Description of On-Board Diagnostics

Fig. 8 ALCL Connector (Located Underdash):

This system does not require the use of special diagnostic equipment for testing. A digital voltmeter with 10 megohm impedance, an ohmmeter capable of low level readings, a tachometer, test light, hand vacuum gauge and pump and jumper leads are required.

To extract a trouble code from the ECM for diagnostic purposes, the Assembly Line Communication Link (ALCL) connector, Fig. 8, is used.

The ALCL diagnostic connector is located in the passenger compartment. Terminal B of the connector is the test terminal and terminal A is the ground used for diagnostic display.

If the test terminal is grounded with ignition on and engine not running, the system will enter the diagnostic mode. In this mode, the ECM will display a Code 12 three consecutive times by flashing the "Check Engine" or "Service Engine Soon" light. A Code 12 consists of one flash, a short pause, then two flashes in rapid succession. After Code 12 is displayed, any stored trouble codes will be displayed three times each by flashing the "Check Engine" or "Service Engine Soon" light, then Code 12 will be displayed again. In the diagnostic mode, the ECM will also energize all ECM controlled relays and solenoids.

After determining the trouble code(s), refer to "Trouble Code Identification."

If the test terminal is grounded with the engine running, the system will enter the field service mode. In this mode, the "Check Engine" or "Service Engine Soon" light will indicate whether the system is in open or closed loop operation. If the system is in open loop operation, the light will flash approximately two and one-half times per second. Closed loop operation is indicated by the light flashing approximately once per second. In closed loop operation, the light will stay out most of the time if system is too lean and remain on most of the time if system is too rich.