1972 AC Vacuum Line routing

[BruthaMan]

Hey Guys,

Working on a 72 for a daily driver (see sig for build) and need some help with the AC vacuum line routing.

I've been scouring through posts via search for the past day, including this one, and I'm still having trouble. Could use your advice.

Vacuum lines were not hooked up to the vacuum canister. I did a t-split on the vacuum line to the distributor, as illustrated in the photo below, in red highlight, and hooked it up to the canister as shown.



In the same photo, above, in yellow highlight, I ran the vacuum line through the cab and down to the ac/heater control unit, to the port on the back of the unit, as indicated in the photo below in yellow highlight.



As illustrated, there are three vacuum ports on the control unit. I next ran a vacuum line from the port highlighted in red, over to the passenger side air vent actuator. Along that same vacuum line, going to the actuator from the red port, I tee'd it off, and ran the t-off back out through the firewall, to the outside actuator under the hood.

The green port, I'm not sure what should connect there. As described above, with the red port going to the actuator, it would never open the inside air vent (on the passenger kick panel).

There is vacuum (suction) on the incoming vacuum line. It's not ultra-strong, but I can put my finger up to the hose and it will "suction" on to my finger.

Could really use some help on identifying the three ports and what should go where. Pretty sure I have it wrong, but have read through a dozen or so posts and diagrams here and still getting no where.

[Riveted1]

I looked through two chassis service manuals and the assembly manual (which is hard to find anything in!) The only diagrams I found were the same as in the other post you were reading. The control unit shown only has 2 vacuum ports. So no idea why yours has 3. Sorry I couldn't help!

[BruthaMan]

Appreciate the effort. I have the manual was well and agree, it's tough to find things. There was a post I read (may be the one linked to, have read through so many today) that had a screen capture of the page from the online manual and showed "page 509 of 1065" in the screen capture, so I turned the manual to the half way point and eventually found the page there.

[shelbert]

I just went through this as well and this is what I learned;

The can gets vacuum from a source as you have done from the manifold.

The yellow line you have run into the cab to the vacuum switch on the a/c controller is correct, (The switch is plugged unless the top lever is in the A/C position. Some of the controls had 2 plugs, some had 3 either way when the A/C switch is in the A/C position is applies vacuum to the other 2 ports in your case.)

The That vacuum is applied to the actuator under the dash near the passenger side kick panel (either of the 2 vacuum prongs it doesn't matter). The 3rd or last vacuum prong line is ran back through the dash to the heater cut off valve and the under the hood actuator. (you will have to T these as well)

I did not however do that with mine, I blocked my 3rd connector off at the controls and put a T under the dash running that one line to both actuators and the heater control valve on the inner fender passenger side. Either of the 2 ways will work.

What this does for you is if you turn on the A/C it shuts the arm under the cowl vent just under the windshield outside (not allowing hot outside air into the cab), at the same time the vacuum closes the heater control valve (turning off the flow of hot water into the heater core to prevent the cold air being heated by the hot coolant water) , and it opens the kick panel vent under the passenger side dash allowing the internal air to recirculate inside the cab for easier cooling.

You should see all this happen just by turning the slide on the control to A/C. Listen carefully when you do it, you will hear a vacuum leak if there is one, also the actuator actuators may be bad with a cracked seal ( one of mine was and it prevented it from opening).

[BruthaMan]

Thank you, Shelbert!. I forgot to mention that I don't have the heater cut off valve. I checked this morning for it, and it's not there. Just straight hoses going to the manifold from the heater box. Is this something worth adding?

I hooked the vacuum line from the manifold directly up to the inside actuator today, and it still didn't open. So either it's bad, or the vacuum is really weak. I didn't check the outside actuator, but will tomorrow morning. Kept thinking I was missing something basic.

Are these actuators available still from the typical parts store?

I appreciate your advice.

[shelbert]

I wouldn't think the heater control or lack of it would be a problem if the flaps and ducts were all good but we are talking about a 40+ year old truck. If you wanted to you could just put a cutoff valve in the heater line and turn it off during the summer to prevent needing it. Then just run the under hood vacuum line just to the actuator.

As far as the actuators, I do not know if they are available still but I took one off a late 70s think a 78 square body at a pull a part ( some were plastic some were metal) I didn't trust the plastic one so I got a metal one. It will mount right up perfectly but the rod that pulls open the door will need to be used from your truck. The one of the 78 was much shorter.

Oh i forgot to mention if you dont use all 3 of the plugs on the A/C switch make sure the other is plugged well or you will get weak vacuum (although it will be hard to miss if its leaking it is very loud under there)

[BruthaMan]

Much appreciated, Shelbert. Will make sure to plug that third port shut. Water shutoff sounds like a good idea. It's hot here (already) from March to late October.

[shelbert]

One more thing that just hit me, you said you put a T on the line going to the distributor, If your distributor is hooked to ported vacuum on your carburetor (and most of them are) you will have none or nearly no vacuum at idle. This may be your only problem. I would probably hook it to a direct manifold vacuum. If your distributor vacuum advance is hooked to manifold vacuum its probably fine.

[JimKshortstep4x4]

Here is the vacuum line schematic for the two port and three port vacuum switches.

Jim

[RenoKeene]

Thanks Jim and shelbert....very good stuff!

[ol'blu72]

Just for reference I found this pic with 3 ports on the valve. It dosent much show where all of them go but might help someone.

[davepl]

I need to bring this topic back up. The repros all have 3 ports, and I think its because the "opens with vacuum" switches need the "opposite" vacuum signal.

With a 2-port, everything (cowl, kick panel, heater valve) all get vacuum when the AC is on. Tha causes the heater valve to close and the two vacuum doors to open.

With a 3-port, you can use a heater valve that is OPEN under vacuum. The third port is backwards from the second port, such that when the AC is on, the cowl and kickpanel get vacuum but the heater valve does NOT. When the heater is on (AC OFF) then the heater valve gets vacuum and opens.

As you can see the 2-port is simpler, and I bet it's simpler to make the valve. If you have a 3-port head and the "closes on vacuum" water valve, you simply don't use the third port (the 'backwards' port) on the control head.

Does this sound right to everyone that's been down this road?