Theory Of Operation

The transmission monitoring system uses three temperature sensors. If any one of the sensors fail, the other two sensors are used for the control and temperature monitoring calculations and the Malfunction Indicator Lamp (MIL) will not be illuminated. The MIL will only illuminate when more than one temperature sensor has a failure.

One of the sensors is located in the oil sump of the transmission and can be monitored with a scan tool under the Oil Temperature Sensor data parameter. This sensor is a 2-wire thermistor with a negative temperature coefficient. This means that as the temperature increases, the resistance of the sensor decreases.

The other two sensors are embedded on the Printed Circuit Board (PCB) in two separate areas of the Transmission Control Module (TCM). The TCM is attached to the valve body and therefore provides the three sensors with a strong temperature correlation. The embedded sensors can not be monitored with the scan tool and cannot be serviced separately.

The oil temperature sensor that is located in the oil sump is normally used exclusively for the transmission control calculations. The embedded sensors located in the TCM are normally used for over-temperature shutdown and temperature sensor rationality monitoring. The following table shows the relationship between temperature and resistance of the Oil Temperature Sensor located in the oil sump.

Because of the three independent signals, it is also possible to detect which sensor caused a rationality failure. As a result the transmission can operate without any restrictions, by using one of the valid sensor signals.