ac controls

[Rick Bollinger]

I have been going through my A/C & heating system. New cables, replaced broken arm and bad fan switch, vacuum lines and cleaned system. This is a new to me truck and don't believe this system has worked in years. The vacuum comes on when the top lever is moved to the left. This opens up the inside passenger vent and closes the outside air. It also turns on the heater valve. But I cannot get the defrosters on with the vacuum on. I am thinking the vacuum is working backwards. It is a 2 port vacuum switch. I could use some help.
Thanks
Rick

[dmjlambert]

Check out this post
http://67-72chevytrucks.com/vboard/s...70#post7392070
2 port control head works with water valve that is closed with vacuum applied.
3 port control head works with water valve that is open with vacuum applied.

[Rick Bollinger]

Thanks that makes sense to me. But everything I have seen says that I have bought the right water valve. So what part do I need for it to normally be open instead of closed? Somewhere down the line someone must have changed the vacuum valve to a 2 port instead of a 3 port? Or is there a cut off date for the 68's to use a different one.
Thanks
Rick

[dmjlambert]

I don't know the different year applications for the water valve and I suppose you need to have a matching vacuum switch. If you had a 3 port vacuum switch you could just plug the water valve port and T into the vent control vacuum line to control a normally open water valve. But you have the opposite problem, and the only easy solution I can think of is T into an always-vacuum line to leave the water valve open all the time, and disconnect that in the summer. I'll bet somebody here on the forum will be able to chime in about what years had the normally closed water valves and which years had the normally open water valves, or give information about if that vacuum switch can be swapped out.

[Rick Bollinger]

Any thoughts?