K 1500 Truck 4WD V8-5.3L VIN T (1999)
Control Assembly: Description and Operation
Heating and Ventilation
Heater Blower Controls Circuit Description
Battery voltage is supplied to the blower motor resistor relay assembly connector C1 cavity G through CKT 540 (ORN) at all times. When the ignition
switch is OFF or the blower switch is in any position except the HI position, there is no current to the coil side of the relay via CKT 52 (ORN) and
remains de-energized. With the blower switch in the HI position the blower resistors are bypassed and voltage is supplied to the coil side of the relay via
CKT 52 (ORN). This closes the contacts of the relay and supplies battery voltage from the Underhood Bussed Electrical Center (UBEC) (FRT HVAC)
fuse to the blower motor. When the blower switch is in the LO or any medium position, the voltage is reduced through the respective resistors. The
voltage reduction will limit the blower speed accordingly. The blower motor is permanently grounded via CKT 1050 (BLK) at G200. The blower motor
resistor relay assembly is located next to the blower motor under the right side of the glove compartment.
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.
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.
