I have an 87 GN and the air conditioning only blows cold air through the floor and out the top defrost vents and not through the center vents. Any setting i put it on.. vent , heat , ac , they all blow air through high and low , but not through the middle vents. I thought there maybe some sort of flap door that's sticking, but i don't know where it is . Any ideas on what's causing this?
I'll bet you popped one of the bladders that opens and closes the vents. Take a look at the check valve that feeds vacuum to the HVAC system. I popped my check valve, and that in turn allowed a whole lot o' boost to get into the HVAC system, and popped one of the bladders. The one that operates the main flapper in the heater box. See if the check valve lets air go in both directions. The check valve is a pancake shaped thing with a vacuum line on either side of it. One side is black, the other is white. It should say VAC on it. There is also one that has one line in, and two coming out. This one is all black, and is near the firewall, close to the Powermaster (brakes). I'm not sure which one is for the heater. I popped them all.
Brian
I had the same problem and have had to do a work-around till I find the real fix.
Here's what I found:
My vacuum control units for the vent controls are good.
There are duct control flaps, which can be seen from under the dash just left of center. [Lay down under steering wheel to view] The bottom unit is easy to see and controls the flow between floor and upper section. The upper [about 10 inches above the lower]controls flow between dash vents and window defrost vents.
I had to activate both with a bypass vacuum line to get dash air.
It's been too hot to work the underlying problem.