Heater Selector Switch

Air Flow Control
Air flow control is accomplished in the following manner:

• Primary control is through the function selector knob , mounted on the heater function selector switch, which is part of the heater control .
• The function selector knob has the following positions: OFF, PANEL, PANEL/FLOOR, FLOOR, FLOOR/DEFROST and DEFROST.
• The heater function selector switch combines a vacuum selector valve with an internal electrical switch.

• The vacuum selector valve directs source vacuum to various vacuum control motors (18A318) . Refer to the System Airflow Schematic and Vacuum Control Charts.
• An internal single-pole electrical switch is also controlled by the selector. The switch controls the electrical supply to the heater blower motor switch (18578) .
• The position of the function selector knob determines the manner in which the system will operate.
• Each position of the function selector knob is detented for positive engagement.
• You probably have a vacuum leak , Pull heater control panel an check for vacuum . A broke vacuum hose or The vacuum selector valve is bad ! Or check for a vacuum hose in the engine compartment being off somewhere !
  

. Air won't blow out of the floor or center vents just defrost ? Do you have manual or automatic climate control ?
Air Flow Control
Air flow control is accomplished in the following manner:

• Primary control is through the function selector knob mounted on the A/C-heater function selector switch, which is part of the integrated control panel.
• The function selector knob has the following positions: OFF, MAX A/C, A/C, PANEL, PANEL/FLOOR, FLOOR, FLOOR/DEFROST and DEFROST. The A/C-heater function selector switch combines a vacuum selector valve with an internal electrical switch.
• The vacuum selector valve directs source vacuum to various vacuum control motors (18A318). Refer to the Vacuum Control Diagram , System Airflow Schematics and Vacuum Control Chart .
• Two internal, single-pole electrical switches are also controlled by the A/C-heater function selector switch. The combination of these electrical switches controls the electrical supply to the A/C clutch and heater blower motor switch (18578).
• The position of the function selector knob determines the manner in which the system will operate.
• Each position of the function selector knob is detented for positive engagement.
• Vacuum Control Motors The vacuum control motors (18A318):
  
  • are located on the A/C evaporator housing, A/C recirculation air duct and heater air plenum chamber (18471).
  • direct system airflow to the vehicle interior as determined by the remote climate control module or manual override.
  • The five-passenger seating configuration has an additional vacuum control motor mounted on the heater air plenum chamber to control airflow to the rear seat A/C register (19893).
  
  

either a fuse or a module

Your selector is controlled either my linkage or pressure tubes. A common problem is for a crack to develop in the hoses and leak the pressure needed to switch to the desired setting. Inspect the hoses and or linkage. that will solve it

It sounds like vacuum is not getting to the vent, heat and A/C controls. Find the vacuum can. It may be under the hood on one of the inner fenders. Trace the lines back to the vacuum block at the intake manifold or fire-wall, and also back to where the line goes thru the fire-wall. Check the can for leaks or obvious holes. They used to be just a plain metal can, but it could be a plastic ball. Check the lines as you go for breaks and rub-thru's. Move the controls thru all settings and listen for vent door movement. (Run the engine momentarily to build vacuum in the system.) Worst case is a bad vacuum motor or air flow position selector switch.

You may have an inoperative switch

Just a suspicion: hissing noise comes from failed/jammed/malfunctioning motor to move the door
in the ducting to route heated air to the hoses/ports for "heater" and "defrost."
Problem: failure of motor/linkage to heater/defrost selector door under the dash on the passenger side.
Also sounds like you have not blown fuse to this selector door.
Also - hissing noise could be air being forced against the closed door (which should be open) - in this
case, you'd want to check the fuse to the heater/defroster actuator door.
The rear heater works since its function is not related to the operation of the front heater/defroster.

U have either lost the vacuum to the mode selector switch (in the dash control) or the vacuum mode selector switch is leaking in the ac/heater control in the dash, u can usually hear a hiss if it is leaking. the vacuum source is the engine, a small hose runs through the firewall on the passenger side to the switch from the eng vacuum source.

Sounds like you have a vacuum leak, those controls are are accuated by vacuum.

there could be an air lock in the system, pluged heator core, a bad thermostate or control modual. try bleeding the air out through the bleeder screew if there is no heat after that have the control modual tested if its okay try having the cooling system flushed then if it still doesnt heat intall new termostate