I am new to this forum so I hope I am doing this correctly. I have a 2000 s10 blazer 4x4 and recently only hot air comes out of the front AC vents. The temperature control changes nothing and the Defrost, AC, floor vent selector changes nothing. Is this a vacuum line issue? Or is it an electrical problem?

 

The air flow to the vents are controlled by vacuum actuators or motors. If you are unable to change the airflow to the various vents, then start looking for a vacuum line break in the engine compartment. Check the lines and connectors.

Are you able to put the vehicle into 4wd?

 

I would go would go with a bad HVAC control unit in the dash. Normally I would say it's a vacuum leak, since air only comes out the dash vents (the "default" position when there's no vacuum). However, the air temperature door is a motor-driven actuator (the rest are vacuum actuators). If it were just a vacuum issue, I would think you'd get more than just hot air to blow out the vents.

Check out this schematic that might help you understand what's going on inside the vents. Kind of hard to understand at first, but the bottom illustration shows the flow of air from outside to the various vent positions, as well as the actuators that control the air flow at each point.

 

Yes it will go into 4wd, 4w auto, 4lo, and 4hi. Will check the vacuum lines.

 

Where is this located? On driver or passenger side? It looks like it too has vaccuum source. When I turn the defrost vent selector, it makes only a small sound behind the dash, but I do not hear much, only a little. I should hear the sound of air in the vacuum system being diverted as the doors open and close? Thank you for the schematic, it helps a lot

 

Temp actuator is in the passenger side, tucked behind the actuator you'll see on your left, after removing the glove box. Try these instructions (which I stole from here):

1. Disconnect battery cable.
2. Open glove compartment, and lift tab in upper left corner to allow the glovebox to swing down completely towards the passenger floorboard. [or just remove the whole glove box, like I usually do]
3. Gently remove the line that enters the "Vac Unit", and then the "Vac Unit" itself by wiggling it upwards off its mount. Once removed gently swing it towards the drivers side and out of your way.
4. You now have access to the Temp Actuator.
5. Carefully remove the electrical connector that feeds the Actuator.
6. Using a 1/4" drive wratchet, and 7/32" socket, remove the two (2) mounting screws. (HELPFUL HINT: For maximum amount of clearance, Keep the wratchect handle facing the drivers side when putting it in place.)
7. There is no screw on the back mounting post, just a pin that holds it steady.
8. Once you've removed the screws you may need to gently pry the Actuator up off the blend door with a screwdriver.
   Installation is reverse of the Removal. The Blend Door and Actuator are both keyed, so theres only one way to put it in. Guide the Actuator onto the Door, then re-secure the mounting screws.

 

I 'cadged' these off another member..

I think you have more than just an actuator problem

 

It now will switch between defrost, AC vent and floor, but I cannot get it to bring cool air, it is only hot air that comes out of the vents. I can now hear the air movement shift as I switch the selector to the different positions. It just started this today. I did not find any vacuum lines not connected to where they go. Is the the temperature actuator fault?

 

Well.. good. Now you only have to deal with the blend door actuator .. read up the thread.
There's several videos on youtube as well

 

I have the same problem. I cannot swich the air flow from vent to defrost or floor but I can change the temperature. There are colored vacuum lines behind the glove box. Is that the actuator?