Vehicle air conditioning is the cooling r refrigeration of the air in the passenger compartment. Refrigeration I accomplished by making practical use of the laws of nature. HEAT GOES TO LESS HEAT. All this means is that heat transfers. If two substances of different temperature are placed near each other, the heat in the warmer substance will always travel to the colder substance until both are of equal temperature.
As a fail-safe, the air handling system is designed to provide defrost when no vacuum is applied to any of the vacuum control motors. This is done to prevent a situation where defrost cannot be obtained due to a system vacuum leak. Instead, a leak in the vacuum control circuit will send all airflow to the defroster outlets. This condition may occur during acceleration (slow vacuum leak), may exist at all times (large vacuum leak), and may happen only when certain specific functions are selected, indicating a leak in that portion of the circuit.