Data Link Connector: Description and Operation

Diagnosis Switch Terminal
There are two diagnosis switch terminals; one included in the monitor coupler in the cabin and the other in the monitor coupler in the engine room. When either diagnosis switch terminal is grounded, a diagnosis signal is fed to ECM which then outputs diagnostic trouble code (self-diagnosis code) and at the same time outputs idle air control (IAC) duty through duty output terminal.

Test Switch Terminal
The test switch terminal is included in the engine-room monitor coupler. When this terminal is grounded, ECM sets the ignition timing to the initial ignition timing.
When both test switch terminal and diagnosis switch terminal are grounded, ECM Outputs A/F duty through the duty output terminal.