when the system is broken it goes into the defrost fail safe mode where the air just comes out of the defrost.
i have a vacuum diagram , i need to find my scanner and post it, have to remind myself to do that.
first thing is check some stuff from the engine compartment.
on the passenger side by the shock tower will be the Vacuum storage canister. look for a hose coming from the engine rear manifold going into the canister. there will be a second hose out of the canister to going to the firewall there will be a plug in the firewall with the hose going into it.
make sure nothing is disconnected visually.
when the car is running and you switch from Heat to A/C are you getting heat and cold from the defront vent?
with the car running, go back to the engine bay on the passenger side by the firewall and engine valve cover are the heater core hoses.
on one hose is a vacuum valve. have an assistant watch that valve or put a cell phone camera on it. go back inside the car and move the tempature selector to heat and note the valve position. next move to cold and see if the valve lever moves.
that valve cuts off the coolant to the heater core and must turn off when the lever is moved to cold. if it doesn't move, that would indicate a vacuum failure or valve failure.
turn the car off and now you will need to work inside and in the engine bay, get a vacuum hand pump.
now we move to minor surgery:
on the back side of the climate controls is a rubber distribution block called the octopus it is a rectangle rubber block with all the vacuum hoses running out of it just like the sea creature. the hoses are color coded, however due to age usually the rubber color codes are VERY hard to see.
looks like this from behind the dash
you can refer to some of my photos here
http://s1031.photobucket.com/user/72hcode/library/Heater%20Box%20AC?sort=2&page=1
first thing is to try and reach behind the dash from the foot well on the passenger side and push on that octopus make sure it didn't fall off or get disconnected.
next use that vacuum hand pump and a piece of hose.
go to the engine bay and disconnect the hose from the intake manifold to the vacuum storage container.
install the hand pump and try to suck the air out and maintain a vacuum. if you pump and pump and it won't hold vacuum then you obviously have a leak in the system. if it holds vacuum,
perform test 2: take the hand pump out and put a clean hose on the input (center), disconnect the output. blow air into the canister center and suck air out,, you should only be able to move air one way, if you can move the air in and out then the reed valve inside the canister is blown and you need to replace the canister.
replacing the canister won't solve your problem but it is the cause of other problems or its failure caused a vacuum clog somewhere else.
connect everything back up go inside the car, remove the glovebox insert(couple of phillips screws) you can see the A/C heater box a little better and some of the vacuum motors.
you can reach all the vacuum motors from the foot wells on the passenger and drivers side. you will need to unplug each motor one at a time and test them for function. they should hold vacuum and move freely and turn on the different flappers and actuators.
you can put the car in Accessory mode (turn key all the way counterclockwise back one from lock) and run the fan motor then as you test each motor you can see if the air is changing paths, from floor to defrost and main vents and if the fresh air recycle is working.
if each motor is working then you know the problem is in the distribution controller for the climate control, that means you will need to remove the center of the dash board, pull the radio and floor console and pull out the climate controls. you will have to pull off the octopus as well.
with the controller out i would test each hose going to the octopus for leaks, it is 40+ year old rubber it can get dry rott, they do not make reproductions so you either have to make repairs to yours or get another one and or run a new hose one at a time to each vacuum motor, time consuming.
now the controller can get a piece of junk stuck inside it, remember that reed valve on the vacuum can, well if the engine backfired or that valve fell apart it gets sucked into the controller and clogs things up, sometimes it is so bad you need to replace the climate controller. so using compressed air in the hose connector tubes for the octopus blow out the controller and move the levers around as you do it to try and get the junk out. the black hose is inlet from the vac can and usually a clog flies out of the can into the controller and clogs the entire system. there is no way to take the controller apart so you have to keep trying to get the clog out, can be pieces of rubber getting inside or carbon from the engine if the valve is blown on the canister in the engine bay.
now again you may never get the clog out, you can use gun barrel cleaners to try and help get inside they sell gun cleaner kits on amazon.
for me i had most of the problems i talked about above,, i had to replace my octopus and replace my climate control controller, because whatever was stuck inside would not come out no matter what i did.
you can see there is a lot of points for failure in this vaccum system for the A/C so you want to test all the parts to make sure.
you could have an issue with a broken or stuck door inside the vent system and the vaccum motor works but cannot open because the mechanical is jammed, in that case you need to pull the top of the dash out to get to the vents.
it is not uncommon for some animal to nest in the heater box or the fan box, and when you turn it on a piece of junk goes flying into the vent mechanical and jams it up. 99% of the time you crack open the heater core and a pile of nuts flies out.
