Here's something I posted on my website WAY back.. 2 years ago..
Heater AC Problems
I had every possible problem with the AC and heater. Finally got it resolved. Hope it helps.
Vacuum
Vacuum leaks and binding doors can cause flappers to not close, not close completely, or be flakey. Air doesn't flow where it should.
There's a hot water shutoff valve to keep the water out of the core when the temperature is in the cold position. The on/off valve is on the heater box in side the car. The shutoff is inline on one of the heater hoses. Vacuum closes it. When it sticks due to age/rust, it usually sticks open. If you have heat, you have an open valve. With the temperature control set to full cold, you should feel vacuum if you disconnect it from the inline shutoff valve. If not, adjust or replace the switch inside the car. Important. You cannot buy a replacement vacuum switch. The one people are selling is for the wiper door on earlier chrome bumper cars. The spring is too strong and will keep you from moving the properly adjusted cable to the cold position. Someone posted a tip where they use the old spring on a new switch. I decided to convert to an electrical shutoff system. Either is fine.
The temperature control is by cable. It might be out of adjustment. You might be blending some hot air into the system. If this cable is not adjusted properly, it won't activate the switch above. There's an adjusting mechanism inline on the cable about midway down the length. You might have to remove the right side console panel to find it. You twist to shorten/lengthen the cable.
On Max A/C, internal air is recirculated. On normal, external air is brought in. This is done with a vacuum controlled valve on the right side near the passengers foot behind the kick plate. If this isn't functioning, you might not be getting the recirculation you expect on Max. An upstream vacuum leak can be a pain to find. I had one of these. Flapper valves were not functioning properly. Turned out to be a leaky hot water shutoff switch. I replaced the mechanical vacuum switch with a microswitch and a vacuum solenoid. Perfect now. I'll have to document this.
With A/C, this vent is the intake for recirculating air. No fresh air flows through it. All fresh air goes through the dash vents. If it is open, you are recirculating what's inside the car. This happens on MAX AC only.
Any vacuum leak is deadly to the A/C on this car. You've got to fix it or none of the flappers work. All vacuum comes through a tiny tube. It enters near the heater box and there's a T connection next to the hot water shutoff switch inside the car on top the heater box. In my car, this valve was leaking slightly. Even a tiny leak bleeds off the vacuum before it can open a valve, door, etc.
The vacuum leak might be causing the defrost door to flip in the wrong direction or the vent / floor to go the wrong way. I can't recall which direction is the default (no vacuum). I'll see if I can find my notes on this. My leak was causing the door between vent and floor to operate slowly or not completely. It was a mess.
otfl
