Introduction to Automotive Computer Controls             

For years, automotive engineers found it challenging to design mechanical, vacuum, and hydraulic systems that could consistently and accurately deliver the needed results.  Systems strayed widely from the ideal air/fuel ratios because they could not monitor combustion or accurately control fuel delivery. They required elaborate emissions controls systems to meet stringent exhaust emissions standards. These systems required a great deal of maintenance and sometimes were complicated, making them difficult to repair. If automotive engineers concentrated on increasing fuel economy and lowering emissions, performance and driveability would suffer. If they focused on reducing emissions, the economy, driveability, and performance would suffer. Electronic engine controls allow automotive engineers to restore good driveability, increase economy, lower exhaust emissions, and increase performance.

Good driveability is what the driver expects from a properly functioning engine, including easy starting, smooth idle, good acceleration through all speeds, instant response, and full power. Driveability concerns arise when a vehicle stalls, hesitates or stumbles, idles rough, surges, lacks power, is hard to start, or fails to start. 

The official start of On-Board Diagnostics I (OBD I) was in 1988 (1). At that time, the California Air Resources Board (CARB) required that all vehicles sold new in California be equipped with a Check Engine Light (CEL). CARB required manufacturers to monitor the items in the area (2) below.

On-Board Diagnostics II (OBD II) was officially required to be equipped on all 1996 and newer passenger cars and light-duty trucks. Some manufacturers installed OBD II on 1994 and 1995 model-year vehicles. CARB required manufacturers to monitor the below items in the area (5).
The basic principles of Computer Fundamentals do not change. The foundation is always Input, Process, and Output. An important note is to look at where the scan tool is placed in the system. It takes processed data from the PCM and decodes it so the technician can read it. Remember that connecting a scan tool means adding another module to the network.

Computer Fundamentals I-P-O principles: Input, Process, and Output apply to every computer today. The sequence of Input, Process, and Output is vital to proper system operation. The basic principles of Computer Fundamentals do not change.

One important note: look at where the scan tool is placed in the system. Sensors and Switches provide the Input to the Processor as raw, non-processed data; the PCM processes the data and issues digital commands to the Outputs. The scan tool decodes the processed data so the technician can read the information.