For 1990-2009 cars

The heating, ventilation and air conditioning 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 HVAC systems. It has three knobs, one push switch and one lever that control temperature, mode, blower speed, A/C operation 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 evaporator and heater cores.

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 A/C switch signals the A/C compressor control module that an A/C request has been made. Upon determination of favorable system conditions by the A/C compressor control module and the Powertrain Control Module (PCM), the A/C system is activated.

The fresh/recirc lever controls the air inlet of the blower. In recirculate mode, the blower takes inside air which is circulated through the HVAC system and delivered into the passenger compartment. In fresh mode, the blower takes outside air which is circulated through the HVAC 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.