Transmission Fluid Temperature Sensor

Description

The TFT sensor is located inside the wiring harness in the fluid pan and is connected to the TCM with two wires. The sensor is a Positive Temperature Co-efficient (PTC) sensor, containing semi-conductor material, which increases its resistance as the temperature increases.

The sensor receives a predetermined current from the TCM on one of the wires and is connected to earth via the TCM on the second wire. The TCM measures the returned voltage and using this information calculates the TFT.

In the case of low fluid temperatures, the TCM prohibits TCC lock-up to promote faster fluid warm up. In the case of high fluid temperatures, the TCM increases the converter lock-up control and modifies the shift programme to reduce fluid temperature.

If the sensor fails, the TCM uses a programmed default temperature value. The default value is derived from the ECT sensor and received on the CAN from the ECM.

There are six diagnostic checks for the TFT sensor, an error is detected if:-
1. The voltage at the positive connection to the sensor is greater than 4.5 V and the voltage at the negative connection of the sensor is less than 0.5 V. This indicates an open circuit condition.
2. The voltage at the positive connection of the sensor is greater than 4.5 V and the voltage at the negative connection of the sensor is greater than 0.5 V. This indicates a short to battery voltage condition.
3. The voltage at the positive connection to the sensor is less than 4.5 V and the voltage at the negative connection of the sensor is less than 0.5 V. This indicates a short to ground condition.
4. The voltage at the positive connection of the sensor is greater than 2.3 V and less than 2.7 V and the voltage at the negative connection of the sensor is greater than 2.3 V and less than 2.7 V. This indicates a short circuit across the sensor.
5. The temperature rise between readings is greater than 10 deg C.
6. The temperature rise since engine start is less than 5 deg C.