A/c climate control

[Chevy Fan Always]

Does anyone know why some unit have 2 vacuum nozzles and some have 3?

[Tdrcreyn4]

They do the same thing just changed throughout the years. The difference is how the vacuum hoses are routed. In particular one supplies vacuum to a hot water valve which should be open with vacuum applied. The other does exactly the opposite. As long as you have the right controller, hot water shut off and vacuum routing you can use either. There is a recent post that shows the differences in hose routing.

[kehstr]

do you have a link to said post?

[john]

Here is a thread that shows both hose routes.
http://67-72chevytrucks.com/vboard/s...ad.php?t=76770

[davepl]

Yes, I am a master of this topic by necessity, ran into issues on my truck. See the other thread for routing.

Long story short, however, they went from a "vacuum switched on" to "vacuum switched off" heater control valve and so needed a third port on the switch that is backwards from the cowl vent. In later years they got both vacuum solenoid and heater control switch on the same circuit, operating in the same direction, so they only needed the one signal. So it becomes a basic vacuum switch in that case, one in and one out.

[Tdrcreyn4]

Here is another post for you with similar info. The 3 port has a separate head for the heater control valve which is open with vacuum on. The two head as mentioned by davepl uses a "T" connector and the water valve is closed with vacuum on. The replies you have now will give you everything you need to know. I guess the recent post I referred to was actually in 2012....



http://67-72chevytrucks.com/vboard/s...d.php?t=518197

[Chevy Fan Always]

Thanks for all the great info. I really appreciate it.

[jjzepplin]

Does this mean that the heater valve was different too?

[davepl]

Originally Posted by jjzepplin

Does this mean that the heater valve was different too?

Yes, a 2-port uses a different valve than a 3-port. It works backwards (on vs off under vacuum).

[3sonsdad]

I realize this is an older post but I was having issues with my 3 port control head vacuum switch so I had to get my vacuum tester hooked up and get this figured out.
My 3 port vacuum switch has part number 7292032-1 cast into the pot metal. These have numbers 1, 2 and 3 cast next to the three ports. Mounted correctly to the air conditioning/ heater sliding control lever assembly under the dash port 1 would be closest to the passenger side, port 2 in the middle and port 3 closest to the driver side.
Port 1 is vacuum in from your vacuum canister under the hood. This has to be vacuum in for this 3 port valve to work correctly.
Port 1 is common with port 2 when the uppermost lever is moved to INSIDE air. In this position vacuum from the vacuum canister draws a vacuum in port 2 and the vacuum line attached to the outside air door diaphragm closing this flap. Vacuum from another line tee'd to the vacuum line attached to port 2 acts on the vacuum canister attached to and opens the recirculating air door on the passenger side kick panel. At the same time the correct water valve is closed from a lack of vacuum in port 3 line.
Port 1 is common with port 3 when the uppermost lever is moved to OUTSIDE air and then remains common as the lever is moved all the way to the right DE-ICE label.
In this position port 3 line vacuum draws the water valve open allowing circulation of coolant. Lack of vacuum in port 2 allows the spring to close the inside kick panel vent and the spring to open the outside air door diaphragm.
My 3 port control head switch could not be taken apart. it appears to be held together with a rivet through the center of the assembly. The spring clip under the large head of the rivet is keeping tension on the two halves sealing the vacuum control head switch as good as possible with 1972 technology.
Hope this helps someone.