Carburetor

NOTE: The Powertrain Control Module (PCM) constantly monitors input devices for malfunctions. However, it only checks for output device faults during a three second period after the ignition is switched "ON" and test connector (green, 1 pin) is grounded.

The PCM is the central component of this computerized fuel, ignition and emission control system. It is a carburetor feedback system with an PCM controlled mixture control solenoid. To maintain fuel economy, good driveability and reduced vehicle emissions, the PCM monitors data from various sensors and controls functions such as air/fuel ratio, emission control devices and on-board diagnostics.

The PCM is a computer that judges air/fuel ratio as rich or lean according to a voltage signal generated by an oxygen sensor. Based on oxygen sensor and other input data, the PCM generates a controlling signal to the mixture control solenoid. The PCM regulates the opening duration (duty cycle) of the mixture control solenoid, maintaining the mixture close to the stoichiometric ratio (14.7:1) and suitability to prevailing driving conditions.

To provide a margin of safety by maintaining driveability in the event of certain system failures, the PCM has a fail-safe mode of preprogrammed values. The output of the PCM will automatically switch to fail-safe mode if a fault is detected in any of these components and/or systems:

^ Barometric absolute pressure sensor
^ EGR position sensor
^ Feedback system
^ Oxygen sensor
^ Vacuum sensor
^ Engine coolant temperature sensor

The system incorporates self-diagnostic capability monitored via a dash mounted malfunction indicator lamp (MIL). When the system detects a fault in a monitored component and/or circuit, the lamp will light and stay on (while the engine is running). To extract codes, the technician can either use a "Scan" tool or cause the MIL to display codes by grounding the test connector (green, 1 pin) in the engine compartment. On Federal specification vehicles, the MIL also comes on after 60,000 miles (96,000 km) and 80,000 miles (128,000 km) to indicate the need for scheduled maintenance.