My 2002 blazer has a strange hissing noise coming from the vicinity of the AC controls when I turn the AC on. I turned off the AC and the hissing faintly continued so I started pressing around different places, found a sweet spot, pressed on it and the hissing stopped. I turned on the AC and the hissing eventually started back so I pressed on the spot and it continued at a lower tone and my AC started to blow harder. Is my AC switch faulty or does anybody know if there is anything that can be knocked loose back there? It's not something that inhibits the functionality of the vehicle but it annoys me and I want to fix it, any ideas would be appreciated.

 

Where was the sweet spot as you mentioned. My guess would be a vacuum leak in the dash somewhere, I'm not sure what the controls look like on the auto climate systems but I'm guessing that vacuum is if not part of the switch it is close by.

 

I have the manual operation AC, I found a thread that showed the back of the HVAC controls and there are quite a few vac connections to it. When I press the Vent Selection Dial on the far right or right next to it on the right side it quietens the hissing down and get rid of it when I turn the AC off. So I'm guessing I am going to have to dig around behind that area to check for leaks. What are the chances of the hole HVAC unit being defective?

 

Had the same problem last year and had to replace the whole unit(2002 Manual controls). Mine would hiss in ac and vent positions but not in the off position, eventually it got worse and hissed in all positions. Inside that switch is like a rubber plate with different paths for vacuum. it's spring loaded and presses against the side with the vac connections. when you turn the switch to different positions it lines up the paths with the correct vac line. I bought the whole assembly(all 3 switches) from the dealer because they didn't sell just the single one. I would pull the hvac controls out of the dash in order to listen to it better and see if it is in fact the switch.

 

Thanks IT Technician, do you happen to know what the part number is for it. I would like to have an idea of what the price is if I can find one online.

 

Nvm, found it on Amazon for 58 bucks. Here is the link for anyone else who needs it. Amazon.com: ACDelco 15-72883 Heater and Air Conditioner Control Assembly: Automotive

 

not a bad price at all. If i remember correctly I paid over $100 for one from the dealer.

 

Thanks for the link. I see the GM switches are a lot like in the 70s. A vacuum cluster on rear of switch. Same as back in the 70s. I wasn't sure if that had changed or not. Now I know thanks. The big difference I'm sure is when they leaked back in the day you could take the vacuum port apart & re-silicone it & fix it.

 

don't mean to hijack your thread, new here! just got my first blazer wednesday! 1998, 4dr here. anyway, my ac controls don't change panels.... theres no hissing or anything. just blows at the feet, face, and defroster. any idea what it could be?? thanks!!!

 

sounds like you have a vacuum delievery problem i would check you lines under the hood. make sure they arent spilt, cracked or rotted.