Transmission Temperature Sensor/Switch: Description and Operation

Automatic Transmission Fluid Temperature Sensor
The automatic Transmission Fluid Temperature (TFT) sensor is a negative temperature coefficient thermistor that provides information to the PCM regarding transmission fluid temperature. The TFT sensor (1) is integrated in the automatic Transmission Fluid Pressure (TFP) manual valve position switch assembly which is bolted to the control valve body. The sensor monitors main line pressure from the inside of the control valve body in order to determine the operating temperature of the transmission fluid. The sensor uses an O-ring seal to maintain fluid pressure in the control valve body.

The internal electrical resistance of the sensor varies in relation to the operating temperature of the transmission fluid. The PCM sends a 5-volt reference signal to the sensor. This measures the voltage drop in the electrical circuit. A lower fluid temperature~ creates a higher resistance in the TFT sensor, thereby measuring a higher voltage signal.

The PCM measures this voltage as another input to- help control line pressure, shift schedules and TCC apply. When the TFT reaches 140°C (284°F), the PCM enters hot mode. Above this temperature the PCM modifies the transmission shift schedules and TCC apply in an attempt to reduce fluid temperature by reducing the amount of heat generated by the transmission. During hot mode the PCM applies TCC at all times in Third and Fourth gears.

Also, the PCM performs the 2-3 and the 3-4 shifts earlier in order to help reduce the generation of fluid heat.