1. Instrument Clusters Topics covered in this presentation:
Analogue Instrument Clusters
Digital Instrument Clusters
Instrument Gauges and Computers

2. Typical Analogue Instrument Cluster
An instrument cluster displays the status of a vehicle’s systems.
Speedometer
Tachometer
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 travelled.
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.

3. Typical Analogue Instrument Cluster
The fuel gauge displays the amount of fuel that is left in a fuel tank. Fuel quantity is measured in litres or gallons. The temperature gauge displays engine coolant temperature.
Temperature gauge
Warning lamps
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, handbrake, battery status, direction indication, main beam, ABS, choke, etc.

4. Typical Instrument Cluster Construction
The instrument cluster is usually made of plastic and is held in position with screws.
PCB
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.
A printed circuit electrically connects components in the instrument cluster. Display illumination bulbs are often inserted from the rear.
Transparent cover or lens

5. Mechanically Driven Analogue 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 gear on 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.

6. 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.
Computer (ECU)
Analogue
signal
The computer (ECU / BCM) processes the input signal and converts the information into drive signals, for either analogue or digital speedometers.
Trigger wheel
Analogue or digital display
Speed sensor
Transmission housing
Some analogue 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.

7. Ignition Coil Driven Analogue Tachometer
When an engine is running, the primary winding of the ignition coil is switched on and off to produce a spark.
Battery
Tachometer
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 analogue pointer in the tachometer to display engine speed in revolutions per minute (RPM).

8. Analogue or digital drive
Sensor Driven Analogue/Digital Tachometer
The crankshaft is fitted with a trigger wheel, which rotates while the engine is running.
Analogue tachometer
Analogue signal
Analogue or digital drive
Trigger wheel
Speed sensor
Computer (ECU)
A speed sensor is positioned next to the trigger wheel. It produces a signal that relates to engine speed.
Digital tachometer
The computer (ECU / BCM) processes the input signal and converts the information into drive signals, for either analogue or digital tachometers.

9. 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 travelled etc.
Fuel consumption
Warning/status lamps
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).

10. Typical Instrument / Body Computer (BCM)
The instrument computer has three main stages of operation:
Input stage
Output stage
Input.
Processing.
Output.
The input stage processes both analogue and digital inputs.
Some input devices require a reference voltage to function. This is provided by the BCM’s power supply.
Analogue
Digital/ switch
Analogue 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.

11. Typical Instrument / Body Computer (BCM)
The “brain” of the computer is the processing stage.
Input 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).
Analogue
Digital/ switch
It compares input data against expected data and switches appropriate output drivers.
CPU
Processing stage
The output stage switches the required actuators.

12. Typical Body Computer Circuits
Fuel Level Circuit
Speaker
Fuel level sensor
The resistance of the fuel level sensor changes with fuel quantity.
Lamp
This causes a corresponding voltage change at the BCM’s analogue input. 5V
The BCM processes the input data and produces an output signal to drive a digital or analogue display.
BCM
The BCM may also switch on a lamp, or use a speaker to warn the driver that the fuel tank is nearly empty.
Digital display

13. Typical Body Computer Circuits
Oil Pressure Circuit
Oil pressure switch
BCM
Lamp
Speaker
12V
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.