Assembly Line Diagnostic Link or ALDL is a proprietary on-board diagnostics system developed by General Motors before the standardization of OBD-2. It was previously called Assembly Line Communications Link or ALCL. The two terms are used interchangeably.
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.
For 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.
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
- 10-pin ALDL connector pinout
- 5-pin ALDL connector pinout
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.
In both versions, ALDL data is sent in a format unique to the model of ECU in the vehicle with little standardization between models, so a proper definition of the data is required to interpret it. Most professional scan tools require a large database of vehicle definitions.
The signaling of ALDL is similar to the RS-232 serial data standard; differing in the voltages used to denote logical one (usually 0 V DC) and logical zero (either +5 V DC or +12 V DC), and that unlike RS-232, 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 those 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.
- "GM 8192/160 Baud ALDL Interface". Tech Edge. Retrieved 17 June 2012.
- Burk, Daniel. "ALDL revealed". Retrieved 4 November 2016.
- Gargano, Peter. "GM 8192/160 Baud ALDL Interface". Retrieved 4 November 2016.
- "TunerPro and TunerPro RT - Professional Automobile Tuning Software". Retrieved 4 November 2016.
- Engh, Stefan. "Engh Motors - Makers of GMTDScan, the PC OBDI/ALDL scan tools for 1994-1995 GM 6.5TD trucks". Retrieved 4 November 2016.
- "ALDL USB Cable". Retrieved 4 November 2016.