HVAC problem

[kd3yt]

2001 Traiblazer - 72,000 miles.

The link "Heater System: Information, Troubleshooting, And Repair" is broken. I did pm administrator.

My problem - Actually started last spring - Using the heater, if I put the control to blow air thru the vents in dash, its blows cool air. IF in rest of positions, blows hot air. Warm weather came and didn't get any work done on it - Was in process of buying house and moving. Yesterday I put the heat on with control set to second position from left (defrost and floor), where I normally set it for heat. I live in the south central mountains of Pa and when we would go up a hill the heater would blow cold air out the dash vents and soon as would top the hill and let off gas, it would go back to warm air through the defrost/floor vents!

The little I know about the hvac system is the vent positions are controlled by vacuum.

My question is, is there only one vacuum line behind switch or one for each position and why would it switch like that when vacuum changed corresponding to engine load.

Any ideas and help greatly appreciated!!

[Blue Bowtie]

There is one small vacuum line supplying the HVAC damper position controls. There is a reservoir (IIRC, it is bolted to the underside of the hood on your Blazer) and check valve installed to maintain vacuum in the HVAC system when climbing hills, hauling loads, etc. The reservoirs tend to develop cracks, and the check valves dry out and fail to seal. Theoretically, you should be able to disconnect the vacuum line from the reservoir sphere a day after shutting off the engine and there should still be vacuum present.

NOTE: If it were TRULY a vacuum, there wouldn't be anything present, right? That's a lot like an oxymoron. So would that really be "Air missing?" Hmmm... I'll have to ponder that one.

Anyhoo, my WAG is that there is a leak in the system which prevents the reservoir from holding vacuum, err, keeping air out. Well, you know what I mean. Do you remember that line you were playing with for the 4WD solenoid vacuum valve? Somewhere in that line is a "T" fitting and check valve which should serve the HVAC controls and reservoir.