1979, First Application of a Digital Computer with Self Diagnostic Codes
The electronic control unit created its own diagnostic codes that could be displayed by flashing sequences of the Check Engine Light (CEL) in the instrument cluster. Software logic was written and hardware added to monitor the performance of various sensors. If performance was outside the range of stored norms, then a code was set in memory and the CEL was illuminated to alert the driver. For a service technician to read out the stored code, the controller was put into the diagnostic mode by means of connecting two terminals in the under-dash ALDL connector. Among others, initial diagnostic codes were included for circuit wiring opens and shorts and the integrity of the coolant temperature, manifold air temperature and manifold air pressure sensors.
After this initial manual enabling of the flashing diagnostic codes by jumping terminals in the under dash ALDL connector, hand held readers were quickly developed to automate the readout process. Later, dealers started installing computer aided maintenance systems (CAMs) equipment. These roll-around IBM PC based computer systems hooked up to the diagnostic ALDL connector and down loaded the controller’s diagnostic history file. If there were any stored codes, regularly updated diagnostic fault trees built into their equipment and displayed on a screen, would lead technicians through the diagnoses and repair of the vehicle. Self diagnostics capability continued to grow in function following this initial introduction. An element of the self diagnostics ability was the history file or log that recorded operating conditions over a period of time. Among other things, this allowed the technician to diagnose intermittent problems that appeared and then later disappeared. Today, the history file capability has grown into such things as the so called crash data recorder, keeping track of the vehicle’s most recent operating conditions just prior to and potentially throughout an accident. With this information, the vehicle through On-Star can call emergency help and transmit location and severity of the accident.
Today, most digital electronics in consumer products employ self diagnostics capability.
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