and the extent of its movement
and thus to what extent it effects the system.
Actuators can be categorised as being:
• A linear solenoid
• A linear motor
• A rotary motor
• A stepper motor
The diagram shown in Bosch, R. Technical Training Book: Electronic Injection
Systems indicates the inputs and outputs of one electronically controlled vehicle
system.
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Magnetism
Prior to commencing the consideration of actuators it would be beneficial to revise the
candidates knowledge of electromagnetism. Some simple experiments involving both
permanent and electromagnets could be employed to restate the principles of
magnetism to include:
• The principle of magnetic attraction and repulsion
• The production of a magnetic flux due to current flowing through a conductor
• The effect of varying the strength of the field current
• The effect on the field polarity when the current direction alters
• The effect of coiling the conducting wire upon flux strength.
Actuators
The actuator operates due to the signal received from the electronic control unit and
operates some form of mechanical mechanism to provide the desired movement of
system components which will result in a modification of the systems operating state.
There are two main classifications of actuator, namely; solenoids and motors,
providing either rotory or linear movement.
Linear Actuators
Solenoid
The majority of automotive applications employ the linear solenoid where the applied
force moves a component in a straight line. Such applications include:
• Electronic Fuel Injector
• Fuel cut-off solenoid
• Idle stabilisation /idle speed actuator
• Fuel pressure regulator
• Hydraulic solenoid valve (within ABS modulator)
• Cruise control solenoid
• Suspension stiffness/height control
The solenoid is an electromagnetic device that produces a linear mechanical force. The
input signal (applied voltage) produces a current flow in the solenoid coil, which in
turn produces a strong magnetic field within and around the coil. The magnetic field
applies an attractive force to the metal armature, pulling the armature into the centre of
the field coil. A strong spring forces the armature back to its rest position when the
applied voltage signal is withdrawn. The solenoid has the disadvantage of the linear
force produced being proportional to the square of the distance between the core and
the magnetic pole piece. This factor limits the effective movement of the solenoid and
restricts its application to a situation where 8mm of movement or less is required. In
addition to this the solenoid is slow in operation relative to the electronics of the
controlling ECU.
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Double acting solenoid
Double acting solenoids may be employed within the control stage of a system where
an alternate position is required as part of the control process as illustrated in the
diagram shown in Thornes, S. Automotive Electronics, Hillier
Examples of solenoid operation and their application within a system
Can be found in Bosch, R. Technical Training Book: Electronic Injection Systems
Electronic Petrol Injector
The function of the injector is to deliver finely atomised fuel into the inlet ports of the
engine. In addition the injector must vary the quantity of fuel injected to suit the
engine operating conditions; this is achieved by varying the time the solenoid is
energised. The ECU varies the quantity of fuel injected relative to the data computed
from the system sensors, it controls the injectors by varying the pulse width of the
voltage signal applied to the bank of injectors which are electrically connected in
parallel. The opening period normally varies between 1.5 and 10 milliseconds. The
injector opening is achieved by solenoid action, the relative movement of the armature
being in the region of 0.15mm.
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Investigation
By applying an oscilloscope across the injector terminals a square- wave pattern will be
displayed which will vary with engine speed/load and with temperature in some designs
of fuel system. The student can observe the variation in injector pulse width and
determine the opening period in milliseconds.
An example of Solenoid application within ABS modulator (hydraulic unit) can be
found in the VAG Journal
The hollow piston within the hydraulic circuit between the master cylinder and the
wheel cylinder is the actuator of a solenoid. The piston will be attracted upwards
towards the centre of the solenoid winding when a voltage signal is applied from the
ECU, the height the piston travels will be dependent upon the value of the voltage
applied and the resulting field current. By varying the field current the position of the
piston can be controlled which in turn determines the operational state of the hydraulic
circuit. In the diagram referred to above the solenoid piston/armature is held in its
lowest position due to spring pressure and hydraulic action can be communicated
between the master cylinder and the wheel cylinder.
As can be seen in the VAG Journal the situation when the ECU has applied partial
field current to the solenoid due to the input of the wheel sensor information. In this
position the pressure in the system is maintained with no increase in pressure at the
wheel cylinder allowed.
The diagram shown in VAG Journal depicts the solenoid in its highest position
(maximum field current) which allows a reduction in pressure at the wheel cylinder by
allowing fluid to escape back towards the pressure reducer due to the action of the
return flow pump.
