yo, Here is the bottom line;
Off- Vacuum is applied to Outside Recirc Door Vac Motor, closing that door to outside Air.
Normal A/C - Outside-Recirc Door is open to outside air. The panel door directs air to the instr panel registers.
The Temperature Control Lever moves a cable connected toTemp Blend Door that mixes heated & un-heated air
In Defrost Mode; with No vacuum to any vacuum "motor" (cannisters looking things), air Flows to Defroster outlets (& air compr clutch is engergized)
Vent - Outside-Recirc Door is open to outside air, as in Normal A/C.
Vacuum is applied to the Panel Door Vacuum Motor, letting air flow only to the instr panel registers.
In the same way, the defroster outlets are closed by the Floor/Defrost Vacuum Motor.
Floor - The Outside-Recirc Door is Open to outside air.
With No Vacuum at the Panel Door Vacuum Motor.
The Panel Door directs air to the floor outlets.
Mix - Vacuum is applied only to Port A of the Floor/Defrost Vacuum Motor positioning the Floor/Defrost Door at mid-position.
Air Flow is split between the floor & windshield.
There is no vac at the Panel Door Vacuum Motor.
This closes the instr. panel registers
So, am thinking that the Floor-Defrost Door is stuck...So. to test, pull & cap or pinch off vac line to the Floor-Def Vac Motor to see if door moves to the Full Defrost position (by feeling air flow).
btw, hear any hissing that indicates a vac leak under isntr panel or in those door areas and control panel?
or much less likely, vac is going to floor bec the floor-def door is stuck or A/C- Heater Function Selector Vacuum Valve (in Chris's diagram below) is messed up
Now, for what is what; my pal, Chris B posted this once on his site;
and a pic of the Blend Door/Foam in an 83..same as your yr; by Steve83
"This is looking rearward over the R wheelwell with the evaporator cover removed beside the R exhaust manifold."
I'll post this now just in case we lose power.