Your mode doors including defrost, floor and vents are vacuum operated and the temperature door or valve as gm calls it is electrically actuated. All are controlled of course by your dash control head. You'll need to check each part of the system independently. Start by looking for a broken vacuum line either under the hood running to your fire wall and from it's entry to the dash to the control head. As far as the electrical controls on the temperature valve you'll need an HVAC controls schematic and a multimeter and possibly a scan tool. You may be able to observe whether or not the shaft of the temp valve is turning by dropping glove box and using a mirror.
Make sure there is enough coolant in the engine ... if you are a few quarts low it will not get to the heater core. If there is enough there is something wrong with your heater door / actuator or cable.
SOURCE: There is no air flow
You need part GM Part number 89018675 or ACDelco 15-73596. The Mode Selector Actuator. What happens is the part is a CHEAP part POORLY designed and when it goes to turn the flaps inside the HVAC unit its the plastic gear slips instead of turning the flap!!!
Just to forward you you might want a deal to install this art for you unless your good at fixing your own vehicle, because you need SMALL sockets and a lot of patience to get this part in!!!
The part is ALMOST directly above the steering shaft for your steering wheel, so theres a lot you got to remove before you will even see them!!!
I only recommend you to have a dealer put this part in for you, unless you have your own service manual!!
Because there is 5 of these darn things and if you pick the wrong one you will have to do it all over again!!!
3 on the Drivers side and 2 on the Passengers side!!
But replacing this part SHOULD solve all your problems!! AS it did for me and i was having the same issues as you have described!!
SOURCE: heater control 98 chevy s10 blazer
These trucks have a problem where moisture builds up in the heater box then freezes. When this freezes and you change your heater settings it breaks the shaft for the blend door. If this is what happened you should be able to look under the dash above and to the right of the gas pedal and you will see two vaccuum accuators one of them you should be able to wiggle around the linkage if it is broke. To fix this you need to remove the entire dash to get the heater box out which involves discharging the AC and draining the coolant because the heater core and evaporator come out. Please rate.
SOURCE: Heat wont blow through heater or defroster vents
common problem, check under dash by the gas pedal if control arms are muving went selecting from ac to heat cuz this it is the control for the vents to go from ac to heat and to defrost.
SOURCE: AIR FLOW STUCK IN DEFORST VENT WILL NOT SWITCH TO FLOOR OR AC VEN
The "MODE" door works off of an electric motor.
SOURCE: heater control stuck blowinf hot air out vents
did you cheked if there is enough refrigrant ( freon ) in your tubes by measuring the pressure . if there is you may need to check your condensor its may be clogged and not letting enough air comes through to cool the refrigran
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