System Description

DIAGNOSIS SYSTEM

1. DESCRIPTION

- When troubleshooting On-Board Diagnostic (OBD II) vehicles, the vehicle must be connected to the OBD II scan tool (in compliance with SAE J1978). Various data output from the vehicle's ECM can then be read.




- OBD II regulations require that the vehicle's on-board computer illuminates the Malfunction Indicator Lamp (MIL) on the instrument panel when the computer detects a malfunction in:

1. The emission control system/components.
2. The powertrain control components (which affect vehicle emissions).
3. The computer.
In addition, the applicable Diagnostic Trouble Code (DTC) prescribed by SAE J2012 are recorded in the ECM memory.
If the malfunction does not reoccur in 3 consecutive trips, the MIL goes off automatically but the DTCs remain recorded in the ECM memory.




- To check DTCs, connect the hand-held tester to the Data Link Connector 3 (DLC3.of the vehicle.The hand-held tester displays DTCs, freeze frame data and a variety of engine data.

The DTCs and freeze frame data can be erased with the scan tool

2. NORMAL MODE AND CHECK MODE

The diagnosis system operates in "normal mode" during normal vehicle use.In normal mode, "2 trip detection logic" is used to ensure accurate detection of malfunctions. "Check mode" is also available to technicians as an option. In check mode, "1 trip detection logic" is used for simulating malfunction symptoms and increasing the system's ability to detect malfunctions, including intermittent malfunctions.

3. 2 TRIP DETECTION LOGIC

When a malfunction is first detected, the malfunction is temporarily stored in the ECM memory (1st trip). If the ignition switch is turned OFF and then ON again, and the same malfunction is detected again, the MIL will illuminate.