I have not received my high speed blower fan relay yet so I thought I would begin checking the lack of air from the ducts in the dash. I removed the center dash panel and the right side front dash. This allowed me access to the vents. Started the engine to check operation and found very very little air at vents with control in max ac or ac, no air at defroster vents when in defrost mode. I removed the upper ducts that carry air from center plenum area. Then checked operation again. All my testing was with engine running.

1. Plenum door went to close position for the defroster mode.
   Found the defroster vacuum valve had the linkage unhooked. hooked it back up and now have
   defroster air.

2. the Pass. kick panel valve working as service manual describes.

3. This is where I believe my problems may be. When in the AC mode the plenum door will only come open about 1 1/2 inches. I can push the door open farther which allows me to actually see the evaporator but when i stop pushing on the door it goes back to the open only 1 1/2 inches which means all air must travel from the heater core side. Should this door be going to a full open position when in Max ac and ac mode or is it operating correctly. There are 2 adjustment areas that I see. the one linkage  adjustment is on the upper side of the case where the door pivot goes thru. The other is just off the vacuum valve where the linkage connects to the backside of the door. Do I make an adjustment here?

The one thing that I can not check is the operation with the ac compressor actually running. I am still waiting for some parts to arrive. I am not sure if that will effect the operations that I have described.
I look forward to any assistance you can give me on this.
Thanks,
Rodney

UPDATE-- I got to spend some time in the garage tonight playing with the vents. I found a few problems that were easily corrected and I now have all vents blowing air to where they need to. The center plenum door is suppose to open the whole way when in AC or Max AC. Mine wouldnt open because the vacuum hose going to the rear side of the vacuum valve was pinched behind the heater case and not allowing vacuum to the rear side of the valve. I had to remove the entire case to fix that. Then I found the vacuum hose on the front side of that valve was mixed with one of the hoses from the defroster vacuum valve. once they were switched the door would close to the halfway position which allows for bi level heat/AC. I didnt have to make any adjustments on the linkage. Now all thats left is to put all the venting back together. I will be installing the heat reflectant foil under the carpet before I put everything back together. I will also be adding the heater control valve to the heater hoses. I really noticed how much heat is in the heater box while working on this with the engine running. Next part to complete is hooking up all the AC hoses and  connections out front and to install the VIR eliminator. then pull to vacuum and charge! getting close now.

Glad you got some of it figured out. It can be a pain sometimes. Keep on pluggin away!

Ain't Corvettes fun??  Troubleshooting is the best !!

did you check to see if the outer plenum door was working properly, its the one on the firewall side, near the inside LF kick panel door, with the heater and ac off the outer door is open and the kick  panel door is closed to supply fresh ambient air to the Astro Vent, with ac on it closes the outer door and opens the inner kick panel door to recirulate cabin air..... I'am a little cofused on item #3 about in ac mode with the door open, it should be closed, cold air is not suppose to go through the heater core, cold air is suppose completely bypass the heater core, to be honest alot of this doesn't sound right to me, I'am looking at a 72 GM manual and I believe 74 is the same, BUT I could be wrong, ........when I speak of the "outer door" I'am not speaking of the kick panel or heater doors, bi-level in my mind means distribution of air hot or cold to both floor and def vents, if the air is to cold during cooling, it is warmed during heating only, but like I said 74 might be different.anips2010-07-28 16:33:04

Joe,
I'll have check the operation again but its going to be 10 days till i do. My wife is making me take a vacation away from the car so I'll be having withdraw for a while. Hard to believe that I would rather stay home but I dont really have a choice!! Maybe by the time I get back the vette will have put itself back together again! I will check on it a soon as I get home though as I need to complete that part so I can move on the the rear and transmission upgrade. Thanks again to everyones input as it makes it so much easier to work on these cars when you have great guidance!
Rodney