Linear Motor
The main difference between the linear motor and the solenoid is reflected in the
difference in construction and the level of applied force. The liner motor employs a
permanent magnet, which increases the magnetic field strength and thus results in the
application of higher linear forces. The force produced is near constant over the
operating range of the motor.
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Rotary Actuators
Permanent DC Motor
The conventional DC Motor remains the most commonly used method of actuating
vehicle systems to include:
• Roller - Cell Fuel pumps
• Electric Windows
• Electric Sunroofs
• Seat position adjustment
• Heater control movement
Examples of permanent magnet DC motors and their application within a system can
be found in Bosch, R. Technical Training Book: Electronic Injection Systems,
The roller-type pump is normally driven by a permanent magnet DC motor. Rotational
speed of the pump moves the rollers outward to seal against the internal walls of the
pump chamber thus creating a number of chambers sealed off from each other. As fuel
is carried round with the rotor the combination of rotor movement together with a
decrease in volume of the chambers causes the increase in fuel pressure. This pressure
must be maintained at a required level in order that the quantity of fuel injected for a
given pulse width can be determined by the ECU, therefore the system incorporates a
fuel pressure regulator. The pump is controlled by the ECU via a fuel pump relay.
Supply to the pump is taken through a ballast resistor which drops the voltage at the
pump terminal to approximately 7 volts. The resistor is shorted out when the engine is
being cranked to compensate for the voltage drop experienced at the battery terminals.
When the ignition is switched on the ECU energises the pump for a short period in
order to initially pressurise the fuel, after this initial period the ECU switches the pump
off by breaking the earth connection for the pump relay until engine cranking occurs.
Due to the high fuel delivery capacity of such pumps an inertia switch is incorporated
into the relay supply so that in the event of an accident or sufficient jolt the inertia
switch will close the supply to the pump relay thus switching off the pump.
Electronic seat height adjustment
The height tilt and rake of the driver’s and passenger’s seat is adjusted by the use of
permanent magnet DC motors in conjunction with a series of drive mechanisms.
Examples of this can be seen in Renault Safra: Engines and Peripherals Workshop
Manual MR302, English Edition.
The seat height adjustment is taken care of by four jack elements, separate adjustment
motors operate the front and rear seat jacks these can either be operated in parallel or
individually. Each motor incorporates a potentiometer (illustrated above) the position
of the potentiometer carrier determines the electrical resistance of each individual
motor and provides an ECU with a means of relating seat position to resistance. Once
driver ‘1’ manually operates the seat to the required position by depressing a memory
button on the control panel together with button ‘1’the ECU records the various
resistance values of each adjusting motor by means of storing a digital code within its
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memory for driver 1. If a second driver wishes to alter the seating position the a
similar manual process occurs with the driver pressing the memory button together
with control button ‘2’. This enters resistance values for driver 2 into the memory of
the ECU. The seat position can be reverted back for driver ‘1’ by depressing the
memory button together with button 1. On receiving this signal the ECU applies
voltage to each motor for the desired time period until each motor potentiometer
matches the resistance values stored in memory.
Stepper Motors
An example can be seen in Mellard, Automotive Electronic Systems, Heinemann
The stepper motor is a digitally driven device with the operation being either linear or
rotational. The stepper motor has increased in popularity with the corresponding
increase in the use of digital electronics. Unlike the conventional ‘DC’ motor the
rotational angular movement of the stepper motor armature can be controlled to rotate
in small incremental degrees. The chief advantage of the stepper motor over the
conventional DC motor is therefore one of controlled motion, both in the number of
steps of movement and in the step rate.
The rotor is a permanent magnet, constrained to rest in a position by the magnetic field
pattern produced by the stator pole pieces. If I1 is reversed, the rotor is forced into
the new position. By switching the polarity of the stator currents in the sequence :+I1
+I2, - I1 + I2 , -I1 - I2, +I1 -I2, so producing a rotating field pattern, the rotor is
moved through 360 degrees. The speed of rotation of the rotor is governed by the rate
at which the stator currents are switched, and the direction of rotation by the sequence
used. Maintaining the currents constant at any point in the sequence holds the rotor
stationary. Step angles are always sub-multiples of 360 degrees, and typical values are:
1.8, 2.5, 3, 7.5, 15 and 30 degrees. Small stepper motors can develop torque up to a
few Newton-metres, but the torque falls off with increasing stepping speed and a
maximum of a few hundred steps per second can be expected.
