See Figures 1 and 2
There are many test procedures that require the connecting a scan tool or connecting a jumper wire to terminals 1 and 3 of the 3-terminal ALDL (the main diagnostic tool connector). The ALDL is used during vehicle assembly to test the engine before it leaves the assembly plant and after the factory to communicate with the ECM. The ALDL is located in the passenger compartment. For most vehicles, it is under the right side of the dashboard, to the right of the glove compartment. Terminal 1 is the diagnostic test terminal and is located on the far left of the connector. Terminal 3, located to the far right of the connector, is the engine ground. When these terminals are jumpered together and the ignition switch turned ON , the CHECK ENGINE MIL will flash Code 12. This indicates that the internal diagnostic system is operating and that specific output device signals are being generated.