Computers and Control Systems: Description and Operation

GENERAL DESCRIPTION
The Computer Command Control System (C3) for vehicles with port fuel injection is a computer controlled multiport electronic fuel injection, ignition and emission control system. The heart of the system is a computer referred to as a "Powertrain Control Module" or PCM (in previous years referred to as Electronic Control Module - ECM.) The PCM monitors operating conditions like engine speed/load and air/fuel ratio through various engine sensors, including an exhaust gas oxygen sensor, and controls the following:

^ Fuel Delivery (pump) and Fuel Injection System
^ Ignition / Electronic Spark Timing (EST)
^ Electronic Spark Control (ESC - knock sensor)
^ Air Conditioning Compressor Clutch
^ Engine Cooling Fan(s)
^ Exhaust Gas Recirculation (digital EGR solenoid valves)
^ Idle Speed Control
^ Evaporative Emission Control System (purge control)
^ [1][2]Torque Converter Clutch. (TCC)
^ Electronically Controlled Automatic Transmission (shift control)

ON-BOARD SELF DIAGNOSTICS - FAULT RECOGNITION
The C3 system has a built-in diagnostic system that recognizes problems in input/output devices and circuits. If a problem is detected, a "trouble code" is stored in the computers memory and the computer turns on the "Malfunction Indicator Light" (MIL), located on the instrument panel. The light will stay "ON" as long as the problem is present. If a problem is intermittent, the light will turn off, but a code will be stored.

NOTE: Some codes will not turn on the SES light.

USING ON-BOARD SELF DIAGNOSTICS - TROUBLE CODES
The MIL can be used to assist the technician in locating and correcting problems within the C3 system. When entered into diagnostic mode, the PCM will display trouble codes by flashing the MIL. Codes are read by counting the number of times the light flashes. See DIAGNOSIS, TESTING AND INSPECTION PROCEDURES/TESTING PROCEDURES/ON-BOARD DIAGNOSTICS for display procedures and trouble code descriptions.

ASSEMBLY LINE DATA LINK (ALDL) CONNECTOR
A special diagnostic connector, called the Assembly Line Data Link or ALDL connector, is provided. This connector is used to enter the computer into diagnostic mode for displaying trouble codes. An electronic "SCAN" tool can also be plugged into the ALDL connector to display trouble codes, monitor engine sensors and switches, and in some cases, operate output devices for diagnosis and testing.

FAIL-SAFE OPERATION
The system also has a built in back-up system to provide near normal driving conditions during system malfunction. When problems are detected in certain systems, the PCM will revert to its fail-safe mode so the vehicle can be driven until repaired.