Stepper motors are commonly found operating the following system components:
• Carburettor choke control
• Engine idle speed control
• Variable power steering
There are three main types of stepper motor:
• Permanent magnet
• Variable reluctance
• Hybrid
Variable reluctance stepper motor
This type has a soft iron rotor with radial teeth and a wound stator having more poles
than the rotor. Diagrams shown in Thornes, S. Automotive Electronics, Hillier,
Illustrate a simplified layout of a 3 phase, 15 degree stepper angle motor; this has
eight rotor teeth and twelve stator poles around which the current flows in one
direction only.
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The number of step positions is determined by:
N =
SR
S− R
Where S= the number of slots in the stator and R= the number of slots in the rotor.
In the example provided in the stepper motor diagram above the number of individual
step positions can be found by:
8 x 12 96
N= 12 - 8 = 4 = 24
360
therefore the step angle = 24 = 15 degrees
Thornes, S. Automotive Electronics, Hillier also show the winding arrangement for
phase 1 of the three phases.
When a current flows through one phase of the stator windings, the rotor aligns itself
to give the shortest magnetic path, that is the path of minimum magnetic resistance. In
each step position the rotor aligns itself with the four stator pools which provides the
motor with more power. An angular position of one step from the position shown in
diagram (b) above is obtained by energising either phase two or three depending upon
the required direction of movement. For a clockwise movement the phases would be
energised in the following order: 3, 2, 1, 3, 2, 1. The angle turned by the rotor by
these six current pulses is 90 degrees with the time for total movement being
determined by the time taken by the control circuit to energise the windings sufficient
to move the rotor to the next step. This type of motor is capable of providing step
angles between 1.8 and 15 degrees.
Hybrid stepper motor
This motor is a combination of both the permanent magnet and variable reluctance type
motors. The construction illustrated in Thornes, S. Automotive Electronics, Hillier
shows that the construction is similar to that of a vehicle alternator. The operation is
similar to that of the permanent magnet motor, the rotor aligns itself so that the
magnetic reluctance is lowest. The hybrid stepper motor has stepping angles as low as
0.9 degrees, a high torque and the ability to operate high stepping rates.
Engine idle speed control
The idle speed on modern vehicles is often controlled by some form of electronic
actuator responding to a voltage signal delivered by an electronic control unit. On
vehicles employing fuel injection the idle speed of the engine can operate at far lower
speeds than that of most comparable carburettor engines and therefore this lower idle
speed is much closer to the stall point of the engine. This situation requires accurate
and speedy adjustment of the engine idle when for example electrical loads are
switched on where relatively large currents are drawn which would in turn increase
engine load and reduce engine idle. The diagram in Bosch, R. Technical Training
Book: Electronic Injection Systems illustrates one such electronically adjusted
system.
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The idle adjuster employs a permanent magnet stepper motor which allows for a
greater or lesser supply of air through the throttle by-pass, according to the deviation
of the actual instantaneous idle speed from the set speed. The control unit supplies the
idle adjuster with a control signal dependent upon engine speed and temperature. Its
rotary spool alters the by-pass cross-sectional area. The idle adjuster has two windings
and a rotational range of 90 degrees. A rotary spool on the armature shaft opens the
by-pass channel to the extent necessary to maintain a preset idle speed regardless of
engine load. The regulation circuit in the ECU which receives the necessary
information about the actual idle speed, compares this with the programmed value and,
via the idle adjuster, adjusts the throughput until actual and programmed idle speeds
coincide.
Alternately a linear stepper motor/solenoid can be employed to achieve the same
results as illustrated Rover Auto Course Distance Learning (Ref C661)
Investigating operation of stepper/linear motor
Checking the action of the stepper motor and the supply oscillations.
• Connect the oscilloscope voltage probe to each of the four stepper motor supply
wires in turn as illustrated in Rover Auto Course Distance Learning (Ref C661)
•
•
•
•
•
Ensure that the oscilloscopes time base in free running set to 20 Volt scale
Turn the ignition ON and OFF when the scope is connected to each stepper motor
supply lead. An oscillation between 12 V and 0 V should appear momentarily
each time the ignition is switched on and off.
Remove the connector from the stepper motor and unscrew the stepper motor
from the throttle body
Refit the connector and hold the motor clear from any obstructions
Instruct a colleague to switch the ignition ON then OFF two or three times and
watch the action of the stepper motor. This can be seen in Rover Auto Course
Distance Learning (Ref C661)
When ignition is switched ON the valve should be seen to index from the fully open
position to the partly closed position. When the ignition is switched OFF the valve
should index to the fully open position.
Variable power assisted steering (incorporating a stepper motor within the power
steering control valve).
