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Instrument Clusters

• Topics covered in this presentation
• Analog Instrument Clusters
• Digital Instrument Clusters
• Instrument Gauges and Computers

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Typical Analog Instrument Cluster
Tachometer
Speedometer
An instrument cluster displays the status of a
vehicles 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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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.
Warning lamps
Temperature gauge
Fuel gauge
The gauges usually have a symbol to represent
their function.
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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Typical Instrument Cluster Construction
The instrument cluster is usually made of plastic
and is held in position with screws.
Instrument housing
The front of the instrument cluster is protected
by a transparent cover or lens.
Speed, fuel and temperature gauges
The instruments are fitted into, or are part of,
the instrument housing.
Support housing
They are aligned / secured in place by a support
housing that may also be a fascia with indication
markings.
Transparent cover or lens
A printed circuit electrically connects
components in the instrument cluster. Display
illumination bulbsare often inserted from the
rear.
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Mechanically Driven Analog Speedometer
The speedometer cable links between the
speedometer head and the transmission housing.
Speedometer head
Flexible inner cable
It has an inner cable that is a flexible wire
with a drive gearon one end. It moves within a
stationary outer housing.
Outer cable housing
When a vehicle is moving, it transfers motion
through a gear linkage and rotates the
speedometer inner cable.
Cable inner wire and gear
Drive 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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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.
Analog signal
The computer (ECU / BCM) processes the input
signal and converts the information into drive
signals, for either analog or digital
speedometers.
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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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.
Tachometer
Battery
The switching rate is proportional to engine
speed.
Switching / ignition pulses
Ignition switch
The tachometer is 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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Sensor Driven Analog/Digital Tachometer
The crankshaft is fitted with a trigger wheel,
which rotates while the engine is running.
A speed sensor is positioned nextto the trigger
wheel. It produces a signal that relates to
engine speed.
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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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.
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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Typical Instrument / Body Computer (BCM)
The instrument computer has three main stages of
operation

• Input.
• Processing.
• Output.

The input stage processes both analog and digital
inputs.
Some input devices require a reference voltage to
function. This is provided by the BCMs power
supply.
Analog voltages are converted to digital signals
by the A/D converter and / or amplifier, ready
for processing.
Weak or noisy digital signals are conditioned,
ready for processing.
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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.
CPU
Processing stage
The output stage switches the required actuators.
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Typical Body Computer Circuits
Fuel Level Circuit
The resistance of the fuel level sensor changes
with fuel quantity.
This causes a corresponding voltage change at the
BCMs 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.
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Typical Body Computer Circuits
Oil Pressure Circuit
When engine oil pressure is correct, the oil
pressure switch is closed.
The voltage at the BCM input is zero.
When engine oil pressure is too low, the oil
pressure switch is open.
The voltage at the BCM input is 12V.
The BCM will switch on a warning lamp or speaker,
when 12V is measured at its input.