l have a 1978 GMC 1/2 ton pickup. There is something wrong with the defroster. When l turn on the defrost it just barely blows. When l change to the floor, it blows like crazy, and when l put it on the vent it blows hard there too. l can hear the blower going full blast when it goes to defrost, but not much air coming out there. A little air comes out the floor and the vent then too. l have dropped the glovebox down to watch the levers work, they do. l have unhooked the lever and did it by hand and still very little going to defrost. Any ideas??

It sounds like there are holes in the vent hose that goes from the heater too the defrost. If the truck also has air conditioning the holes could be anywhere in the duct system and just leaking when the heat is directed too the defrost. You will need too replase the ducts for the entire system if you do the job right otherwise with a system that old it is just a matter of time until it goes in another area.

l had a similar problem with a 69 corvette and after removing the dash l found that over time the plastic duct work had shifted and the air was not being routed to the vent.

Have worked on & rebuilt many of these over the years. l have found more than l can count on both hands that had a bunch of crap. ( pencils, pens, candy, paper, etc. etc. ) down inside the defroster. One my own truck once it sat for a several weeks in the fall. l went to drive it. the heater controls did not work correctly. Upon investigation mice had filled the entire heater box & defroster with small dog biscuits from the bag l keep in the truck.
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