The temperature sensor should be connected to the evaporator in a location where both liquid and vapor refrigerant will be flowing so that an accurate saturation temperature can be measured. Typically, one of the bends on a fin-tube evaporator just downstream of the expansion device is an ideal location.
This temperature reading is used to optimize the air flow and speed of the PSC blower motor by activating one of the three speed taps of the motor. Since the X1 only provides a single operating speed, no temperature reading is necessary and therefore no temperature sensor is used.