wondering (before i buy) if anyone on here would have any idea on what this could actually be instead of throwing money at it. But what could be the issue of air coming out but only on "DEFROST" nothing else. It will not come out on the body vents just the glass vent for the windshield and the floor. Any ideas?? When i changed the heater core i also changed the blend door. Everything was hooked back up exactly by a friend who is ASE CERT. He asked me if the switch worked before we did the swop, and infact yes it did. There is nothing blocking the vents.... ANY IDEAS anyone please!!!

 

most vehicles when they have no vacuum going to the control head will default to defrost. soooo i would start there, try to find the line going into the firewall. see if it has it there, then maybe at the control head then at the actuator under the dash, if you can get at it

unless you have electric controls then ignore all that but pretty sure s10 heater controls are vacuum

 

my heater was stuck on high heat and only worked on defrost because of a bad heater resister, cheap and easy fix under and hour and less than $20

good luck

 

Do you think that could be the issue though @burt222??? I have my A/C hook up and recharged by the same friend and it still works. I use it daily now because its starting to get warmer here in central pa. Also @abig84, I have not checked the lines yet, I will check them today as far as I can trace them. I have no idea where the line would be under the dash, but I guess when im looking at it, Ill figure that out. LOL.

 

Sorry for the delayed response, but it was the vaccum lines. They were all rotted a and falling apart as i pulled them out of the engine.... but new issue, or is this normal, but whenever I am driving and I take the rpm's over 2000-2500 i lose vent flow and it defaults to defrost.... any ideas or is that normal for a 95' ls blazer???

 

Sounds like the check valve in the system either isn't installed or isn't working properly. It should prevent low vacuum pressure in the engine (i.e. when under acceleration) from affecting the vacuum pressure in the HVAC & 4wd (if equipped). It should be installed to allow the engine to pull a higher vacuum on the HVAC/4wd system and allow the vacuum to stay in that system if the line between the engine & the check valve gets disconnected.

 

Okay sooo where is that loacated??????

 

so i think i changed the right part called the check valve, it had three vaccum lines going into it. buttttt i still lose air out of my vents at 2000+ rpms... anymore ideas????

 

If the check valve didn't cure it and it is installed correctly (single port side towards engine, double port side towards vacuum accessories), then you have a leak somewhere in the vacuum lines.