No matter what I set the control to,I only feel air coming from the defrost vents.

What exactly controls that feature?

I too have this same prob and would like to know the answer

Defrost is the default setting. You have a vac leak somewhere--the culprit is often at the switch. Stick at ear to the control head and give a listen. Since the problem is common to our cars, a search should locate more info.

JamesC

definitely a vac leak. i did it to myself on my last engine swap - forgot to hook up the 2 vac lines that came through the firewall

I assume the vacuum for the HVAC comes from the engine? Where would the line be located under the hood? The car came with a carbed sbc,so I sure the PO didnt hook it up properly.

On an 85 Trans, it's the connection right on the intake between the carb and distrib. Rubber hose goes from there to the vacuum tank by the wiper motor and brake booster, then a thin hard plastic vac line from that tank through the firewall.

My own switch leaks too on about half the positions. Hear it hissing, and have to switch back and forth a few times and leave it for a while before it finally switches. I just capped it off at the intake and live with it being stuck on defrost, most useful setting anyway.

Probably this line here https://www.thirdgen.org/forums/inte...ml#post4686134 Not connected at either end?

I wonder if same place on 85 camaro

Here's the piece that was problematic on my 85:

https://www.thirdgen.org/forums/atta...duct010707.jpg

JamesC

about center of the firewall and half way down, you should have a bundle of wires. in this bundle, you'll have 2 hard vacuum lines. if they are not hooked up to anything, there's your issue

And don't hook both of them up to vacuum like I did during my motor swap, because then when you want to use the heat, all you will hear is vacuum noises inside the car lol. One is vacuum into the car for the controls and the other is vacuum out to the heater core valve near the air solenoids.

James. How did you know that piece was a problem? Did you just start replacing stuff?

I could hear the leak/hiss when I stuck my ear close. I replace mine, but here's link that provides some interesting info:

https://www.thirdgen.org/forums/inte...ctor-disc.html

JamesC

Any pics or diagrams of what/how the two lines coming out of the firewall connect to?

Thanks,
Jay

hi! same problem on my 88 iroc; heater stuck on defrost but I hear no hissing from the control pannel. i guess the leak is under the hood.
Where would the line be located under the hood of a 350 TPI?

Thanks a lot

same problem but i only hear the hiss when mines on the low setting

If you hear it, it's probably the switch inside the car, and you hear it on low because the fan isn't loud enough to cover the vacuum sounds. If you heard no hissing at all then the problem is with the vacuum line coming from the intake to the HVAC system inside the car.

This has been an issue on mine, I fixed that nut on the selector but that didn't fix anything, I do hear a hiss coming from the control panel tho, but I'm pretty confident I fixed the selector nut properly, any ideas?

Must be a pissed off snake

I hate when that happens.

🐍 🐍 🐍 or it could be a vacuum leak causing the hissing 🤔