Vintage air Cold out defrost hot out dash vents.
 

I am confused as to why my A/C compressor in my 69 GMC comes on when I switch to defrost and works great ! When I move the lever back to the left the compressor shuts off. What am I missing? Thanks !

Re: Vintage air Cold out defrost hot out dash vents.
 

You may need to calibrate it.
On Vintage Air Gen IV systems using factory controls, it is necessary to calibrate the system to your specific
control panel. This procedure ensures that the stroke of your control panel levers or knobs is translated into
precise control of the fan speed, temperature blend and mode door position. Please carefully read and understand
these procedures before beginning. The procedure may be repeated as many times as necessary to get it right.
In preparation for calibration, you will need to attach the supplied white ground jumper wire to a suitable chassis
ground. This jumper wire must be easily connected to the gray programming wire located in the main Gen IV wiring
harness next to the compressor relay. During the calibration procedure, you will connect the white jumper to the
gray program wire, which will “teach” the Gen IV ECU the upper limits of the control levers or knobs. The blower
will momentarily change speeds, signaling that the upper limits have been “learned”. You will move the levers or
knobs to opposite extreme positions of their travel and then disconnect the white jumper. The blower will again
change speeds, signaling that the lower limits have been learned and that the calibration procedure is complete.

Re: Vintage air Cold out defrost hot out dash vents.
 

I can't speak to the Vintage Air specifically, but OEM use the AC compressor on defrost mode (with heat ON) because the AC system will remove moisture from the air and help with the defrost function.