K 1500 Suburban 4WD V8-6.5L DSL Turbo VIN F (1999)
Housing Assembly HVAC: Description and Operation
HVAC System - Manual
General Information
AIR DISTRIBUTION SYSTEM DESCRIPTION
The electronically-actuated doors control airflow. At various positions of the rotary knobs on the control assembly, the mode doors mix and direct
cooled, heated, and outside air through the air distribution duct.
Ducts and Outlets
A one-piece air distributor duct directs air to the passenger compartment. The air distributor duct attaches to the instrument panel with snap-in
style retainers and 3 screws.
In the case of poor air output, inspect the air distributor duct for the following obstructions.
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Leaves
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Dirt
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Other objects that may fall into the air distributor duct from the passenger compartment
Front Ventilation Controls
Vehicles with heating and air conditioning and vehicles with heat only provide front air delivery and temperature controls. The heater only option's
heater control module operates similarly to the HVAC control module except that the A/C pushbutton and temperature dial MAX detent features are not
provided for the control of the recirculation door. An auxiliary cooling fan is provided for vehicles with air conditioning and 7.4L gasoline engines.
Refer to Engine Cooling Fan Description - Electric in Engine Cooling.
The following are powered by CKT 141 (BRN) when the ignition switch is in the RUN position.
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The front mode door motor
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The front temperature door motor
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The recirculation door motor
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The HVAC Control Module
Grounding for the door motors is passes through CKT 150 (BLK) to ground G202. CKT 141 (BRN) powers the HVAC control module when the
ignition switch is in the RUN position. Grounding passes through CKT 150 (BLK) to ground G200 for the IP. The HVAC control module ventilation
controls for the front mode door and the recirculation door are disabled when the module front blower switch is in the OFF position. The ventilation
controls are enabled when the front blower switch is in the HI, M2, M1, or LO positions. The front temperature door is not dependent on the front
blower operations. The front temperature door can be controlled only when the recirculation button on or the A/C is in the max position.
The mode doors and the temperature doors are positioned in proportion to the variable voltage signal received from the HVAC control module. The
temperature dial at the HVAC control module adjusts a rheostat which provides a voltage signal proportional to the dial position. CKT 733 (LT BLU)
carries the voltage to the front temperature door motor. The front temperature door motor responds to the voltage signal. The mode dial at the HVAC
control module adjusts a rheostat. The mode dial provides a voltage signal proportional to the dial position. CKT 454 (WHT) carries voltage to the front
mode door motor. The front mode door motor responds to the voltage signal.
The recirculation door is a 2-position door. The recirculation door is in the recirculation position only when input CKT 1614 (DK GRN) is grounded
through the HVAC control module. The recirculation door is grounded when the auxiliary cooling fan A/C pressure switch operates the auxiliary cooling
fan for air conditioned vehicles with 7.4L gasoline engines. Refer to Engine Cooling Fan Description - Electric in Engine Cooling.
The system will be completely shut off with the ignition switch placed in the OFF position. With the ignition switch in the OFF position, CKT 71 (DK
GRN) provides B+ voltage to pin 3, C2 of the HVAC Control module which activates an internal relay removing B+ voltage from the system.
The recirculation door is normally in the outside air position but moves to the inside air position when the input wire is grounded. The LED on the
HVAC control module is illuminated when the recirculation door is grounded through the HVAC control module.
Rear Auxiliary Ventilation Controls
The rear auxiliary air delivery and the temperature controls provided with the combined rear auxiliary heater and air conditioner option, work
independently of the ventilation controls used for the front of the vehicle. The front system provides a recirculation door and the door controls.
The rear auxiliary system does not have the recirculation door and the door controls. The rear auxiliary mode door and the rear auxiliary temperature
door are exclusively controlled from either of the 2 auxiliary HVAC control modules. The front auxiliary HVAC control module blower switch is located
above the IP. The front auxiliary HVAC control module blower has a permissive position called REAR. The REAR position enables control from the
rear auxiliary HVAC control module. Relay contacts in the auxiliary HVAC control logic module change state in order to enable control input from the
ventilation controls when in the REAR position with the ignition switch in the RUN position. The ventilation controls are located at the rear auxiliary
HVAC control module. The logic module relay is de-energized when the front auxiliary HVAC control module blower switch is NOT in the REAR
position. The logic module relay controls inputs from the front auxiliary HVAC control module that control the doors.
Each control module produces a variable voltage signal to the door motor. Each door motor is powered from CKT 341 (BRN) when the ignition switch
is in the RUN position. Door grounding passes through CKT 150 (BLK) to ground G400. The variable voltage signal operates the door motor to a
