I have a 2004 Blazer. Approximately in the middle of the firewall is a valve?? switch?? which I believe controls the heater vent position. It has a vac. line coming from the drivers side of the vehicle to the unit and a vac. line that goes out anddown behind the intake on the passenger side, or visa-versa. There is an electrical connection. I first thought it was a canister purge valve but am pretty sure that is not the case. About the time I broke one of the vac. connections off, the heater started blowing heat from the AC vents, floor etc. no matter where the selection **** was set. I took it to PepBoys today and after 45 minutes of hunting they could find nothing that even was close to this thingy.
Any help will be greatly appreciated. Probably a dealer item but I would like to know what I'm looking for before I go there.

 

I take it you have the auto 4wd feature? Yes...then it is celoniod that 'takes the place' of the transfer case switch as we know it. Vacuum to one side of it all the time and then when you put it in 4wd, vacuum goes thru it over to the servo(below the battery box and then pulls in the cable, locking in the front wheels.) Not sure why you'd be looking at replacing it especially if it works as described-then it is not your issue. jpps looking back at your post, it does not control heater/vent position-but to be sure take a pic and post it here and we all will confirm what indeed it is. And another thing-if it doesn't have vacuum to either side of it you may have lost your vecuum source to this as well as your heater/vents as well-thus your issue. But if you have vacuum to one side of this celonoid and putting it in 4wd 'bridges' vacuum thru it then this is proper operation. My suspicion is inside the vehicle-control head, under dash 'modules', or in dash doors.(that is if door operation is your problem)

 

Yup, that's what it is. Front axle actuator electronic vacuum switch.

 

OK guys, that gives me something to go on. The only reason I was replacing it is because I broke one of the vac. connectors off. I havae tried to super glue the end back on and will try it this a.m.. The thing is just so brittle you look at it wrong and another part breaks. I'll keep you posted as to what happens and again---THANKS!