Heater troubles

[KojaK 79k10]

Hey everyone, got a 79' k10 silverado w/AC (gone now) and i have problems with my heater, it only works on floor weather its on defrost, vent or floor. I have checked my switch operation and the cable moves smoothly. the blower also works on high, medium and low. It shuts off when switched over to a/c like it should because a/c is gone so i don't know what could be wrong. there is a vent flap behind dash that has the rod to it broken but i ran the heater and switched it manually with my hand and it did nothing.

Any input would be highly appreciated because it is getting very cold here in Wyoming and it is imperative that i resolve this problem.

[DnStClr]

You may not be getting a vacuum to the switching valve that is located on the control panel. There should be a small tan or grey tubing that supplies the vacuum to the valve.There are other colored tubings at the valve too so you might have to trace that tubing back to it's source, which should be at a tee on the intake manifold or a vac. cannister. Make sure the tubing is ok and has good connections. If there is vac. at the source tubing, you might have a sticky or bad actuator at whatever position you select on the control switch. Each position has it's own actuator.The actuators have a diaphram in them that can fail over the yrs..  they're hard to get to and need the vac. signal to work.

[VileZambonie]

Yes it's probably a vacuum control issue. You should have a hose coming through the firewall and going to a canister. the canister needs a vacuum supply source from the intake manifold.