I replaced the entire control unit and moved parts around to get a good rotary control valve, BUT it is still stuck on defrost. What's up? Where does the engine vaccum come from. Recently redid the intake gaskets, maybe I forgot to hook something up. Help Please!! What is up behind the dash that I could check?

 

The vaccum line coming off the back of my intake manifold was crimped. I wasn't getting any vaccum to the controls. Fixed it now and know how the controls work. Now I can replace my A/C compressor and I'll be in business.

 

you know, i asked over and over where the sopurce for the vacuum for the hvac controls came from and here i find it in tthis thread... back of intake! finally !!!!

thanks..

 

Back of the intake near the external HEI coil on my car. I had pinched it in the coil bracket and didn't realize it. I had read your post and figured out the interior controls workings, but still couldn't figure out what was wrong. Then I realized that I had no vaccum....went to the vaccum source and voila...it works. Good Luck!!