See Figure 1
There are many test procedures that require connecting a scan tool or connecting a jumper wire to terminals A and B of the 12-terminal ALDL/DLC (the main diagnostic tool connector). The connector is used during vehicle assembly to test the engine control systems before the vehicle leaves the assembly plant. The connector is also used throughout the vehicle's life to communicate with the computer control module.
When using a jumper wire at the connector to activate trouble code read out, the jumper must be connected during certain ignition switch conditions. On carbureted vehicles, the jumper should be connected with the ignition switch turned ON to begin code display. For fuel injected vehicles, the jumper should be connected with the ignition switch OFF, then with the jumpers in place, the switch should be turned ON to begin code read out. In both cases, the engine should not be running.
The diagnostic connector is located in the passenger compartment. For most vehicles covered by this manual, the connector is located under the far left side of the dashboard. Terminal B is the diagnostic test terminal and is normally located on the top of the connector, second from right. Terminal A, located on the top right of the connector (to the right of terminal B), is the engine ground. When these terminals are jumpered together under the proper conditions, the MIL will flash Code 12. This indicates that the internal diagnostic system is operating. Terminal M, located on the bottom, far right of the connector, is usually the serial data terminal and is used by diagnostic scan tools to obtain and display information.