System Description

DIAGNOSIS SYSTEM DESCRIPTION



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

The Malfunction Indicator Lamp Is Located In The Instrument Cluster:

OBD II regulations require that the vehicle's on-board computer lights up the Malfunction Indicator Lamp (MIL) on the instrument panel when the computer detects a malfunction in the computer itself or in drive system components which affect vehicle emissions. In addition to the MIL lighting up when a malfunction is detected, the applicable diagnostic trouble codes prescribed by SAE J2012 are recorded in the ECM memory.
If the malfunction does not recur in 3 trips, the MIL goes off but the diagnostic trouble codes remain recorded in the ECM memory.
To check the diagnotic trouble codes, connect the OBD II scan tool or TOYOTA hand-held tester to Data Link Connector 3 on the vehicle. The OBDIl scan tool or TOYOTA hand- held tester also enables you to erase the diagnostic trouble codes and check freezed frame data and various forms engine data. (For operating instructions, see the OBD II scan tool's instruction book.)
Diagnostic trouble codes 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. It also has a check mode for technicians to simulate malfunction symptoms and troubleshoot. Most diagnostic trouble codes use 2 trip detection logic* to prevent erroneous detection and ensure thorough malfunction 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 only)

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 drive test, this second detection causes the MIL to light up.