Data Link Connector (DLC) Pinout: A Comprehensive Guide

The Data Link Connector (DLC), also known as the Assembly Line Diagnostic Link (ALDL), is a crucial component for diagnosing vehicle issues. This article provides a comprehensive guide to understanding DLC pinouts, focusing on the different connector types and their respective functionalities. Understanding the pinout configuration is essential for effectively using diagnostic tools and retrieving trouble codes.

Understanding Data Link Connector (DLC) Variations

General Motors (GM) vehicles predominantly utilized two main DLC types: a 12-pin connector and a less common 5-pin connector. Lotus employed a 10-pin connector. Each connector features a specific pin arrangement, identified by letter designations.

12-Pin DLC Pinout

The 12-pin DLC, the most prevalent in GM vehicles, has the following layout (viewed from the connector’s front):

F E D C B A
G H J K L M

Important Note: The 12-pin DLC does not supply battery voltage.

10-Pin DLC Pinout

Lotus vehicles used a 10-pin DLC with this configuration:

A B C D E
K J H G F

5-Pin DLC Pinout

The less common 5-pin DLC layout is as follows:

A B C D E

Observe that the letter “I” is omitted in all configurations, and the pin order varies between connector types. Crucially, the function assigned to each pin also differed depending on the vehicle model.

Essential DLC Pins for Diagnostics

While various pins provided vehicle-specific data and control functions, three core pins were universally used for basic diagnostics:

• Ground: Provides a ground connection for the diagnostic tool. The “A” terminal in the 12-pin connector commonly serves as the ground. Ensuring a reliable ground connection is critical for successful communication with the scan tool.
• Battery Voltage: Supplies power to the diagnostic tool (Not present in the 12-pin connector).
• Data: Transmits diagnostic data between the vehicle’s Engine Control Module (ECM) and the scan tool. In the 12-pin connector, the “E” terminal is dedicated to data transmission.

Retrieving Diagnostic Trouble Codes (DTCs)

Diagnostic trouble codes (DTCs), often referred to as “check engine light codes”, pinpoint malfunctions within the vehicle’s systems. To retrieve these codes using the DLC:

1. Initiate Diagnostic Mode: In a 12-pin DLC, jumpering terminals “A” (ground) and “B” initiates diagnostic mode. This action prompts the check engine light to flash, signaling the ECM is ready to communicate.

2. Interpreting Flash Codes: The check engine light will flash a sequence of codes. For example, a single flash followed by a pause and two quick flashes represents code “12,” indicating the ECM is in diagnostic mode. Each code is flashed three times before progressing to the next stored code. If no codes are present, code “12” repeats continuously.

This image illustrates a typical ALDL connector, highlighting its physical appearance and connection points. Understanding the connector’s layout is the first step in successfully accessing vehicle diagnostic data. Consult your vehicle’s specific documentation for precise pin assignments and diagnostic procedures.