ALDL

📖 YOUCANIC Automotive Glossary

ALDL (Assembly Line Diagnostic Link), sometimes called the Assembly Line Communications Link (ALCL), was GM’s pre-OBD-II diagnostic connector used from the early 1980s through 1995. The ALDL was a 12-pin rectangular connector typically located under the dashboard. On GM vehicles, jumping pins A and B with the key on would put the ECM into diagnostic mode, causing the check engine light to flash stored DTCs as a series of blinks. The ALDL also supported serial data communication for GM’s proprietary scan tools. Other manufacturers had their own pre-OBD-II connectors with different shapes and pin configurations.

The ALDL was replaced by the standardized 16-pin OBD-II DLC connector in 1996. DIYers working on pre-1996 GM vehicles can still use the ALDL flash code method for basic diagnostics without any scan tool — turn the key to ON (engine off), jumper pins A and B, and count the check engine light flashes. Code 12 (one flash, pause, two flashes) is the starting code indicating the ECM is in diagnostic mode.