ALDL

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.

There were at least four different connectors used with ALDL. 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. Lotus implemented a 10-pin connector. The pins are given letter designations in the following layouts (as seen from the front of the vehicle connector):

12-pin ALDL connector pinout:
F E D C B A
G H J K L M

10-pin ALDL connector pinout:
A B C D E
K J H G F

5-pin ALDL connector pinout:
4A B C D E

Note the difference in pin ordering between the connectors and the fact that the letter I is not used. Unfortunately, the definition of which signals were present on each pin varied between vehicle models. 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. No battery voltage is present in the 12 pin ALDL connector.

The earliest implementations of ALDL were unidirectional and transmitted serial data at 160 baud using PWM. 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.

Later versions were bidirectional and operated at a much faster (but incredibly slow compared to today's standards) rate of 8192 baud. Implementations using the 8192 baud rate were primarily request-driven, meaning that the main diagnostic data was not transmitted until a request was made. Some idle data transmission of trivial parameters, however, existed in many vehicles. Bidirectional communication also allowed many other functions to be performed via ALDL, such as actuator tests, parameter overrides, and in some cases even reprogramming of the ECU itself. Multiple devices could be placed on the ALDL data line for primitive networking and communication. Many later 8192 baud vehicles, for example, had airbag control, ABS, and even climate control units sending data on the same serial bus.

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