For 1990-2009 cars

The air conditioning system delivers air that has been heated or cooled and dehumidified for occupant comfort. It also provides ram air, power ventilation, windshield defrosting, and side window defogging.

During most operating conditions, fresh outside air is drawn into the ventilation ducts by the blower. Additional outside air (ram air) is also forced into the vehicle by forward movement of the vehicle.

Within the ventilation ducts, the air is pushed by the blower motor through the evaporator, then through the heater core, and into the passenger compartment. For air conditioning or defroster operating modes, the refrigeration system will chill the evaporator to almost freezing. As the air passes through the evaporator, the air temperature drops which causes moisture in the air to condense on the evaporator core fins. This moisture leaves the vehicle by gravity feed through a drain hole, dehumidifying the vehicle.

From the evaporator case, some or all of the air may pass through the heater core, which is warmed by engine coolant. Any ambient air which is not to be heated bypasses the heater core and then mixes with the heated air before entering the passenger compartment. To maximize air conditioning cooling, the fresh/recirc control lever can be set to recirculation. This way, most of the air entering the blower during this mode is taken from the passenger compartment (recirculated) where the air is likely to be cooler than the outside air.

For heater-only or ventilation modes, the operator can shut off the refrigeration system so that cooling is not provided. The operator can turn off the A/C system by setting the A/C switch to off. The amount of heating is governed by the proportions of the air directed through or around the heater core in the same manner as with air conditioning. The heater control unit assembly in the instrument panel gives the operator a means of selecting the desired operating mode, air temperature, and blower speed when the engine is running.