The "Check Engine" or "Service Engine Soon" light on the instrument panel is used as a warning lamp to tell the driver that a problem has occured in the electronic engine control system. When the self-diagnosis mode is activated by grounding the test terminal of the diagnostic connector, the check engine light will flash stored trouble codes to help isolate system problems. The Electronic Control Module (ECM) has a memory that knows what certain engine sensors should be, under certain conditions. If a sensor reading is not what the ECM thinks it should be, the control unit will illuminate the check engine light and store a trouble code in its memory. The trouble code indicates what circuit the problem is in, each circuit consisting of a sensor, the wiring harness and connectors to it and the ECM.
The Assembly Line Diagnostic Link (ALDL) is a diagnostic connector located in the passenger compartment console. It has terminals which are used in the assembly plant to check that the engine is operating properly before shipment. Terminal B is the diagnostic test terminal and Terminal A is the ground. By connecting the two terminals together with a jumper wire, the diagnostic mode is activated and the control unit will begin to flash trouble codes using the check engine light.
When the test terminal is grounded with the key ON and the engine stopped, the ECM will display code 12 to show that the system is working. The ECM will usually display code 12 three times, then start to display any stored trouble codes. If no trouble codes are stored, the ECM will continue to display code 12 until the test terminal is disconnected. Each trouble code will be flashed three times, then code 12 will display again. The ECM will also energize all controlled relays and solenoids when in the diagnostic mode to check function.
When the test terminal is grounded with the engine running, it will cause the ECM to enter the Field Service Mode. In this mode, the service engine soon light will indicate whether the system is in Open or Closed Loop operation. In open loop, the light will flash 2 1 / 2 times per second; in closed loop, the light will stay out most of the time if the system is too lean and will stay on most of the time if the system is too rich.
The vehicle may be driven in the Field Service mode and system evaluated at any steady road speed. This mode is useful in diagnosing driveability problems where the system is rich or lean too long.
Trouble codes should be cleared after service is completed. To clear the trouble code memory, disconnect the battery for at least 10 seconds. This may be accomplished by disconnecting the ECM harness from the positive battery pigtail or by removing the ECM fuse.