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A Look At Automotive On-Board Diagnostics

You may not know it but your late-model car has an astonishing array of sensors to make sure that its engine and electronic systems all run at peak efficiency, while keeping emissions to a minimum. This increasing use of electronics in vehicles has also lead to improvements in the way a vehicle can be maintained. With On Board Diagnostics (OBD), the performance of critical engine components can be easily monitored.

By John Clarke

All new cars sold in Australia from 2006 onwards are required to comply with the Australian Design Rules (ADR) to include On-Board Diag­nostics. This is called OBDII and the “II” indicates that this is the second generation OBD standard.

OBD is not new and has been around for more than 20 years. The first OBD standard adopted in California (USA) was introduced primarily to monitor the condition of vehicle emission sensors.

Early OBD systems included a Malfunction Indicator Light (MIL). This used a rudimentary blinking light system where the number of blinks could be counted. This blink count could be cross-referenced against a list to find the problem indicated. Over the years, there have been many refinements and improvements.

The latest OBDII standard is far more complex and includes a data link connector for connection of an OBDII scan tool which can be a dedicated hand-held unit. Alternatively, it may connect to a laptop computer via a cable that includes some form of signal processing and which is used in conjunction with special software. Either approach can be used to retrieve information from the vehicle.

Click for larger image
Fig.1: the engine management unit (ECU) accepts inputs from a range of sensors and controls the ignition timing, fuel injectors and EGR valve. The OBDII scan tool plugs into the ECU and displays a range of diagnostic information to aid troubleshooting.

On-board diagnostics are possible due to the computer systems in modern cars. Most readers would know about the car’s ECU (Engine Control Unit) which is dedicated to controlling such components as fuel injectors, the exhaust gas recirculation (EGR) valves and the ignition timing. Computerised engine control is vital to ensure optimum fuel efficiency.

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