ALDL

Assembly Line Diagnostic Link or ALDL was a proprietary on-board diagnostics system developed by General Motors prior to the standardization of OBD-I.

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.

Diagnostic connector

There were at least three different connectors used with ALDL. General Motors implemented both a 5-pin connector and a 12-pin connector, with the 12 pin connector 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.