A/C thermostat dial is a switch, no difference in cooling output between barely on and cranked.

I hope you checked the screen in the expansion valve. It is known to sometimes clog.

did you add a hi/low pressure cut out switch to the refrigerant drier? They were common on most AC systems but not on our cars. Easy to install and simple to wire  The high pressure would happen on an icing condition that blocks refrigerant flow, and the low pressure would happen in a refrigerant leak. Both triggerpoints will open the power to the compressor preventing damage

The expansion valve coil that clips to the evaporator line is in charge of temperature control by regulating the flow of refrigerant.

The switch commonly referred to as the temperature switch is technically a thermostatic switch. It has a capillary tube that is inserted into the evaporator coil.

It is designed to open the compressor power circuit if it senses a low enough temperature that might cause atmospheric humidity to freeze on the coil. By disabling the compressor the system temperature rises and the potential icing condition is avoided  

less than full clockwise rotation tells the switch to open the compressor power circuit at a higher temperature than an icing temperature  

there is likely nothing wrong and no troubleshooting required

Larry

Thanks Larry, that's super helpful.

No, I didn't install a pressure switch. I guess I should have. My A/C shop guy is great but he gets this "oh no! not the Pantera again" look whenever I come by.

I guess I'll terrorize him one more time.

"Got to 32 degrees but I noticed no difference between the dial being barely on, just past the detent, to all the way clockwise."

John,

Is the compressor cycling at all?  I wonder if the tube coming out of  the knob/switch is correctly seated in the evaporator.  If it's not then the compressor will run continuously and the knob position will have no effect.

Hi Steve. Good questions! I can't tell if the compressor is cycling. Unlike the old York, I can't hear the Sanden start and stop with the engine running and I've replaced the engine cover and back panel. I'll stick my endoscope back there and see if I can watch the clutch.

The tube from the thermostat is going through a hole with a grommet into the evaporator but I can't tell where or how it contacts the evaporator.

There seem to be two lengths used for that bourdon tube that inserts in the A/C box- a short and a longer one. And a few are 'coiled' which means the necessary hole will have to be bigger. Fresh duct tape along the edges of the box might increase efficiency a little. Some condensate drain hoses are kinked where they go from the bottom of the box thru the front firewall. I routed mine straight down thru the inside of the console and thru the floor. Then I used the now-unused drain hole for routing the wiring for a dropped battery.