(This system is employed on the Renault Safrane.)
This system which varies power assistance depending upon vehicle speed provides:
• Increased assistance during parking manoeuvres
• Progressively reduced assistance whilst driving as speed increases.
The system comprises a variable hydraulic valve in place of the conventional power
assisted steering valve. The control of this hydraulic valve in undertaken by a stepper
motor.
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It is not the intention here to explain in any detail the operation of the power-steering
system this can be applied if power steering is chosen as part of outcome ‘four’, system
investigation. The following is therefore an outline of the system operation in order
that the role played by the stepper motor may be investigated.
On vehicles employing conventional power steering systems the level of assistance
provided is dependent upon the applied driver effort to the steering system. As the
driver applied a force a torque reaction would be set up within the steering column, the
applied torque relative to the resistance at the road wheels would twist a torsion bar.
The more this torsion bar twisted the greater the assistance provided. With the
variable power steering system the determining factor as to the level of assistance
provided is dependent upon information supplied to the steering ECU by two
independent vehicle speed sensors. The lower the vehicle speed the greater the
assistance provided. Four diagrams illustrate the components which make up this
system these can be found in Renault Safra: Engines and Peripherals Workshop
Manual MR302, English Edition
The stepper motor controls the quantity and directs the flow of oil from the engine
driven pump into the assistance chambers within the steering rack, the voltage applied
to the stepper motor windings controlling the degree of rotation and hence the quantity
of oil delivered. The sequence of powering the motor windings determining the
direction of the required assistance (right or left turn) determined by movement of the
steering wheel.
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Outcome 4
Any electronically controlled vehicle system can be selected for investigation as part of
outcome 4. It will prove economical in terms of time and provide unit integration with
aspects of the ‘Secondary Chassis Systems Unit’ if anti-lock braking systems is
selected as the chosen system.
The system chosen for investigation here is the ‘Bosch 2E’ arrangement. This system
is utilised by the following vehicle manufacturers:
• Alfa
• Audi
• Lancia
• Peugeot
• Renault
• SAAB
• Vauxhall
• Volvo.
There are two main types of Bosch ABS system utilised within these manufacturers
vehicles, the only difference relating to the position of the ECU. One system has a
separate ECU, while the other has the ECU incorporated within the modulator unit
assembly. Both types are represented in the diagrams shown in Autodata.
The vehicle employed for investigation in this example is the Renault Safrane, which
employs the ABS system with the integrated ECU.
For reference purposes the Safrane system orientation can be found in Renault Safra:
Engines and Peripherals Workshop Manual MR302, English Edition.
In addition to the braking elements:
• Brake calipers
• Tandem master cylinder
• Servo unit
• Pressure limiting valve (rear brake lines).
Which are conventional, the ABS system comprises:
The system components
Hydraulic modulator
The hydraulic modulator is controlled by the ECU and regulates hydraulic pressure in
each of the brake circuits. The modulator is located between the master cylinder and
the brake calipers. It contains a solenoid operated valve for each brake circuit, (in the
case of the Renault Safrane there are three solenoid valves one for each front circuit
and one for the rear circuit) and an electrically operated pump with pressure reduction
chambers.
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Diagrams shown in VAG Journal represents the hydraulic modulator complete with
solenoid valves, pump relays and hydraulic return pump. Also seen here a simplified
version of the hydraulic modulator showing one of the solenoid valves, the pump with
pressure reduction chamber.
Wheel speed sensors and ring-gear
Speed sensors are fitted adjacent to a ring gear attached to each respective hub. The
sensors convert the speeds of the individual wheels into analogue signals which are
transmitted to the system ECU. This is shown in VAG Journal.
Electronic Control Unit
The ECU receives signals from all four wheel speed sensors, the signals are converted
by the analogue to digital converter into binary code. When this code has been
compared with memory, a voltage signal is generated and delivered to the hydraulic
modulator solenoid valves depending on the system conditions. The diagram shown in
Autodata illustrates the basic wiring of components into the ECU.
System operation
By monitoring the signals from the speed sensors during braking the ECU is able to
determine the point at which a particular wheel is about to lock. To prevent wheel
lock the ECU will activate the appropriate solenoid valve in the hydraulic modulator to
regulate the pressure within the relevant brake circuit. The solenoid valves operate in
three stages which maintain, decrease. Or allow an increase in pressure within the
brake circuit to which they are connected. In the first stage the brake circuit pressure
is determined by brake pedal pressure. At the second stage the solenoid valve cuts off
the brake circuit from the master cylinder which maintains pressure within the circuit.
