1. Instrument Clusters Theory Support Automotive – Displays & Accessories 1 of 13 Instrument Clusters Topics covered in this presentation: Analog Instrument Clusters Digital Instrument Clusters Instrument Gauges and Computers

1. Instrument Clusters Theory Support Automotive – Displays & Accessories 2 of 13 Typical Analog Instrument Cluster The speedometer is usually situated in a prominent position. 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). An instrument cluster displays the status of a vehicles systems. The rev limit (red area) indicates the maximum engine speed allowed, before damage can occur. Tachometer Rev limit Odometer Pointer Speedometer

1. Instrument Clusters Theory Support Automotive – Displays & Accessories 3 of 13 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. Typical Analog Instrument Cluster The gauges usually have a symbol to represent their function. 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. Fuel gauge Warning lamps Temperature gauge

1. Instrument Clusters Theory Support Automotive – Displays & Accessories 4 of 13 Typical Instrument Cluster Construction 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. 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. The instrument cluster is usually made of plastic and is held in position with screws. PCB Support housing Transparent cover or lens Speed, fuel and temperature gauges Instrument housing

1. Instrument Clusters Theory Support Automotive – Displays & Accessories 5 of 13 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. At the speedometer head in the instrument cluster, a small magnet rotates. The resulting magnetic force moves the speedometer pointer. Flexible inner cable Outer cable housing Speedometer head Transmission housing Cable inner wire and gear Drive gear

1. Instrument Clusters Theory Support Automotive – Displays & Accessories 6 of 13 Trigger wheel Transmission housing Speed sensor Analog or digital display Computer (ECU) 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. 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. Analog signal When a vehicle is moving, the sensor produces a signal that relates to vehicle speed.

1. Instrument Clusters Theory Support Automotive – Displays & Accessories 7 of 13 Battery Ignition switch Switching / ignition pulses Tachometer Coil switching device Ignition coil 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. The tachometer is connected to the minus (switching) terminal of the ignition coil. The switching pulses trigger the analog pointer in the tachometer to display engine speed in revolutions per minute (RPM). The switching rate is proportional to engine speed.

1. Instrument Clusters Theory Support Automotive – Displays & Accessories 8 of 13 Analog or digital drive Trigger wheel Speed sensor Computer (ECU) Digital tachometer Analog tachometer Analog signal Sensor Driven Analog/Digital Tachometer The crankshaft is fitted with a trigger wheel, which rotates while the engine is running. The computer (ECU / BCM) processes the input signal and converts the information into drive signals, for either analog or digital tachometers. A speed sensor is positioned next to the trigger wheel. It produces a signal that relates to engine speed.

1. Instrument Clusters Theory Support Automotive – Displays & Accessories 9 of 13 Fuel consumption Warning/status lamps Climate control Tachometer Main display 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).

1. Instrument Clusters Theory Support Automotive – Displays & Accessories 10 of 13 Digital/ switch Analog Input stage Processing stage Output stage Input. Processing. Output. The input stage processes both analog and digital inputs. Weak or noisy digital signals are conditioned, ready for processing. Typical Instrument / Body Computer (BCM) Analog voltages are converted to digital signals by the A/D converter and / or amplifier, ready for processing. Some input devices require a reference voltage to function. This is provided by the BCMs power supply. The instrument computer has three main stages of operation:

1. Instrument Clusters Theory Support Automotive – Displays & Accessories 11 of 13 Digital/ switch Analog Input stage Processing stage Output stage CPU Typical Instrument / Body Computer (BCM) The brain of the computer is the processing stage. The CPU stores input data in random access memory (RAM). The output stage switches the required actuators. CPU operation is controlled by a software program, stored in read only memory (ROM). It compares input data against expected data and switches appropriate output drivers.

1. Instrument Clusters Theory Support Automotive – Displays & Accessories 12 of 13 Fuel level sensor Lamp BCM Digital display Speaker 5V The resistance of the fuel level sensor changes with fuel quantity. 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. Fuel Level Circuit Typical Body Computer Circuits This causes a corresponding voltage change at the BCMs analog input.

1. Instrument Clusters Theory Support Automotive – Displays & Accessories 13 of 13 Oil pressure switch BCM Lamp Speaker 12V Typical Body Computer Circuits When engine oil pressure is correct, the oil pressure switch is closed. The BCM will switch on a warning lamp or speaker, when 12V is measured at its input. When engine oil pressure is too low, the oil pressure switch is open. The voltage at the BCM input is zero. The voltage at the BCM input is 12V. Oil Pressure Circuit