AC only blows through bottom vents only??

[TheObiJuan]

it is on a 92 ty that this is occuring btw. and no, it is not because the ac control is set to floor vents. any ideas on how to fix this? could it be that the mechanism that switches the flow from reg vents to floor vents is broken? tia.

 

[jwaller]

you have a vac leak. possibly under the hood at the firewall connection. if thats not it, it could be the controls themselves or even the vac can that moves the door vent itself. just listen for the leak and you should be able to find it pretty easy.

 

[GAShan]

Most likely the lower of the two vacuum line connections on the underside of the front of the upper intake manifold has come off or is leaking. Common problem.

 

[Ian Turgeon]

right.. follow the line that comes off the intake manifold in front.

if theres no leaks (goes to egr, cruise control, and vac ball) then the large black check valve is bad (or their is a leak inside the truck)

 

[TurboRob]

Another possible trouble area is the HVAC control assembly in the dash.

On mine, I had the same problem accompanied by a hissing sound from the HVAC controls in the dash depending on which setting I chose (AC, Max AC, Heat, etc.)

I wound up replacing the entire control assembly from gmpartsdirect for about $70, but you should be able to replace just the vacuum controller on the assembly for about $10.