Accessing and Reading Diagnostic Trouble Codes

Fig. 2 ALDL connector terminal identification:

The Assembly Line Communication Link (ALCL) is a diagnostic connector located in the passenger compartment (Figure 2). It has terminals which are used in the assembly plant to check that the engine is operating properly before it leaves the plant. Terminal "B" is the Diagnostic terminal, and it can be connected to terminal "A", or ground, to enter the Diagnostic mode.

If the Diagnostic terminal is grounded with the ignition "ON" and the engine stopped, the system will enter Diagnostic Mode. In this mode the ECM will:

1. Display a code "12" by flashing the "Check Engine"/"Service Engine Soon" light (indicating the system is now operating). A code "12" consists of one flash, followed by a short pause, then two flashes in quick succession. This code will be flashed three times. If no other codes are stored, code 12 will continue to flash until the Diagnostic terminal is ungrounded:

The engine should not be started with the Diagnostic terminal grounded, because it may continue to flash a Code 12 with the engine running. Also, if the test terminal is grounded after the engine is running, any stored codes will flash, but code 12 will flash only if there is a problem with the distributor reference signal.

2. Display any stored trouble codes by flashing the "Check Engine"/"Service Engine Soon" light. Each code will be flashed three times, then code "12" will be flashed again. If a trouble code is displayed, the memory is cleared, then the engine is run to see if the code is a "hard" or "intermittant" failure. If it is a "hard" failure, a Diagnostic Code Chart is used to find the problem. If it is an intermittant failure, the charts are not used. A physical inspection of the applicable system is made.

3. Energize all ECM controlled relays and solenoids (with some exceptions, as noted in "Component Systems"). The mixture control solenoid is pulsed for 25 seconds or until the engine is started, whichever occurs first.