Chevy Silverado. 305 Cid. Question = Hello JK.  Please help me, I'm at my wit's end.  My 1995
Chevy truck is having AC/vent problems.   
The fan speed and fan work correctly.  The heater control and heating work correctly. The fresh
air/recirc button never illuminates or has any affect.  The AC button never illuminates or turns on
the    AC compressor.  The vent selector has no affect.  Air only comes out of   the floor vent,
regardless or the setting.  

I just replaced the dash control unit with a new one, and it didn't change the symptoms at all.
I checked and even checked continuity on the related fuses.

I jumped the AC high pressure sensor and it didn't change a thing.  I hot wired the AC compressor
on, and nice cold air blew out of the vent immediately, so I assume the AC system is working
correctly.  I've gotten in the dash and reseated all the plugs I could reach.  When I swap between
the new and old control units nothing changes at all.

Once in a while one of the servos will move, but rarely.  The air might come out of the upper  dash
vents when I first start the truck but switches to floor vent within a couple seconds.  Sounds like one
of the other flaps might move once in a while, but rarely.

Can you please tell me if these symptoms ring a bell with you, of at least offer a suggestion on how
to troubleshoot this thing?

Sounds like you have several different problems here. For the vents switching, sounds like a  
vacuum problem. There is a vacuum reservoir tank that hold vacuum to move the vents  actuators.
If vents switch when hitting gas, or when idling, then need to find source of vacuum leak.  Either the
tank or the hose that feeds the switch. The compressor not coming on is electrical. There is also a
low pressure switch, a compressor   relay for the system. Make sure to
check ALL fuses also.

1999 Chev. Silverado 6.0 Vortec
Work done to repair this problem = mechanic replaced the motor in middle of
heater/ac but did not fix the problem.

Question = The heater/ac makes a thumping noise in the dash on the passenger
side. It seems to be in the fresh air/inside air motor or solenoid that changes the
door. It acts like it is trying to open or close the door but is hindered. It thumps,
thumps, thumps then stops, after a few minutes it does it again. If the button is
pushed for inside air it seems like a grinding noise. It also does it when changed
from heat to AC. Where is the motor or solenoid which changes that door?
GM Tech JK:
To replace the recirculate actuator, you need to remove the top of the dash. You
may not need to replace the actuator though. This door actuator does not have
anything to do with the temp. So a noise in that door can.t be this motor.
Try a reset...

First, disconnect the battery for a few minutes. Then reconnect, start truck, and
let it run for 3 minutes without touching any of the A/C controls. This is a sort or
reset for the electric door actuator motors.

To see that door and motor, remove the glove box, release the tabs on the inside
of the glove box, you do not have to take it off, just open all the way past the
stops that keep it from swinging al the way down.

When you do this, you should be able to see the recirculation door, it has foam on
it with a plastic grill. Turn on the a/c, push on the door and turn the system to
recirculation and ses if it moves, and if this is the one giving you the noise.
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