Transmission Control Module Malfunctions

Transmission Control Module Malfunctions

Checksum Failures - P0605/P1603
To diagnose errors with the Electrically Erasable Programmable 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.

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 a default action and inform the driver of the problem, via the instrument pack message center.

Watchdog Mechanism - P0606
The watchdog monitoring function serves two tasks. 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. A fault is present 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).

Combination of Substitute
The transmission has two ways of monitoring vehicle speed for shift scheduling and torque converter control. One way is for it to use it s own OSS sensor and the other way to use the individual rear wheel speeds that are transmitted by the ABS module on CAN. If the TCM detects a problem with either of the above signal inputs the transmission will perform all related control functions using the alternative/substitute value. If both systems fail then a double fault DTC will be logged by the TCM.

This DTC logs because two diagnostics with priority 2 have logged simultaneously. The only diagnostics with priority 2 within the TCM software are the OSS P0720) and ABS wheel speed diagnostics (P1799).

Substrate Temperature Sensor
The TCM is situated within the transmission on the valve body. As the TCM controls a number of high power solenoids and is surrounded by transmission fluid it can get very hot. If the temperature of the hardware rises above a pre-determined level the TCM will be shut down and the transmission will enter mechanical limp-home mode. Prior to the TCM shutting down the TCM will log a fault code. A temperature dependent resistor mounted on the processor performs monitoring of the substrate temperature.

Plausibility Checking
The TCM detects a fault if an excessive voltage jump is identified between any two consecutive measurements. Also, with the engine started from cold the TFT will start to rise. Therefore the substrate or fluid temperature will also start to rise because the TCM is surrounded by transmission fluid. If the engine and OSS is higher than a set threshold for a predetermined length of time without the substrate temperature rising above a set threshold a fault will be detected.