Diagnosis System
- DESCRIPTION
When troubleshooting OBD II (On-Board Diagnostics) vehicles, the intelligent tester or OBD II scan tool (complying with SAE J1987) must be connected to the DLC3 (Data Link Connector 3) of the vehicle. Various data in the vehicle's ECM (Engine Control Module) can be then read.
OBD II regulations require that the vehicle's on-board computer illuminates the MIL (Malfunction Indicator Lamp) on the instrument panel when the computer detects a malfunction in:
- The emission control systems and components
- The power train control components (which affect vehicle emissions)
- The computer itself
In addition, the applicable Diagnostic Trouble Codes (DTCs) prescribed by SAE J2012 are recorded in the ECM memory. If the malfunction does not reoccur in 3 consecutive trips, the MIL turns off automatically but the DTCs remain recorded in the ECM memory.
To check DTCs, connect the intelligent tester or OBD II scan tool to the DLC3. The tester and scan tool display DTCs, freeze frame data, and a variety of engine data. The DTCs and freeze frame data can be erased with the tester and scan tool (see DTC CHECK / CLEAR ).
- 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 as an option for technicians. In check mode, 1 trip detection logic is used for simulating malfunction symptoms and increasing the system's ability to detect malfunctions, including intermittent problems (intelligent tester only) (see CHECK MODE PROCEDURE ).
- TRIP DETECTION LOGIC
When a malfunction is first detected, the malfunction is temporarily stored in the ECM memory (1st trip). If the same malfunction is detected during the next subsequent drive cycle, the MIL is illuminated (2nd trip).
- FREEZE FRAME DATA
Freeze frame data records the engine conditions (fuel system, calculated engine load, engine coolant temperature, fuel trim, engine speed, vehicle speed, etc.) when malfunctions are detected. When troubleshooting, freeze frame data can help determine if the vehicle was moving or stationary, if the engine was warmed up or not, if the air-fuel ratio was lean or rich, and other data from the time the malfunction occurred.
- DATA LINK CONNECTOR3 (DLC3)
The vehicle's ECM uses the ISO 9141-2 communication protocol. The terminal arrangement of the DLC3 complies with SAE J1962 and matches the ISO 9141-2 format.
DATA LISTSymbols Terminal No. Name Reference terminal Results Condition SIL 7 Bus "+" line 5 - Signal ground Pulse generation During transmission CG 4 Chassis ground Body ground 1 Ω or less Always SG 5 Signal ground Body ground 1 Ω or less Always BAT 16 Battery positive Body ground 9 to 14 V Always HINT:
The DLC3 is the interface prepared for reading various data from the vehicle's ECM. After connecting the cable of the intelligent tester or OBD II scan tool, turn the ignition switch ON and turn the tester or scan tool ON. If a communication failure message is displayed on the tester or scan tool screen (on the tester: UNABLE TO CONNECT TO VEHICLE), a problem exists in either the vehicle or tester. In order to identify the location of the problem, connect the tester to another vehicle. If communication is normal: Inspect the DLC3 on the original vehicle.
If communication is impossible: The problem is probably with the tester itself. Consult the Service Department listed in the instruction manual.
- BATTERY VOLTAGE
Battery voltage:
11 to 14 V
If the voltage is below 11 V, recharge or replace the battery before proceeding.
- Malfunction Indicator Lamp (MIL)
- The MIL is illuminated when the ignition switch is first turned ON (the engine is not running).
- The MIL should turn off when the engine is started. If the MIL remains illuminated, the diagnosis system has detected a malfunction or abnormality in the system.
HINT:
If the MIL is not illuminated when the ignition switch is first turned ON, check the MIL circuit (see ).