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 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. Later versions were bidirectional and operated at a much faster rate of 8192 baud. Implementations using the 8192 baud rate were request-driven, meaning that no data was transmitted unless a request was made. Conversely, the slower 160 baud implementations repeatedly transmitted sensor data as long as the correct diagnostic resistor is connected to the ALDL port. ALDL data is sent in a format unique to the model of ECU in the vehicle.
The signaling of ALDL is very similar to the RS-232 serial data standard; differing in the voltages used to denote logical one (usually 0VDC) and logical zero (either +5VDC or +12VDC), and that unlike RS232, both transmit and receive functions are on the same conductor. Schematics are available on the internet for devices that can be used to convert the ALDL voltages to that of the RS-232 standard, allowing the raw data to be read with a computer having a serial port and the proper software.
Multiple scanner software programs are available. TunerPro RT is one of the most flexible and most popular. It covers most US applications.  94-95 6.5 Turbo Diesel scanner software is also available  Direct USB to ALDL cables and even Bluetooth modules are available from suppliers like Red Devil River.