Does anyone have the slightest idea how the vacume operated selector switch system works???

I am having a problem,I cant get air to blow out the dash vents. Air comes out the lower plenums at a VERY reduced rate. How do I trouble shoot to fix??????

Car is a 72.

Nothin.

Well I tore the whole dash apart and still dont understand why this isnt working.
I might have a bad part(dont have a clue what its called) thats on top of the assembly where all the vacume lines meet. Sounded like it leaked. will replace if I can,or replace the whole controle unit.

Any ideas? Anyone??

I have a 70 SS with air and have worked some on that system.

This could be caused by a bad vacuum valve or no/defienct vacuum in your system. First check to be sure your vacuum canister under the hood is holding (not leaking) vacuum. Also check the lines for leaks. The dash control you mentioned could also be the culprit but I doubt it. I would not touch that unit until all other items below have been checked, unless there is outward evidence of damaged hoses at the manifold. The dash control manifold has a lot of little lines and could be easy to damage or reverse position.

If I had to guess based on my knowledge of the system, I would say that your problem may be the cowl plenum valve. I would test this valve first. It supplies the volume of air for the system. See below for location and testing procedure.

Next, if the 72 system is the same as 70 you should have 4 vacuum valves. The locations are as follows.
1. cowl plenim area passenger side under the plastic grills (plenim).
2. Behind passenger kick panel (side shroud)
3. A/C, Heater diverter door. Door is inside ducting coming through firewall from evaporator.
4. Defog valve.

Cant remember the location of the last 2 valves, but one is just to the right of the gas accellerator pedal.

If you are sure you have vacuum supply, begin to check the valves.
With the engine running begin slowly switching through the various control lever postions, while listening at the various valves for activation. Have a friend switch will you listen. Stay at one valve try all dash positons, then move to next and repeat. If none of the valves are activating then lean towards supply. IE vacuum tank or main vacuum supply line or dash control.
You can also just hook a temp hose to the valves and suck in with your mouth. You should be able to activate the valves in this fashion and listen for them to open and close. If the valves are good you then need to check the vacuum supply lines that hook to dash control or the control itself.

If you purchase a wiring diagram for your car (year one) it will show the layout of the a/c system. It includes the vacuum lines routing and connection. The lines should be color coded at the dash switch.

You can fix this but you will have to work it through the steps discussed to isolate the problem. Hope this helps, Stein...

Yes this helps ALOT!!!!!!!!!!!!!!!!!!
THANK YOU!!!!!!!!!!!!!!

I have already decided that at least PART of my problem is the rotary select switch( diverter valve control??) It was leaking and when I took it apart,I found the spring loaded clip slightly broke. In the process of inspecting this part I broke the crap out of that clip,so now its no good for sure.
So thats PART of my problem.

Now,what is and where do I find the Vacum Canister?? I dont beleive I have one,so I might need to get one,to have proper vacume pressure?? Does anyone know the correct amount of vacume?(I have a vacume gauge,thinking I could measure vacume?)

THX for the help!!!:)

Below is a shortcut to a website. It shows a picture of the engine compartment click on it. You will see the vacuum canister mounted on the side of the evaporator box. It sits above the valve cover. That is a pic of a 70 but I think the 72 is similar. Some of the cannisters where shaped/looked like a tin can. If yours is missing you will need to locate a used one. They do sell repos (Hinshaws Chevelle 336 586-0802) but I dont know if that is right for your car. Ask for James he may be able to help you. As far as your question about vacuum amounts you should have a fiting on your intake. It is located at the back of the intake manifold behind the carb. This supplies the proper amount of vacuum to the cannister which then supplies to the dash control and then to the valves. It doesnt require alot of vacuum to power the a/c equipment. If your power brakes work ok, then the a/c will have plenty of vacuum. Hope this helps, Stein...


http://goldenclassics.com/cgi/displayphotos.cgi?stockno=1569&year=1970&make=CHEVY&model=Chevelle%20SS%20%20454&photodirectory=1569&thumbnail_url=L10.JPG

Yup,I dont have that. I have a very small canister type thing in the same genaral area. it is inline and about the size of a small fuel filter. SMALL fuel filter.

I need to investigate this further. THX!!

Im not sure what the canister looks like on a 72 or if it reguires one. Best thing to do is find a picture of a 72 a/c car on the internet. Has your a/c and heating ever worked on the car?

Did you buy the car with the system inop?
If it worked for you before, than I doubt parts are missing...

No,bought it inop.

I have seen pictures of 2 or 3, 72 cars with air, that dont have the canister.

http://adcache.collectorcartraderonline.com/10/3/0/79499630.htm
http://adcache.collectorcartraderonline.com/10/7/4/78491274.htm

Perhaps they did away with the canister on 72 cars. I would reseach this more before buying any parts.

It appears that some changes occured on the design of the 72 cars a/c system compared to the 70.

The good news for you is perhaps your system is not in as bad shape as you think. Sorry for the pump-fake, Stein...

You problem might not of been within your main control unit but since you broke the piece now it is but maybe its part of the inside Air box under the dash on the passenger side there is a flapper door inside it which changes from heat to air and also divertes the air to the main ducts there is a plastic piece with a little pin and a screw it is common piece to brake might want to check it out you will need to take the main diverter tube off which is held on by 2 screws to see the piece.

I have seen pictures of 2 or 3, 72 cars with air, that dont have the canister.

http://adcache.collectorcartraderonline.com/10/3/0/79499630.htm
http://adcache.collectorcartraderonline.com/10/7/4/78491274.htm

Perhaps they did away with the canister on 72 cars. I would reseach this more before buying any parts.

It appears that some changes occured on the design of the 72 cars a/c system compared to the 70.

The good news for you is perhaps your system is not in as bad shape as you think. Sorry for the pump-fake, Stein... If you look in the engine of the second link,you will see a small white fuel filter looking thing located inline where your vacume canister is. That is how mine is. it is also how some tech members cars are,so Im thinking Im good to go on that part.

You problem might not of been within your main control unit but since you broke the piece now it is but maybe its part of the inside Air box under the dash on the passenger side there is a flapper door inside it which changes from heat to air and also divertes the air to the main ducts there is a plastic piece with a little pin and a screw it is common piece to brake might want to check it out you will need to take the main diverter tube off which is held on by 2 screws to see the piece. Wow dude,breathe:D
You could possible be right. But the peice I broke already had a leak. So really I just a made bad peice completely useless;)

Ill check into what your saying though. Just to be sure.
Where exactly is the plastic peice with the pin? Is it on the door itself??
I do know part of my problem (well the part im dealing with at the moment) is that the door you described isnt moving. Its kinda stuck in the middle,so I have air coming out of everywhere at once. When I close the door manually,I have a good quantity of air coming out of the vents though.