Automatic Transmission Fluid Temperature Sensor
The automatic transmission fluid temperature (TFT) sensor controls TCC and line pressure. The TFT sensor attaches to the upper control valve assembly. The TFT signals the transmission control module (TCM) regarding transmission fluid temperature. The sensor is a negative temperature coefficient thermistor which receives a five volt signal from the TCM.
When the temperature is cold, the sensor resistance is high; therefore, the TCM sees a high signal voltage. As the fluid warms, the sensor resistance decreases, the voltage drop across the sensor becomes lower.
The TFT sensor may be tested with an ohmmeter across pin terminals L and M of the 20-way transmission connector. Nominal ohm readings at specified temperatures are given in voltage levels at the TCM. Refer to Transmission Fluid Temperature Sensor Specifications .
The temperature sensor mode operates in the following two ways:
- Hot mode enable: Above 130°C (266°F) - TCC ON in 3rd gear at 48 km/h (30 mph) with 0-10 degree throttle angle; and 4th gear at 63 km/h (39 mph) with 0-10 degree throttle angle. This reduces transmission temperature by decreasing the amount of heat generated in the torque converter. It also provides maximum cooling by routing transmission fluid directly to the transmission cooler in the engine radiator.
- Super hot mode disable on the 275 HP RPO LD8 ENGINE: Above 140°C (284°F) - TCC OFF with 4th gear inhibited. Super hot is a condition where a hot engine adds heat to the transmission through the transmission cooler. By inhibiting TCC and 4th gear operation, the engine works under less load. This reduces the heat added to the transmission fluid by the transmission cooler. As both the engine and the transmission cool off, TCC and 4th gear are permitted again.
A TCM internal shunt circuit comes into play as the temperature increases beyond 50°C (122°F). As the temperature decreases, the internal shunt comes out at 40°C (104°F). There is a 10°C (18°F) overlap.