C 1500 Truck 2WD V8-5.3L VIN T (1999)
Housing Assembly HVAC: Description and Operation
Heating and Ventilation
General Information
Within the heater module is a series of valves called the temperature valve, the vent valve and the defroster valve. Valves are hinged components that
direct airflow through the various sections of the heater module. The valves provide the proper airflow for the selected operating mode.
Heater System Description
The heater system provides heating, ram air, power ventilation, and the windshield defrosting. The blower fan draws outside air from the plenum at the
base of the windshield into the heater module. The forward movement of the vehicle forces additional outside air, or ram air, into the heater module.
Within the heater module, the air is heated. The heated air is routed through the ducts to the proper outlets for discharge into the passenger compartment.
A control assembly in the instrument panel allows the operator to control the blower speed, the mode of operation, and the temperature of the air coming
from the heater system.
The heater module contains components that heat the air, and direct the air to the proper ducts and outlets. The components are enclosed inside a two
part housing. The front housing mounts to the front of the cowl panel in the engine compartment. The rear housing mounts to the rear of the cowl panel
in the passenger compartment under the instrument panel.
Bi-Level Mode
When the mode knob moves to the Bi-Level position, the defroster valve moves to uncover more of the opening to the defroster. An increasing amount
of air flows to the defroster nozzle, and a decreasing amount of air flows to the heater duct.
Defrost Mode
When the mode knob is in the Defrost position, the defroster valve moves to fully uncover the opening to the defroster nozzle and restrict the opening to
the heater duct. The defroster nozzle receives the maximum airflow, with only a little amount of airflow coming from the heater duct.
General Information
The heater system provides the following 5 basic operating modes:
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Upper Mode
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Bi-Level Mode
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Lower Mode
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Defog Mode
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Defrost Mode
Lower Mode
When the mode knob is in the Lower position, the vent valve partially closes. The defroster valve remains closed. The blower fan heats the air and
directs the heated air to the defroster valve. The defroster valve sends most of the airflow to the heater duct and some to the defroster nozzle.
Upper Mode
With the mode knob in the Upper position, the vent valve opens so air can flow to the instrument panel outlet ducts.
Temperature Valve and Heater Core
The temperature of the air coming from the heater duct and the defroster nozzle is regulated by controlling the relative amounts of warm and cool air
coming from the heater module. The temperature valve determines the amount of the total airflow that passes through the heater core.
When the temperature knob in the control assembly is at the extreme left side of travel, the full BLUE, or cold position, the temperature valve stops all
the airflow through the heater core. The heater duct and the defroster nozzle receive only unheated air.
When the temperature knob moves away from the full BLUE, or cold position, the temperature valve opens to allow an increasing amount of air to pass
through the heater core. At the same time, the temperature valve reduces the amount of unheated air that enters into the mixture. The result is a very
responsive control of the temperature of the air discharged through the heater ducts and the defroster nozzle.
When the temperature knob reaches the full RED, or hot position, to the extreme right end of travel, the temperature valve diverts all the airflow through
the heater core for maximum heating.
Hot coolant from the engine moves through the heater core and returns to the cooling system when the engine is running.
