On-Car Diagnostics

ALDL Connector:

The ECM incorporates a diagnostic program which, when activated, will flash codes stored in the trouble code memory through the "CHECK ENGINE" lamp. A diagnostic connector, located under the instrument panel, is used to activate the self-diagnosis system. The connector, allows access to system circuits and contains a test terminal which, when grounded with the ignition on and engine stopped, causes stored trouble codes to be flashed on the "CHECK ENGINE" lamp.

When the diagnostic terminal is grounded (jumper terminal "A" to "B") with the ignition on and engine stopped, codes are displayed in numerical order, always starting with Code 12 which indicates that the self-diagnosis system is operating properly. Trouble codes, are displayed in the following manner: Code 12 is displayed by one flash, a short pause, then two flashes and a longer pause. Code 12 will be displayed 3 times in this manner, then any additional codes stored will be displayed 3 times each in a similar manner until all codes have been displayed. Once all codes have been displayed. The process will be repeated as long as the test terminal remains grounded. DO NOT ground the diagnostic terminal until the ignition is "ON" and engine is "OFF" (not running). After trouble codes have been noted, the ground wire should be removed from the test lead and the engine should be started. When the engine is started and there are no other codes except Code 12, the "CHECK ENGINE" light should go out after approximately 5.0 seconds, indicating that the ECM has not detected a malfunction. However, if the ECM detects a current malfunction, the "CHECK ENGINE" lamp will remain illuminated while the engine is running. A trouble code indicates a possible system malfunction. If a trouble code can be obtained, even if the "CHECK ENGINE" lamp is "OFF" when the engine is running, a "SYSTEM PERFORMANCE CHECK" should be performed and the indicated circuit should be inspected.

Use the appropriate diagnostic charts if trouble codes can be obtained with the "CHECK ENGINE" lamp "OFF." A System Performance Check and a thorough physical inspection of the circuit involved should be carried out if any of these codes are evident. Repair as necessary.

Removing battery voltage to ECM terminal "R" for a period of 10 seconds will clear all stored trouble codes. This usually can be accomplished by removing the ECM fuse, if equipped, from the fuse block. In some cases it may be easier to disconnect the negative battery terminal. However, doing so will result in deprogrammed ETR radios, electronic clocks and Trip Computers.