During the third stage the valve moves further and allows the fluid within the circuit to
enter a reduction chamber, at the same time the ECU activates an hydraulic pump
which forces fluid above the valve, back to the master cylinder against brake pedal
pressure. The operation of the solenoid valves and pump and the consequent increase
and decrease of fluid pressure occur between 4 to 10 times per second and will result
in a slight in a slight vibration through the brake pedal.
The ECU incorporates a safety circuit which can detect and store any incorrect signals
or faults in the system. Should a fault occur in the system the ECU will switch off the
ABS and illuminate a warning lamp to alert the driver. In addition the ECU will switch
off the ABS system at road speeds below approximately 5 MPH, to allow the vehicle
to be brought to rest.
When the ABS system incorporates electronic traction control, excessive spinning of
one driving wheel to the front or rear axle will result in the brake of the spinning wheel
being applied. When the speed of the two driving wheels equalise, the applied brake
will be released.
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System Testing
SAFETY PRECAUTIONS
• Do not connect or disconnect electrical components of the system with the
ignition switched on.
• Always ensure electrical connections are making good contact, particularly the
ECU and modulator connections.
• Do not connect a 12 volt supply direct to the ECU or solenoid valves
• Do not connect or disconnect diagnostic or measuring equipment from the system
whilst live.
• Always ensure battery voltage is between 11.5 and 13.5 volts before commencing
any system checks.
A number of the checks to be performed require connections to the ECU to be made.
Great care must be taken not to damage the delicate connecting pins within the
connector plug. If accessible, the test connections should be made at the back of the
plug (as illustrated in Autodata) once the plug has been removed.
Alternatively a breakout box as illustrated in Autodata could be used which will make
this type of testing easier and result in less damage to system connections.
General test equipment
Although special test equipment may be available the test procedures employed here
utilise mainly digital or analogue multi-meters.
The majority of tests use the multi-meter, but although analogue or digital meter
displays can provide sufficient information to make accurate checks and diagnosis for
the majority of components in the ABS system, it is possible to enhance the testing of
wheel speed sensors by the use of an oscilloscope, with a low voltage facility.
Autodata diagnostic manual illustrate recommended test procedures for the selected
vehicle. These test procedures correspond to those recommended by the vehicle
manufacturer.
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Investigation of system operation
The following procedure is not one which would be expected to be applied within
normal diagnostic investigation but is employed here to enhance the candidates
understanding of the system operation. In addition it provides the candidate with the
opportunity to view ECU control of the system hydraulic solenoids.
Investigation
Ignition switched OFF. Connect a mains signal generator to three of the four wheel
speed sensors, with a separate signal generator to the remaining sensor. Adjust the
signal generators to a frequency above 200Hz with a voltage amplitude of 7 volts
which represents a vehicle speed in excess of 5MPH (approx. 30 mph). With ignition
switched on brake pedal depressed and a voltmeter connected across the appropriate
solenoid/pump connections from the ECU gradually reduce the frequency/amplitude of
the single signal generator. Observe the system operation and record the reading on
the voltmeters. Gradually reinstate the frequency to that of the others and observe the
system.
If the frequency of the signal provided for the single wheel speed sensor is reduced
gradually then the first stage to be witnessed should relate to the solenoid being
powered. As the frequency continues to be reduced the next stage should see both the
solenoid and pump being powered. As the brake pedal is released the ECU should cut
off the supply to the hydraulic modulator.
Automotive Engineering Vehicle Management - Higher
40
PUBLICATIONS
Automotive Electronic Systems
Trevor Mellard
Heinemann Newnes
Oxford OX2 8EJ
ISBN 0 434 91257 3
Fundamentals of Automotive Electronics
V. A W. Hillier
Stanley Thornes Ltd
Cheltenham GL5O 1YW
ISBN 0 7487 2695 0
Understanding Automotive Electronics
W. B. Ribbens and N. P. Mansour
Texas Instruments
Dallas PO box 225012, MS - 54
ISBN 0 672 27017 X
Computerised Engine Control
D.H. King
Delmar Publishers
Albany New York PO box 15b 015
ISBN 0 8273 4059 1
Robert Bosch GmbH Stuttgart
D 7 Stuttgart 1, Postfach 50
Various Publications
Autodata Ltd
Priors way
Maidenhead
Berkshire
SL6 2HP
Various Publications
VAG
AG Wolfsburg
Germany
Automotive Engineering Vehicle Management - Higher
41