Diagnosis System

When troubleshooting OBD II (On-Board Diagnostics) vehicles, a hand-held 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.

Fig 1: Identifying MIL

Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

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:

1. The emission control systems and components
2. The power train control components (which affect vehicle emissions)
3. The computer itself

In addition, the applicable DTCs (Diagnostic Trouble Codes) prescribed by SAE J2012 are recorded on 3 consecutive trips, the MIL turns off automatically but the DTCs remain recorded in the ECM memory (see DIAGNOSTIC TROUBLE CODE CHART ).

Fig 2: Connecting Hand-Held Tester Or OBD II Scan Tool To DLC3

Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

To check DTCs, connect a hand-held 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 ).

The diagnosis system operates in the 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 (hand-held tester only) (see CHECK MODE PROCEDURE ).

Fig 3: Identifying DLC3 Connector Terminal

Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

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 record the engine condition (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.

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.

Fig 4: Identifying DLC3

Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

HINT:

The DLC3 is the interface prepared for reading various data from the vehicle's ECM. After connecting the cable of a hand-held tester or OBD II scan tool, turn the ignition switch to 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: 11 to 14 V 

If the voltage is below 11 V, recharge the battery before proceeding.

1. The MIL is illuminated when the ignition switch is first turned to ON (the engine is not running).
2. 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 to ON, check the MIL circuit (see MIL CIRCUIT ).