Adjusting Mode Knob Doesn't Change Direction of Airflow

When I adjust my Mode Knob to any of the settings such as Upper, Bi-Level, Floor, Defog or Defrost all I get is airflow coming out of all vents at the same time. I've changed my blower motor resistor in the past but I think it only handles the amount of airflow (speeds 1 to 5). Could this be a problem with the actual control unit in the dash or is it something more complex? Any advise is greatly appreciated! (1997 Pontiac Grand Prix GT 3.8L V6)

Most loss of vent control or airflow problems is a bad or loose accessory vacuum line connection.

Check the plastic vacuum line that runs from the back of the upper intake manifold along the firewall and over by the battery/main fuse box. Look for damage or a loose coupler.

I agree with bnaylor. The modes are controlled by vacuum, and a cracked/disconnected line will cause what you describe. Have you recently done any work around the battery area under the hood? The HVAC vacuum supply line runs near it.

Okay, bnaylor and richtazz hit it right on the money. I followed the vacuum line connection starting from the back of the upper intake manifold along the firewall and over by the battery. When you get to the battery the vacuum line disappears under the washer fluid reservoir. Once I removed the bracket and reservoir I found that the line comes to a junction of some sorts where it "T's" off into two directions. Fortunately (or perhaps unfortunately) I found that the line that goes beside the battery and under the fuse box and then into a large conduit of wires heading back to the firewall had a crack in it. So, I took some electrical tape and taped it up real good then started up the engine and tried it out. Sure enough all the modes started working again. Many thanks to bnaylor and richtazz.

Now I need some additional advice. The tape is only temporary for now, of course. What I need to know is if there is a way to just cut away the part of the vacuum line that is cracked and then somehow splice it together using new line and couplers? Unfortunately there is no extra slack in the line so I'll need to purchase some new line and perhaps couplers from somewhere (any idea on part numbers?) I'm not all that anxious to try and replace that whole line that goes into that conduit of wires. I tried following it back to the firewall and it's not very accessible at all. Any tips in order to make the easiest most affordable fix would be greatly appreciated.

You can do the repair in one of two ways. You can replace the entire bad section with rubber hose that slides over the plastic snugly, or, if it goes through an area that the larger OD rubber hose won't fit through, you can buy a repair section of the plastic line (HELP!/Motormite offers it) and use vacuum couplers (or short sections or the previously mentioned hose) at each end.

I have the exact same problem with the exact same car. Is the reason described the only cause that it could be and was it eventually repaired as suggested