i have a 98 k1500. when i got the truck the previous owner was using needle nose pliers to adjust the temp. the ac was working though. after letting the truck sit for a year i got a used head unit and installed it but it would only blow on the floor. the truck sat for another 2 years. last week i put a new motor and trans in and today bought a brand new head unit from the dealer. when i first installed it, it wouldnt do anything but light up i tapped on the blower motor a few times and now it blows only on the floor no matter what position the switch is in and no cool air. my concern isnt the no cold air it probably needs freon but that shouldnt affect the selector switch. what could it be? would a bad blower do this?

 

Well if the fan is blowing, it should be okay, Could be bad actuator(s), ground or wiring issues. I'd rent a gauge set for the A/C to see what pressures they show. Normal at 75-80 F is 30-40 PSI on the low-side and 175-210 PSI on the high side. With R-134A, too much pressure can reduce cooling efficiency, so don't add too much. It might not hurt to put an ounce of PAG-150 compressor lubricant in as well.

 

Sounds like your blend doors aren't working properly. They have actuators that control them that could be bad