Why does air conditioner cycles on and off?

It is normal for air conditioners to cycle on and off when they are controlled by a thermostat. Air conditioners only operate in full "on" and full "off" mode, and nothing in-between. The thermostat senses ambient air temperature; if the temperature rises above the setpoint, it sends a signal to the air conditioner to turn on, and the air conditioner will run as long as it takes for the ambient temperature to drop to a certain temperature below the setpoint. The temperature difference between the turn-on and turn-off is called the system hysteresis, and is typically 2°F, although on some electronic thermostats this value can be changed by the user. By making the hysteresis value larger, the air conditioning system will cycle less frequently, but run for a longer period when it comes on. Having a small hysteresis value may be important for comfort, to keep the room temperature in a narrower range.