Later versions were bidirectional and operated at a much faster (but incredibly slow compared to today's standards) rate of 8192 baud. Some 160 baud models constantly transmitted sensor data on startup, while others started transmitting data when placed in diagnostic mode with a resistor connected to the ALDL port. The earliest implementations of ALDL were unidirectional and transmitted serial data at 160 baud using PWM. No battery voltage is present in the 12 pin ALDL connector. There were generally only three pins used for basic ALDL -ground, battery voltage, and a single line for data-, although other pins were often used for additional vehicle-specific diagnostic information and control interfaces. Unfortunately, the definition of which signals were present on each pin varied between vehicle models. Note the difference in pin ordering between the connectors and the fact that the letter I is not used. The pins are given letter designations in the following layouts (as seen from the front of the vehicle connector): General Motors implemented both a 5-pin connector and a 12-pin connector, with the 12 pin connector (Packard/Delco/Delphi part number 12020043) being used in the vast majority of GM cars. There were at least four different connectors used with ALDL. In the late 1980s the system was migrated to the ADTS system (ALDL Development and Test System) which utilized an industrialized IBM PC with custom interface hardware. 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.įor the assembly plant test system computer that was connected to this vehicle connector and known by the same name, see the article IBM Series/1. ALDL was largely used by manufacturers for diagnostics at their dealerships and official maintenance facilities. This system was only vaguely standardized and suffered from the fact that specifications for the communications link varied from one model to the next.

It was previously called Assembly Line Communications Link or ALCL. Assembly Line Diagnostic Link, Assembly Line Data Link, or ALDL is a proprietary on-board diagnostics system developed by General Motors before the standardization of OBD-2.

Gm obd1 diagnostic