Automatic Transmission Fluid Temperature Sensor determines the fluid temperature inside the automatic transmission unit. Transmission fluid temperature sensor is often used for diagnostic purposes to check ATF (Automatic Transmission Fluid) at the correct temperature. The main use of transmission fluid temperature sensor has been as a failsafe feature to downshift the transmission if the ATF becomes extremely hot. This input allows the TCU to modify the line pressure and solenoid pressures according to the changing viscosity of the fluid based on temperature, and also to determine regulation of the torque converter lock-up clutch.

Transmission Fluid Temperature sensor measures the temperature of the transmission fluid. It provides input to the engine control module (ECM) to modify shift patterns for smoother shifting in automatic transmission, and also provides overtemperature protection by locking the torque convertor.

The ATF (Automatic Transmission Fluid) temperature sensor converts the fluid temperature into a resistance value which is input into the ECM. The ECM applies a voltage to the temperature sensor. The sensor resistance changes with the transmission fluid temperature. As the temperature becomes higher, the sensor resistance decreases. One terminal of the sensor is grounded so that the sensor resistance decreases and the voltage goes down as the temperature becomes higher. The ECM calculates the fluid temperature based on the voltage signal.

The symptoms of a bad transmission fluid temperature sensor are the possibility of automatic gear shifting problems and engine light ON (or service engine soon warning light).