Question about 1990 GMC Sierra

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Heater works defroster does not

The heater works just fine but when i turn on the defroster the heater still stays on.i am not sure if it is the ducts or the controls.

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  • clalexy Jan 11, 2009

    I have the same problem. The heater blows great but when I move the lever to defrost the heat to the floor slows down and the defrost baerly bolws. It is something to do with the control doors in the heater duct. 1990 GMC Pick Up.

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It sounds as though you're having problems with your controls...if it were the ducts...you'd get some kind of a air current..even a faint one

hope this helps

Posted on Jan 03, 2009

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Could be faulting acutar motor for the hot air door. stocks5958.gif (1) Heater Core Cover (2) Heater Core (3) A/C Defroster Seal (4) Upper HVAC Module Case (5) Dash Seal (6) A/C Valve (7) Heater/Defroster Valve (8) Temperature Valve (9) A/C Valve Lever (10) Mode Valve Actuator (11) Heater/Defroster Valve Lever (12) Mode Valve Cam (13) Lower HVAC Module Case (14) Rear Heater Duct Seal (15) Temperature Valve Actuator Screw (16) Temperature Valve Actuator (17) HVAC Module Wiring Harness (18) Blower Motor Resistor/Relay Assembly (19) Blower Motor Acoustic Insulator (20) Blower Motor (21) Blower Motor Fan (22) Passenger Compartment Air Filter Access Door (23) Dash Seal (24) Evaporator Block Seal (25) Evaporator Core (26) Passenger Compartment Air Filters (27) Air Inlet Valve (28) Air Inlet Housing (29) Air Inlet Housing Seal (30) Air Inlet Actuator (31) Heater Core Pipe Seal

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Make sure it's on max.

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A coolant system should be flush every two years.

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