Heater Mode

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 sliding 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 A/C.

The HVAC control module receives power from the rear fuse block on the ignition 3 voltage circuit and battery positive voltage circuit. The ground circuits provide ground for the module.

Air Temperature Actuator
When the temperature switch is moved to a desired temperature, a signal is sent from the HVAC control module to the air temperature actuator through the air temperature door control circuit. When the logic inside the actuator receives the signal, the air temperature actuator moves into the desired mode. The motor opens the air mixture door to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature.

The rear fuse block provides. power to the air temperature actuator through the ignition 3 voltage circuit. Ground is provided by the low reference, the HVAC control module and ground circuits.

The air temperature actuator is a DC motor. Zero volts drives the actuator in one direction while 12 volts moves the actuator in the opposite direction. A 12 volt signal is sent out over the air temperature door control circuit, from the HVAC control module, to the air temperature actuator. When the air temperature setting is selected, the logic inside the actuator is used to determine the air temperature door position signals value. The motor moves the air temperature door to the desired temperature. When servicing the actuators, make sure you use the correct service part number. There are specific parts for the left and right side of the HVAC module. Using. the wrong part will effect air temperature operation. Circuit 733 (LT BLU) is a feedback circuit used only by the component supplier during their manufacturing process. It is not connected in the vehicle and is not used for diagnostic purposes.

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