For 1990-2009 cars

The heater system delivers warm air to the passenger compartment after the engine reaches normal operating temperature. Outside air and air from the passenger compartment are passed through the heater core. Hot engine coolant heats this air, which travels to the passenger compartment via ducts and outlets.

The heater control unit allows the operator to operate the heater and ventilation system. It has three knobs and one lever that control temperature, mode, blower speed, and fresh/recirc settings.

The temperature control knob controls the temperature of the air delivered to the passenger compartment. This control knob opens and closes the air mix door, controlling the amount of air which passes through and around the heater core.

The mode control knob allows the driver to select air outlets: ventilation bi-level, heat , heat and defroster mix, and defroster.

The blower speed control knob controls a four-speed blower motor and fan. It allows the driver to regulate the amount of air, cold or warm, delivered to the passenger compartment.

The fresh/recirc lever controls the air inlet of the blower. In recirculate mode, the blower takes inside air which is circulated through the heater system and delivered into the passenger compartment. Recirculate mode is helpful in heating up the passenger compartment faster. However, during normal heater operation use the fresh mode to prevent window fogging. In fresh mode, the blower takes outside air which is circulated through the heater system and delivered into the passenger compartment.