on a 1978 GMC Jimmy 4x4 does the defrost work with the AC?, i have the belt off and no defrost!

The compressor is used in defrost but it will still blow air regardless of the ac being operable. The cable or door that moves to select the vents is likely the issue

Thanks for the feed back, when actuating the vent lever i can hear a noise that sounds like a door dropping. but i don't hear a fan sound at all. does the door dropping signal the fan to turn on? and where is the fan located under the dash or in the engine compartment? maybe it's not getting power?

The fan is on the passenger side of the engine compartment on the firewall. That same lever should activate the fan then you can change the speed of it with another lever

thanks for the input,found a coroted wire to the fan, now i have fan but had to by pass heater core due to leak.( California not to cold)new issue smog test:( fail on egr system,egr replaced years ago i am thinking vacuum sensor any trouble shooting ideas?