B Series Truck 4WD L4-2606cc 2.6L SOHC (1992)
Powertrain Control Module (PCM): Description and Operation
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. The MIL will only display trouble codes on
California specification vehicles.
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 fuel injectors that regulates "ON" time. Because fuel pressure remains
constant, the ECU controls air/fuel ratio adjustments by varying injector "ON" time.
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:
^ Airflow sensor
^ Atmospheric pressure sensor
^ Feedback system
^ Intake air thermosensor
^ Oxygen sensor
^ Throttle sensor
^ Water thermosensor
The ECU 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 utilize a "Scan" tool or cause the MIL to display codes by grounding the test connector (green, 1 pin) in the engine compartment.