When I have my heater or A/C on, no matter what setting I have it on, I can only get air coming from the floor vents, and/or defroster vents. When I put the 'slider' on the dash vent setting, i only get air coming from the floor vent and the side window defroster vents, but nothing from the actual dash vents. I've briefly looked for a hose or something that has slipped off, but it's pretty tight in there and thought I would ask here before I rip my dash apart.
You didn't mention which vehicle... we'll go on the assumption that you have a vacuum-operated vent system.
A question... does the venting air ever change when you move the slider around? My system defaulted to what you describe when the vacuum line to the vacuum reservoir got melted... this was right by the battery (I used a torch to melt solder on some battery connections, dangerous, I know). Same air venting, no matter what position I had the control (knob, on mine). My cruise control wouldn't work, either... dead giveaway on a vacuum problem.
My vacuum reservoir is underneath the passenger side front bumper on my '00 XJ. All I had to do was splice the tubing with a piece of 1/4" ID rubber hose and all fix.
I went ahead and bought a vacuum pump bleeder-tester... but I like tools, you might not want to spend $30 on one. I didn't need it to fix my problem... if all your lines are OK, then you're going to have to start testing reservoir vacuum (the pump has a gauge!). The pump can be used to test vacuum motors, too... this is the diaphragm that moves the vent doors, changing your air path.
I have the FSM for my Cherokee, as well as both Haynes and Chilton for earlier years. I suggest you get at least one of these resources. __ Steve .
I want to thank you for posting this information. I used this as a guide and it led me right to the problem, which was also in the vacuum tube below the battery. Thanks again Jim Shaw St. Louis, MO
MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.