Description of On-Board Diagnostics

General Description
The automatic transmission control system in this vehicle is controlled by Transmission Control Module (TCM). TCM has an On-Board Diagnostic system which detects a malfunction in this system and abnormality of those parts that influence the engine exhaust emission. When diagnosing automatic transmission troubles, be sure to have full understanding of the outline of "On-Board Diagnostic System" and each item in "Precaution in Diagnosing Trouble" and execute diagnosis according to Diagnosis by Symptom.





On-Board Diagnostic System
For automatic transmission control system, TCM has the following functions:
- When the ignition switch is turned ON with the O/D cut switch turned OFF and no malfunction in A/T control system detected, the "O/D OFF" lamp lights for about 2 sec. after the ignition switch is turned ON and then goes OFF for the bulb check. lithe O/D cut switch is ON at this time, however, "O/D OFF" lamp remains ON to let driver know that gear is not shifted to overdrive gear (4th gear).
- When a malfunction in A/T control system (and/or a malfunction which gives an adverse effect to vehicle emission) is detected, it flashes "O/D OFF" lamp, stores the malfunction DTC and at the same time transmits DTC of that malfunction to ECM. (Based on this signal, ECM turns ON malfunction indicator lamp (MIL, "CHECK ENGINE" lamp) and stores DTC/freeze frame data then in its memory.
- If no malfunction is detected in the next driving cycle after detecting above malfunction, TCM does not flash "O/D OFF" lamp and does not transmit DTC of that malfunction to ECM, however, DTC stored in TCM memory will remain. If ECM receives no DTC of malfunction for 3 driving cycles continuously, it turns OFF MIL although DTC stored in ECM memory will also remain.

Freeze Frame Data
Refer to Powertrain Management / Computers and Controls for more data.

2 Driving Cycle Detection Logic
When a malfunction is detected during the initial driving cycle, it is stored in TCM memory and at the same time TCM transmits a data to inform that a malfunction occurred to ECM. At this time, ECM stores a corresponding pending DTC and freeze frame data in its memory. (Neither lighting of MIL nor flashing of "O/D OFF" light takes place then.) If the same malfunction is also detected in the next driving cycle, TCM flashes "O/D OFF" light and ECM lights MIL when it receives a data informing occurrence of a malfunction from TCM.