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):
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.
This is the simplest form of the circuit, and the one that we have chosen to use. We have tested it and it performs quite well in every application where we have used it. This circuit requires that you have a source for the 12V DC power. In our case, this comes from either the cigarette lighter plug or from
The Environmental Protection Agency standardized vehicle diagnostics in 1996 so that fault codes would become universal accross makes and models. Before this came into effect, each manufacturer had its own system. Ford, for example, had Electronic Engine Control and the self-testing terminals were located under the hood. The ALDL outlet was something specific to all General Motors vehicles, Chevrolet included.