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In my 75, when I turn the heater control to defrost or heater it works fine. But when I put it on AC or vent, bi-level it won't blow any air. I'm getting ready to put my stock ac compressor in and am trying to figure out if there is a specific actuator or fuse for those vents. Any body had this problem? Any help will be appreciated as it is 84 here today. Thanks
From: SUFFIELD CT USA 2023 C8 CORVETTE UN-MODIFIED FINALIST
2023 C8 of the Year Finalist - Unmodified
When my '77 was stuck on heat, no defrost,vent or air it was a broken nipple on the back of the heater control panel causing a vacuum leak. You could here it after turning off the engine!
Sounds like a vacuum problem. If there is NO vacuum getting to the AC control the default is for all air to come out the lower heater outlet but you said you get air out of the defrost so NO vacuum is probably out.
Wilcox site has the AC/heater vacuum schematics ,you might look at those.
A bad actuator,hose off or leaking rotary valve could cause your problem.