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TCM Monitoring Functions As explained above the TCM monitors all input and outputs to identify possible failures. If a fault is detected the TCM takes the appropriate action to ensure the transmission enters a safe mode of operation, without sacrificing transmission durability or driver safety. Supply Monitoring If the battery voltage is either too great or too low, the TCM will detect a fault condition. For the TCM to be able to identify this fault, the engine must be running and the transmission fluid temperature sensor must be functioning correctly. Solenoid Supply Monitoring While the solenoid operating transistors are being activated, checks are run for open circuits, shorts circuits to ground and short circuits to supply. The monitoring function evaluates the voltage characteristics during the switch on process checking for the above faults.
All solenoid outputs are fully protected. The processor and the appropriate fail-safe action taken can quickly identify open and short circuit faults. Sensor Supply Monitoring The sensor supply voltage is a stabilized supply. This supply is monitored by the micro-processor by an Analogue to Digital Converter (ADC). If the voltage is out of the valid tolerance a raise a diagnostic trouble code (DTC) is set and the appropriate fail-safe action is performed. Electronically Erasable Program Read Only Memory (EEPROM) Monitoring To diagnose errors with the electronically erasable program read only memory (EEPROM) the TCM calculates 4 checksums continuously: If the processor identifies discrepancies in any of the four checksums the TCM will engage mechanical limp-home mode.
The TCM can diagnose errors within the EEPROM. Diagnosis is only performed during TCM initialization. There is no fail-safe mechanism associated with this function as the EEPROM is mainly used for the storage of fault codes and transmission calibration adaptions. If a fault occurs the TCM is able to perform default action and inform the driver of the problem, this is by the instrument cluster message centre. Watchdog Monitoring The watchdog monitoring function has two functions. Firstly it checks that it is possible to inhibit output control by the activation of the solenoid supply transistor. Secondly the watchdog checks that the safety circuit is functioning correctly.
During initialization the watchdog checks that it is possible to inhibit control of the pressure regulator and solenoid valves by switching the solenoid supply transistor. There is a fault if activation of the solenoids cannot be inhibited by the watchdog (NB. The supply to the solenoids can still be inhibited by the high side switch responsible for control of each solenoid i.e. One safety path is lost). |