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1. Instrument Clusters
Theory Support
Automotive – Displays & Accessories
Instrument Clusters
Topics covered in this presentation:
 Analog Instrument Clusters
 Digital Instrument Clusters
 Instrument Gauges and Computers
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1. Instrument Clusters
Theory Support
Automotive – Displays & Accessories
Typical Analog Instrument Cluster
Tachometer
Speedometer
An instrument
cluster displays
the status of a
vehicle’s systems.
Rev
limit
The speedometer
is usually situated
in a prominent
position.
Pointer
Odometer
It has a pointer that lines up with numerical values,
which indicate vehicle speed in miles per hour (MPH).
The speedometer also has an odometer to measure distance traveled.
The tachometer displays engine speed in revolutions per minute (RPM).
The rev limit (red area) indicates the maximum engine speed allowed,
before damage can occur.
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1. Instrument Clusters
Theory Support
Automotive – Displays & Accessories
Typical Analog Instrument Cluster
The fuel gauge displays the amount of fuel that is left in a fuel tank. Fuel
quantity is measured in gallons. The temperature gauge displays engine
coolant temperature.
Temperature gauge
Warning lamps
The gauges
usually have a
symbol to
represent their
function.
Fuel
gauge
Warning lamps are usually positioned around the perimeter of an
instrument cluster. They may have red, green, blue or orange lenses.
They are used to indicate system information such as oil pressure, parking
brake, battery status, direction indication, hi beam, ABS, choke, etc.
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1. Instrument Clusters
Theory Support
Automotive – Displays & Accessories
Typical Instrument Cluster Construction
The instrument cluster is
usually made of plastic and is
held in position with screws.
The front of the instrument
cluster is protected by a
transparent cover or lens.
The instruments are fitted
into, or are part of, the
instrument housing.
PCB
Instrument
housing
Speed, fuel and
temperature
gauges
They are aligned / secured in place
by a support housing that may also
be a fascia with indication markings.
A printed circuit electrically connects
components in the instrument cluster.
Display illumination bulbs
are often inserted from the rear.
Support
housing
Transparent
cover or lens
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1. Instrument Clusters
Theory Support
Automotive – Displays & Accessories
Mechanically Driven Analog Speedometer
The speedometer cable links between
the speedometer head and the
transmission housing.
It has an inner cable that is a
flexible wire with a drive gear
on one end. It moves within a
stationary outer housing.
When a vehicle is moving, it
transfers motion through a gear
linkage and rotates the
speedometer inner cable.
Drive gear
Speedometer head
Flexible
inner cable
Outer
cable
housing
Cable inner wire and gear
Transmission housing
At the speedometer head in the instrument cluster, a small magnet
rotates. The resulting magnetic force moves the speedometer pointer.
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1. Instrument Clusters
Theory Support
Automotive – Displays & Accessories
Electronically Driven Speedometer
The transmission housing contains an output shaft with a trigger wheel.
A vehicle speed sensor is positioned next to the trigger wheel.
When a vehicle is moving, the
sensor produces a signal that
relates to vehicle speed.
The computer (ECU /
BCM) processes the
input signal and
converts the information Trigger
wheel
into drive signals, for
either analog or digital
speedometers.
Computer (ECU)
Analog
signal
Analog or digital display
Speed sensor
Transmission housing
Some analog systems do not use a computer. The frequency of the
incoming signal is converted to a voltage, which is used to drive a stepper
motor that deflects the pointer.
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1. Instrument Clusters
Theory Support
Automotive – Displays & Accessories
Ignition Coil Driven Analog Tachometer
When an engine is
running, the primary
winding of the ignition
coil is switched on and
off to produce a spark.
Battery
The switching rate is
proportional to
engine speed.
Tachometer
Switching /
ignition pulses
Ignition
The tachometer is
switch
connected to the minus
(switching) terminal of
the ignition coil.
Coil switching
device
Ignition coil
The switching pulses trigger the analog pointer in the tachometer to display
engine speed in revolutions per minute (RPM).
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1. Instrument Clusters
Theory Support
Automotive – Displays & Accessories
Sensor Driven Analog/Digital Tachometer
The crankshaft is fitted with a trigger wheel, which rotates while the
engine is running.
Analog
tachometer
Analog
signal
Analog or
digital drive
Trigger
wheel
Speed sensor
A speed sensor is positioned next
to the trigger wheel. It produces a
signal that relates to engine speed.
Computer
(ECU)
Digital tachometer
The computer (ECU / BCM) processes the input signal and converts the
information into drive signals, for either analog or digital tachometers.
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1. Instrument Clusters
Theory Support
Automotive – Displays & Accessories
Typical Digital Instrument Cluster
The digital instrument cluster is lightweight and easy to read.
It can have digital displays for speed, fuel level, climate control, distance
traveled etc.
Warning/status lamps
Fuel consumption
Climate
control
Tachometer
Main display
There will also be lamps for warning / vehicle status, and direction indication.
The displays may be LCD, LED, or vacuum fluorescent. Incoming data is
processed by the Body Computer Module (BCM).
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1. Instrument Clusters
Theory Support
Automotive – Displays & Accessories
Typical Instrument / Body Computer (BCM)
The instrument computer has
three main stages of operation:
 Input.
 Processing.
 Output.
Input
stage
Output
stage
The input stage processes
both analog and digital inputs.
Some input devices
require a reference
voltage to function.
This is provided by the
BCM’s power supply.
Analog
Digital/
switch
Analog voltages are converted to digital signals by the
A/D converter and / or amplifier, ready for processing.
Processing
stage
Weak or noisy digital signals are conditioned, ready for processing.
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1. Instrument Clusters
Theory Support
Automotive – Displays & Accessories
Typical Instrument / Body Computer (BCM)
Input
stage
The “brain” of the computer
is the processing stage.
Output
stage
CPU operation is controlled
by a software program, stored
in read only memory (ROM).
The CPU stores input
data in random access
memory (RAM).
Analog
Digital/
switch
It compares input data against
expected data and switches
appropriate output drivers.
The output stage switches the
required actuators.
CPU
Processing
stage
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1. Instrument Clusters
Theory Support
Automotive – Displays & Accessories
Typical Body Computer Circuits
Fuel Level Circuit
The resistance of the fuel
level sensor changes with
fuel quantity.
Speaker
Fuel level
sensor
This causes a
corresponding voltage
change at the BCM’s
analog input.
The BCM processes the input data
and produces an output signal to
drive a digital or analog display.
The BCM may also switch on a lamp,
or use a speaker to warn the driver
that the fuel tank is nearly empty.
Lamp
5V
BCM
Digital display
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1. Instrument Clusters
Theory Support
Automotive – Displays & Accessories
Typical Body Computer Circuits
Oil Pressure Circuit
Speaker
When engine oil pressure is correct,
the oil pressure switch is closed.
Lamp
The voltage at the BCM input is zero.
When engine oil pressure
is too low, the oil pressure
switch is open.
12V
Oil pressure
switch
The voltage at the
BCM input is 12V.
BCM
The BCM will switch on a warning lamp or
speaker, when 12V is measured at its input.
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