My Blazer heating system is working odd. It blows only hot air to cabin. Where cant I find the valve wich controls hot and cold air?
This problem came out of nowhere. I had my ecm reprogrammed because of immobilizer wich wouldnt let me start my car. I don`t believe that it`s the problem. I had my battery unhooked while ecm was loose. well that cause something like that? Heating just wont react at all my attempts to adjust air temperature.

 

Sounds like a vacuum line off or broken. Could also be the blend door in the cabin but if the controls don't work I'd lean towards a vacuum leak.
Use the search function for vacuum leaks, heater controls etc.

 

You're looking for a motor-driven actuator that moves the hot/cold air blend door. Should look like this:



There are different ones for automatic temperature control and manual temperature control. Here's what I think are the instructions for a 1998 Blazer, stolen from Josh Barber and automotivehelper.com:


REPLACING THE TEMP ACTUATOR ON A 98 CHEVY BLAZER 4X4

http://i16.photobucket.com/albums/b40/JoshB_/relay2.jpg

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.
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 ammount 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 just went through the same things all of the previous guys experienced. I've read the same instructions in several places so I figured they must be fairly accurate. In reality, there's a few little things they left out which for me made the difference between a one hour job the way it's described and a ten hour job the way the dealer describes.

First of all, you need to be a contortionist for this job. Access is very tight. To make the job easier get your floor jack out and get the vehicle as high off the ground as you can.

If you can actually operate a ratchet pointed towards the drivers side you must have barbie sized hands. The ratchet and socket combination won't fit on the drivers side screw, there's just not enough vertical clearance.


Since I had a 7/32" socket all I needed was a 1/4" ratchet and all should be well according to the instructions. Wrong. I have five 1/4" ratchets, I could not operate any of them up inside this little cavity you have to work in. The solution was a 1/4" flex ratchet from Autozone. It fit on the right side mounting screw and backed it right out. The left side was a different story. As the bolt started to back out the duct pushes the ratchet away and with every turn the angle increases to the point that the socket will no longer turn the bolt. A wrench would fit on the bolt if you have one. I didn't so I went in search of one and it turns out that they don't exist, 1/4" is the smallest available. The socket fit by itself but I couldn't get enough grip to turn it because it's too smooth. The answer, I got a piece of sticky back DA paper and cut a thin strip, Put some small folds in it to get more grip, and wrapped it around the socket. I got it into place and wha la, the bolt was in my hand. Saved myself 9 HOURS!!!!