83 Mustang Gt HVAC Control Issues

[83-Steaman]

I have a 83 GT with factory a/c. I decided to start it up today. I went to adjust the heater controls to defrost, it wouldn't move past "High/Low" on the HVAC control. It feels like something is jammed. As well, the heater fan quit working. If I wiggle it to MAX on A/C, the fan begins to work on all speeds. I'm not sure where to start. I know the whole defrost, heat, vent... works on vacuum so I don't think a cable is jammed. As far as the fan not working, the resistor must be good if all speeds work on MAX A/C.
It all worked when I parked in in my garage last fall. The only work I'm currently doing is replacing both power window motors.
Any help would be appreciated
83-Steaman

[tbirdman]

Only the temperature selector is controlled by a cable. They are known to bind, but can be adjusted. The function selector in the middle has a vacuum harness on the back, no cable. There is a vacuum reservoir behind the passenger side front fender, and the vacuum line runs from the hvac box to the reservoir along the passenger side fender apron. If you lose all hvac functions except for defrost (which is the default setting), there is a loss in vacuum somewhere between the hvac box and the reservoir. If you have operation in functions besides defrost, it most likely isn't that.

When you turn on the fan in max AC, does it work on all speed settings? I believe the fan speed selector is harnessed separately from the other parts of the hvac controls. If you are having trouble with the fan coming on, it's most likely either the switch at the control unit, or the blower motor resistor.

[KevinK]

When the selector jams or gets stuck, it’s usually the worst case problem. The vacuum switching is done with a sliding block device. The bracket retaining it is plastic. Once that breaks, it’s time for a new hvac control unit. There is nothing to leverage the block to slide it and it lost contact with the mating surface to seal the vacuum.

If someone developed a metal replacement bracket, I believe they could be repaired.

[83-Steaman]

About the fan, I do have all the speeds when I turn it to MAX A/C. but the fan doesn't work in any other position. I should be able to hook up a hand-held vacuum pump to see if I can hold vacuum on the system. Could there be a bad ground for the fan not to work?

83-Steaman

[KevinK]

Check the blower resistors.

[tbirdman]

I would probably change the blower motor resistor first. Easiest and cheapest fix to try and see if you can get the fan working on all speed settings. The fan wouldn't work at all (I think) if there was a bad ground somewhere. Since the fan does work (albeit only slightly), it sounds like there is power between the switch and the resistor. The resistors are known to go bad often. I have a 2002 F-150 and have changed the blower motor resistor three times. I've changed the one in the Mustang as well.