How to Extract Codes & Perform Tests

Fig. 3 ALDL connector location:

A three terminal connector, located near the ECM connector, is used to actuate the trouble code system within the ECM. This is the ALDL connector (Assembly Line Diagnostic Link) or ALCL connector (Assembly Line Communications Link), Fig. 3. Two terminals of this connector (A and C) are used to activate the trouble code system in the ECM. With the ignition switch turned ON, a jumper wire between terminal A and C will activate the trouble code system. The ``Check Engine'' light will begin to flash a code 12. Code 12 consists of one flash, a short pause, then two flashes. There will be a longer pause and a code 12 will repeat two more times. This check indicates that the self-diagnostic system is operating. The cycle will then repeat itself until the engine is started or the ignition is switched OFF. In most cases, codes will be checked with the engine operating since no codes other than code 12 or 51 will be present on initial key ON (ignition switch ON).
Remove ground from test terminal before starting engine.