Assembly Line Diagnostic Link or ALDL was a proprietary on-board diagnostics system developed by General Motors. ALDL was previously called Assembly Line Communications Link or ALCL. The two terms are used synonymously.
It is sometimes referred as a predecessor to, or a manufacturer's proprietary version of OBD-I and OBD-II diagnostic systems. There are several varieties of this interface, changed with power train control modules such as aka PCM, ECM and ECU). Different ALDL versions had slight differences in pin-outs and baud rates. Earlier interfaces used a 160 baud rate, while later went up to 8192 baud and used bi-directional communications to the PCM.
ALDL was largely used by manufacturers for diagnostics at their dealerships and official maintenance facilities. The connector usually locates under the dashboard on the driver's side of left-hand drive (LHD) vehicles, but the location is not standardized.