DC motor fan speed control with Temperature Sensor
I am assuming that whatever temperature you are measuring is not rising when the motor is on.
I see that you are sharing the power/ground between your temperature sensor IC and the motor.  Motors are extremely noisy from an electrical viewpoint and it's almost certainly causing your temperature sensor to malfunction.
It would be best to separate the temp sensor power/ground connections from the motor power/ground connections completely.  Power the motor from an external supply instead of sourcing it from the Arduino.
Another thing that might help is to add some bypass capacitors as close as possible to the motor and to the temperature IC across the Vcc and GND lines.  This will help to smooth out the power in spite of the noise that the motor itself generates.  Generally you want at least one of two types, an electrolytic capacitor and a ceramic capacitor.  These are both inexpensive if you don't already have some on hand.  I'll suggest a 10 uF electrolytic and a 0.1 uF ceramic capacitor near both the motor and the temp sensor.