Hey i got a 2000 s-10 blazer 4x4 4.3 with a heat problem. I seems the heat is comming out of only the defrost. I have checked vacuum lines under hood and behind dashboard and found no problem. I had glanced underneath around the transmission but saw no problem with the lines.I read there may be a switch on the tranny where 3 or 4 vacuum lines connect and can get oil in them that will cause the vacuum system to go into fault mode. That switch and the actual heat selection switch is the only things i can think of. Any got any ideas. Thanks.

 

When you checked the vacuum lines at the HVAC control module, did you find any transmission fluid? And was there adequate engine vacuum on the supply line to the HVAC control module?

 

I never found the exact location of the module, i found 3 or 4 lines running on the top of the tranny and i just went out and pulled one line off of a "Y" junction and tranny fluid poured out.I do belive theese are all vacuum lines.Do you know the exact where abouts of the module?And do i need to replace all the lines with fluid in them and the module or just the module?Thanks man.

 

Ok... If you found transmission fluid there, then the vacuum switch on the transfer case has gone bad. You likely also have a failed input seal on the transfer case. Pull the fill plug on the transfer case. If transmission fluid pours out, you need to pull the t-case and replace the input seal.

Now, if you haven't pulled the vacuum lines off of the HVAC control module under the dash, then you should do that fairly quickly to see if transmission fluid is there as well.