Climate control, variable speed

[Jred]

I have a 1970 Avanti II that came with AC but was removed by a previous owner at some point. 
The knob that controls how fast the air blows, it will adjust from low to high but it won’t shut off completely. If the car is on and the knob is adjusted to what I assume is the off position(all the way low), there’s a slight breeze. I can also hear the motor rotating when the engine is off but power is on. 
would it be safe to bet the issue lies under the vertical dash area where the knob is?

[Jim78]

The rotary fan control has an "OFF" position when it is fully counterclockwise.  It is possible that the switch is broken.  Another possibility is that the PO mixed up the wiring at the back of the fan and temperature controls.  That's easy enough to check by removing the four screws that hold the vertical panel and tipping it forward.  You may need to loosen the screws on the horizontal panel a little to get the bottom tab of the vertical panel loose.   A picture of the correct wiring is attached.

But, before you get into removing the panel...  This may be a dumb question, but are you sure that it is the A/C fan that you are hearing?  There is another blower motor in the heater that is controlled by the 2-speed switch in the overhead console.  You might want to check that first.

 

[Jred]

Thanks @Jim78, very helpful 

Yeah I did make sure that I was hearing the motor while the overhead heater switches were in the off position. Wanted to make sure of that before posting, ha.

[Desert Driver]

That is one nasty rheostat on my '71. Let that sucker cool down after you shut it off to remove it. That thing will burn your fingers. Ask the man who owns one!

[Jred]

14 hours ago, Desert Driver said:

That is one nasty rheostat on my '71. Let that sucker cool down after you shut it off to remove it. That thing will burn your fingers. Ask the man who owns one!

Read this too late. Unscrewed the the cover  when I got home and touched it right away. Hotter than hot. No wonder warm air was constantly blowing out of this area. 

[Jim78]

7 hours ago, Jred said:

Read this too late. Unscrewed the the cover  when I got home and touched it right away. Hotter than hot. No wonder warm air was constantly blowing out of this area. 

By design, these controls reduce voltage to the motor to control the fan speed.  The quick and dirty way to do this is by adding variable resistance to the circuit.  The down side to this method is that the variable resistor turns the excess power into heat.  That is why these controls are typically located in the air stream.  Mine had the additional problem of corrosion that caused poor contact between the wiper and the resistance coil that caused erratic speed control and even more heat.  Replacement switches are available, but a bit expensive.  I was able to carefully disassemble mine and clean and adjust it.  Works as good as new.

[Jred]

Looks like my problem was the knob wasn’t able to depress for one reason or another and wouldn’t fully turn into the off position. 
It began to function properly after I removed it. Wondering if adjusting the nut that holds the piece in had anything to do with it. 
@Jim78 what did you you use to clean the corrosion up? 

[Jim78]

22 hours ago, Jred said:

Looks like my problem was the knob wasn’t able to depress for one reason or another and wouldn’t fully turn into the off position. 
It began to function properly after I removed it. Wondering if adjusting the nut that holds the piece in had anything to do with it. 
@Jim78 what did you you use to clean the corrosion up? 

I used an electrical contact cleaner (aerosol can) and a fine wire brass brush.  If you disassemble the switch, know that the wiper is spring loaded, so open it carefully and note the order that the parts (wiper, washer, spring) go in.