Description of On-Board Diagnostics

DIAGNOSIS SYSTEM

a. Description




- When troubleshooting OBD II vehicles, the only difference from the usual troubleshooting procedure is that you need to connect the OBD II scan tool complying with SAE J1978 or TOYOTA hand-held tester to the vehicle, and read off various data output from the vehicle's ECM.

- OBD II regulations require that the vehicle's on-board computer lights up the MIL on the instrument panel when the computer detects a malfunction in the computer itself or in drive system components which affect the vehicle emissions. In addition to the MIL lighting up when a malfunction is detected, the applicable DTCs prescribed by SAE J2021 are recorded in the ECM memory.
If the malfunction does not reoccur in 3-trips, the MIL goes off but the DTCs remain recorded in the ECM memory.




- To check the DTCs, connect the OBD II scan tool or TOYOTA hand-held tester to the DLC 3 on the vehicle. The OBD II scan tool or the TOYOTA hand-held tester also enables you to erase the DTCs and check freeze frame data and various forms of engine data. (For operation instructions, see the OBD II scan tool's instruction book.)

- DTCs include SAE controlled codes and Manufacturer controlled codes.
SAE controlled codes must be set as prescribed by the SAE, while Manufacturer controlled codes can be set freely by the Manufacturer within the prescribed limits.

- The diagnosis system operates in normal mode during normal vehicle use, and also has a check mode for technicians to simulate malfunction symptoms and perform a troubleshooting. Most DTCs use 2-trip detection logic (*) to prevent erroneous detection. By switching the ECM to check mode when troubleshooting, the technician can cause the MIL to light up for a malfunction that is only detected once or momentarily. (TOYOTA hand-held tester) (See step 3.)

- *2-trip detection logic:
When a logic malfunction is first detected, the malfunction is temporarily stored in the ECM memory. If the same malfunction is detected again during the second test drive, this second detection causes the MIL to light up.




b. Inspect the DLC 3.
The vehicle's ECM uses V.P.W. (Variable Pulse Width) for communication to comply with SAE J1850. The terminal arrangement of the DLC 3 complies with SAE J1962 and matches the V.P.W. format.

HINT: If your display shows "UNABLE TO CONNECT TO VEHICLE" when you have connected the cable of the OBD II scan tool or the TOYOTA hand-held tester to the DLC 3, turned the ignition switch ON and operated the scan tool, there is a problem on the vehicle side or tool side.
- If communication is normal when the tool is connected to another vehicle, inspect the DLC 3 on the original vehicle.
- If communication is still not possible when the tool is connected to another vehicle, the problem is probably in the tool itself, so consult the Service Department listed in the tool's instruction manual.