For 1990-2009 cars

### Heating

The purpose of the heater is to supply heat to the interior of the vehicle. The vehicle operator can determine the level of heat by adjusting the temperature control, located on the HVAC control module, to any setting. The temperature control can change the vehicle's air temperature regardless of the HVAC mode setting, heater, or OFF.

When the air temperature switches are pressed, the HVAC control module monitors the position of the air temperature door through the air temperature door position signal circuit. The air temperature actuator receives its power and ground through the air temperature door control A and air temperature control B circuits. The polarity is switched to acquire the desired temperature.

A 5 volt reference signal is sent out over the 5 volt reference circuit to the air temperature actuator. When a desired temperature setting is selected, a variable resistor is used to determine the air temperature door position. That variable resistor is grounded through the low reference circuit.

If the air temperature actuator or wiring fails, the air temperature LED graph on the HVAC control module will flash. Power and ground are provided to the HVAC control module by the ignition 3 voltage, battery positive voltage and ground circuits.

#### Engine Coolant

Engine coolant is the key element of the heating system. The normal engine operating coolant temperature is controlled by the thermostat. The thermostat also creates a restriction for the cooling system that promotes a positive coolant flow and helps prevent cavitation.

Coolant enters the heater core through the inlet heater hose, in a pressurized state. The heater core is located inside the HVAC control module. The heat of the coolant flowing through the heater core is absorbed by the ambient air drawn through the HVAC control module. Heated air is distributed to the passenger compartment, through the HVAC control module, for passenger comfort. The amount of heat delivered to the passenger compartment is controlled by opening or closing the HVAC control module temperature door. The coolant exits the heater core through the return heater hose and recirculated back through the engine cooling system.

#### Air Temperature Actuator

When the air temperature switches are pressed, the HVAC control module monitors the position of the air temperature door through the air temperature door position signal circuit. The air temperature actuator receives its power and ground through the air temperature door control A and air temperature control B circuits. The polarity is switched to acquire the desired temperature.

A 5 volt reference signal is sent out over the 5 volt reference circuit to the air temperature actuator. When a desired temperature setting is selected, a variable resistor is used to determine the air temperature door position. That variable resistor is grounded through the low reference circuit.

If the air temperature actuator or wiring fails, the air temperature LED graph on the HVAC control module will flash. Power and ground are provided to the HVAC control module by the ignition 3 voltage, battery positive voltage and ground circuits.