I hope I get some responses to this post because I realize that it is your winter-time. I live in Australia and it is still hot here which brings my problem to light. I finally got my ac going and that is great. My problem=The system works great when I put a rubber hose to by-pass the heater core but when I put it back the way it is supposed to be, it doesnt send out near as cold air. I would like to find out the answer to the problem because as I have by-passed the heater core then I dont have a warm air defrost. Any help or do you need more info.? Thanks in advance.

 

You may have a blend door not closing or the blend door seal may be deteriorated.
Coolant flows through the heater core all the time, (no water valve) and there is a blend door that routes air either through the core or bypasses it.
I don't think GM added any water valves to third gens, but I may be wrong.
The blend doors are vacuum operated.
Hope this helps.

 

Thanks for the help. Would I be wrong to add another valve on the other line to close off the heater core when I want to? That way if I have a valve on each line that I can close or open then I wouldnt have to bypass the core each time with a short piece of hose. I could probably get two electrically operated valves so that I wouldnt have to make the switch manually. Sound weird? It probably is a pain to find the blend door to fix.

 

All you need is one valve to stop the flow--it doesn't need to be bypassed. Later third-gens got the heater valve, but they sometimes break, leaking fluid all over the place. The later ones have a vacuum operated switch controlled by the A/C controller.

 

I realize that youre supposed to use one valve to stop the flow, but when the one valve that I have is shut off the air conditioner doesnt work near as well. When I by-pass the heater core with a small hose thus eliminating the core altogether, then the air conditioner works great. I know it sounds weird, but it must have something to do with the water in the core sitting there. Any ideas?