Method and system for inferring and controlling transmission fluid temperature

A method for controlling transmission fluid temperature in a vehicle powertrain that comprises an engine, a power transmission and a hydrokinetic torque converter with an engine-driven impeller and a turbine, a torque converter lock-up clutch for connecting the turbine and the impeller at high speed ratios, the torque converter and the transmission forming power flow paths from the engine-driven impeller to vehicle traction wheels, a transmission fluid cooler in a cooling fluid flow circuit that includes the transmission, the torque converter, the cooler and coolant flow lines between the torque converter and the cooler, and an electronic control processor with repetitive control loops, the method comprising the steps of:

• measuring transmission fluid temperature;
• determining a delta steady-state temperature for the coolant at a coolant flow exit port for the torque converter that is not greater than a predetermined temperature;
• computing speed ratio for the torque converter;
• computing in successive processor control loops an inferred coolant temperature at the coolant flow exit port based on computed inferred coolant temperature in a preceding processor control loop;
• determining whether the lock-up clutch is failed;
• and
• limiting engine torque as a calibrated function of current inferred temperature and speed ratio when the lock-up clutch is failed, whereby temperature at the coolant flow exit port for the torque converter is maintained at a temperature level that is not greater than a predetermined value.