Automatic Transmission Fluid Temperature (TFT) Sensor
The TFT sensor (36) is part of the automatic transmission wiring harness assembly (33). The TFT sensor is a resistor, or thermistor, which changes value based on temperature. The sensor has a negative-temperature coefficient. This means that as the temperature increases, the resistance decreases, and as the temperature decreases the resistance increases. The TCM supplies a 5-volt reference signal to the sensor and measures the voltage drop in the circuit. When the transmission fluid is cold the sensor resistance is high and the TCM detects high signal voltage. As the fluid temperature warms to a normal operating temperature, the resistance becomes less and the signal voltage decreases. The TCM uses this information to maintain shift quality and torque converter clutch apply quality over the operating temperature range.
If transmission fluid temperatures become excessively high [above approximately 140°C (284°F) (calibratable in the TCM)], the TCM will disable ECCC function and command lock up mode. Applying the TCC serves to reduce transmission fluid temperatures created by the fluid coupling in the torque converter with the TCC released.
Above approximately 149°C (300°F) (calibratable in the TCM), the TCM will set a transmission fluid temperature code. This causes the TCM to use a fixed value of 140°C (284°F) (calibratable in the TCM) as the transmission fluid temperature input signal.