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Assembly Line Diagnostic Link or ALDL was a proprietary on-board diagnostics system developed by General Motors prior to the standardization of OBD-2. For the assembly plant test system computer which was connected to this vehicle connector and known by the same name see the article IBM Series/1.

ALDL was previously called Assembly Line Communications Link or ALCL. The two terms are used synonymously.

This system was only vaguely standardized and suffered from the fact that specifications for the communications link varied from one model to the next. ALDL was largely used by manufacturers for diagnostics at their dealerships and official maintenance facilities. The connector is usually located under the dash on the driver's side of left-hand drive (LHD) vehicles, though this location was not standardized.