B Series Truck 2WD L4-2184cc 2.2L 2200 2-bbl (1990)
Engine Control Module: Description and Operation
Carburetor
The ECU is the central component of this computerized fuel, ignition and emission control system. It is a carburetor feedback system featuring an ECU
controlled air/fuel solenoid. To maintain fuel economy, good driveability and reduced vehicle emissions, the ECU monitors data from various sensors
and controls functions such as air/fuel ratio, emission control devices and on-board diagnostics.
The ECU 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 ECU generates a controlling signal to the air/fuel solenoid. The ECU regulates the opening duration (duty cycle) of the air/fuel
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 ECU has a fail-safe mode of preprogrammed values.
The output of the ECU will automatically switch to fail-safe mode if a fault is detected in any of these components and/or systems:
Atmospheric pressure sensor
EGR position sensor
Feedback system
Oxygen sensor
Vacuum sensor
Water thermosensor
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 ("CHECK") and stay on (while the engine is running). To extract codes, the technician can
either utilize 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 and 80,000 miles to indicate the need for scheduled maintenance.
NOTE: The ECU 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.
