Hello, everyone! I'm going to ask what's probably an obvious question, so you'll have to bear with me here, as this is my first vehicle I've worked on before...

On my 04 blazer, with manual AC controls, I hear a hissing sound only when the AC control is in the "off" position. When the AC control is in the off position, the idle gets noticeably rougher. The positioning for the off position isn't very precise, meaning I can leave it halfway between Max AC and Off, unlike the other controls which snap to the proper position. I can still switch between all vent positions successfully.

My understanding is that these things are vacuum powered, and that the AC Control essentially switches between different vacuum lines to operate the different controls. It seems to me like there's a pretty good vacuum leak if it's affecting the idle this much. For now I just keep it away from the off position.

So... the question:
Where should I begin when diagnosing this? Is there a way to check functionality of the control unit itself vs the hoses? Or Is it more of a "pop the dash cover off and look under there" sort of thing?
If anyone's gone through this already, I'd love to hear any advice.

Thank you!

 

So, last night I messed with the blazer. And I think I'm a little closer to figuring it out. The trick is, the blazer idle gets worse with the AC off in PARK, not when in gear. Having the AC on helps smooth things out while in park. So perhaps the AC on/off situation is more of a symptom of a different problem.

Hopefully this weekend I'll do some reading and see what I can find. And of course, advice is always appreciated.

 

Sounds to me that there is a vacuum leak in your HVAC system behind that control panel. An engine tends to run rough with low vacuum when idling.