1988 Chevrolet Silverado 1500 ex-cab 2wd v8 5.0L?

Question

My defroster doesn't work

heat comes out just fine from the vents and floor, but when i switch to defrost to defog my windshield, the fan runs but nothing comes out

though a little will come out from the floor vent

just to add, the AC unit doesn't work... but it does blow 'cool' air when i set it to cool...

but the cool air doent come out the vents above the dash either

Answer 1

Sounds like you lost vacuum to the heater control switch.Should be a small vacuum hose going into the firewall that supplies vacuum to switch.  

Answer 2

The problem is called misdirection. It is due to a loose or broken vacuum line which supplies the control head inside the truck. The line comes out the firewall around the evaporator case on the passenger side engine compartment. It shouldn't be difficult to find or repair

Answer 3

Since air comes out of both the floor ducts and dash vents depending on the vacuum switch position, the first and easiest thing to check is if the ductwork to the defroster vent has been knocked loose or is broken. There also could be a problem with one of a couple vacuum diaphragms to the blend doors or the hoses that control them. Not likely to be a problem with the dashboard vacuum switch or anything before that if it does switch between 2 of the 3.