Understanding how the AC control lever works

[davepl]

Could someone tell me what happens on a properly set up truck when the control lever is moved from "Inside" to "Outside"? My expectation is that "Inside" recirculates air, so it closes the kick panel and cowl doors for "Inside" and opens them for "Outside".

My problem is that in 1970 you only have a single vacuum port on the control head (2 ports, one in, one out). You can apply vacuum to all three or none of them. So if you close the doors by removing the vacuum signal, you also open the heater valve.

So you can have the doors open and the heater valve closed (cool fresh outside air) or the doors closed and the valve open (recirclated heated air).

None of that gives you the "Inside" mode of recirculated cold air. I'm at a loss how it's even possible, since the water valve closes with vacuum present.

In summary:

UNDER VACUUM:
- Heater valve closes
- Kick panel and cowl doors open

NO VACUUM:
- Heater valve reopens
- Kick panel and cowl doors close

How do you make an "Inside" mode of cool recirculated air out of that? I actually have the 3-port control head, and only using 2 of them, but the truck didn't originally have the 3-port (which is intended for the water valve that opens on vacuum), and it worked from the factory!

Let's stick solely with the simpler 2-port head for now; that's intended for use with the water valve that closes under vacuum (unless I'm wrong on that key point).

My only flailing guess is that the heater valve is actually OPEN during Recirc AC, but the airflow takes a different path, so it doesn't matter. But I have no idea.

[davepl]

The above post is INCORRECT but too old to edit (post 30 mins). Mods feel free to nuke it in favor of this second updated one, if possible.

I had this super-complicated in my mind because I didn't realize the cowl door CLOSES with vacuum while the kick panel OPENS with vacuum. Now that I've got my head on straight, can someone check my math for correctness here?

My expectation is that "Inside" recirculates air, so when set to INSIDE, the cowl door closes (vacuum present) and the kick panel door opens (vacuum present) and the heater valve closes (vacuum present). So you get inside air drawn in from the kick panel and re-cooled.

Then, when you move the switch to OUTSIDE, the vacuum goes away, so, the cowl door opens (no vaccum), the kick panel door closes (no vacuum) and the heater valve opens (no vacuum). You get outside air drawn in from the cowl and the heater core is available again to mix with the cold air.

Thus on INSIDE you get cold recirculated air and the heater core is turned off. On OUTSIDE the heater core is re-enabled so you can mix air, what they call Bilevel in the manual.

And now let's get fancy and talk about 2-port and 3-port control heads. In each case one of those ports is engine vacuum, so there's either 1 or 2 control ports. I'll still call them 2 port and 3 port for consistency.

On the 3-port, they used a water valve that OPENS with vacuum. That means when vacuum is being sent to the cowl and kick panel, they needed an OPPOSITE vacuum signal, and that's the third port.

On the 2-port, they used a water valve that CLOSES with vacuum, so it can be just connected to the same vacuum signal as the doors. That eliminates the need for the third port entirely.

I believe the functionality and connections to be:

TWO PORT:

- When set to INSIDE, Vacuum ON Port 2
- Vacuum ON (Port 2) closes outside door
- Vacuum ON (Port 2) opens the inside door
- Vacuum ON (Port 2) closes the hot water valve
- And so, cold air recirculated from inside the cab

- When set to OUTSIDE, Vacuum OFF Port 2
- Vacuum OFF (Port 2) opens the outside door
- Vacuum OFF (Port 2) closes the kick panel door
- Vacuum OFF (Port 2) opens the hot water valve
- And so heater core active again, outside air mixed hot and cold by temp lever

THREE PORT:

- When set to INSIDE, Vacuum ON Port 2, Vacuum OFF Port 3
- Vacuum ON (Port 2) closes outside door
- Vacuum ON (Port 2) opens the inside door
- Vacuum OFF (Port 3) closes the hot water valve
- And so, cold air recirculated from inside the cab

- When set to OUTSIDE, Vacuum OFF Port 2, Vacuum ON Port 3
- Vacuum OFF (Port 2) opens the outside door
- Vacuum OFF (Port 2) closes the kick panel door
- Vacuum ON (Port 3) opens the hot water valve
- And so heater core active again, outside air mixed hot and cold by temp lever

And that, kind sirs, is my Chevy Truck Air Conditioning Manifesto. Can anyone who knows how the system actually works check that this is correct?

[chipflyer]

I think this should help. Also, they made 2 different shutoff valves one that shuts off flow with vacuum and one that shuts the water off without vacuum.

Jeff

[davepl]

Thanks, the diagrams are helpful ( I had them when I wrote it, nice to have them here ). I did cover the two different water valves and the changes it required (if I'm right).

[tommys72]

So what's the difference with water valves? What differs to make you use one or the other?

[Davidf]

I think operation principle is same for both 2 and 3 port vacuum valve. Water valve is normally open on both versions. Only difference is multiple vacuum source so one failed diaphragm doesn't cause everything to loose vacuum. Could definitely be wrong on this.

[chipflyer]

No, they are different. Take a look at both diagrams carefully, one (the 3 valve) is open with vacuum applied the other (2 valve version) is closed with vacuum applied. I had replaced mine with a new one and had hot air mixing with my A/C on. Pulled the vacuum line and water stopped circulating. ended up finding the correct valve and it all works perfectly now.

Jeff

[davepl]

With a 3-port you could use either water valve (because you have both ON and OFF available at the control head) but with a 2-port you can only use the "closed under vacuum" style, since that's the only signal available.

Check my tables, I think they're right...

[chipflyer]

Check my tables, I think they're right...

I'll take your word on that!

[davepl]

Can anyone confirm at least this one part? I just want to make sure I'm chasing the right rabbit here, to coin a phrase. This is how I'm going to -try- to make it work, but want to ensure that's how it's -supposed- to work.

TWO PORT:

- When set to INSIDE, Vacuum ON Port 2
- Vacuum ON (Port 2) closes outside door
- Vacuum ON (Port 2) opens the inside door
- Vacuum ON (Port 2) closes the hot water valve
- And so, cold air recirculated from inside the cab

- When set to OUTSIDE, Vacuum OFF Port 2
- Vacuum OFF (Port 2) opens the outside door
- Vacuum OFF (Port 2) closes the kick panel door
- Vacuum OFF (Port 2) opens the hot water valve
- And so heater core active again, outside air mixed hot and cold by temp lever