The technical run down on this problem begins and ends with a Low Charge of Refrigerant. Now as strange as it may seem given all that Cold Fog Blowing out of the Air Vents... Here is what is actually happening ...and WHY:
When the System Charge of R-134A drops below a level in which a sufficient amount of Refrigerant Vapour is available as a Super-Heated gas to be compressed by the A/C Compressor... Low Vapour Charge is then propelled through the Condensing Coils which causes the Vapour to change state from a Gas into a Liquid and give up all of its collected heat but suffers from what is called a Premature Vapour Expansion "FLASH-OVER" because an insufficient mass of Liquid Refrigerant is present to completely fill up the Liquid Line.
Normally... the Liquid Refrigerant follows the Liquid Line into either a Fixed Orifice Tube or a TXV (Thermostatic Expansion Valve that is normally NOT stuck OPEN) and thence... into a series of Small Expansion Tubes, This action should happen GRADUALLY enough to allow the Liquid to expand into a Gas incrementally... in way that normally drops the temperature of the Evaporator Coils down to around 40-45 Degrees Fahrenheit. However... in the case of a Low Refrigerant Charge "FLASH-OVER"... the expansion happens so suddenly and prematurely... that it will Drop the Temperature of the Evaporator Coils down below the Freezing Point of Water at 32 Degrees Fahrenheit. It is the normal condition of the Evaporator Coils to be cold...cold enough to allow Moisture in the Air Column to condense and fall down into the Condensate Drain as Liquid Water. But if that Moisture FREEZES... ICING The Evaporator is inevitable.
When this happens... all of that Warm, Moisture Laden Cabin Air being blown over the Evaporator Coils will very quickly Freeze any Moisture being carried in that column of air passing through and portions of the Evaporator Coils will Freeze SOLID and become blocked with whatever moisture there is to be present inside the closed cabin of the vehicle.
If you have ever opened your Freezer Door and seen a similar "Cloud of Cold Fog" forming... or perhaps had a Fresh, Ice Cold Soda on a Hot Summer's Day... and allowed your breath to pass over the Ice Cubes settled in the top of the Glass and seen the same Water Vapour in your exhaled air condense over the Cubes and form a Small Cloud.... the exact same thing is what is happening under your dashboard. So If you left the Car-SUV Door standing open long enough while watching this "Cloud" of moist air blowing out of the AC Vents... eventually it will STOP as soon as enough moisture has passed over the Evaporator Coils to Freeze it up into a Solid Block of Ice. The result will be that the interior of the cabin will become uncomfortably warm until the Ice collected on the Evaporator melts and the whole cycle repeats itself:
THIS is what the condition looks like...and while this Evaporator Coil is inside of a Residential Air Handler Evaporator Section... (The Home Interior Temps were in the 90's !) THIS is the common outcome of having a Low Refrigerant Charge in the System ...and all of the aforesaid Thermodynamics still apply:
Back in the Day...when Detroit made cars with AC Systems that provided Mechanics with an Easy to Diagnose Tool to determine if the refrigerant was LOW and WET due to moisture being present inside of the system... A "Sight Glass" was usually positioned along the edge of the Fender Well mated to the Liquid Line as it exited the Condenser Coils. This "Sight Glass" was a sturdy, leak-proof High Pressure Resistant Mineral Quartz Lens that allowed for seeing any "Bubbles in the Stream" and some had a centred coloured Dot that would show Yellow if any Moisture was present in the Liquid Refrigerant Stream... or remain a Pale Green Dot if the system was de-hydrated with no invasion of damp, residual air in the lines.
The "cool" part (pun intended here... ) was that if you could see a mass of Tiny Gas Bubbles flowing towards the Orifice Tube or TXV... then you would know to add Freon (Back then it was the now BANNED CFC --> R-12) and gradually add small increments until the system cycled "Bubble Free". Here are some images of the ones used in Commercial and Residential Split AC Systems and those with Heat Pumps, as well. This device is usually connected just Downstream of the Liquid Line Filter exiting the Compressor Pump Unit and as close to the King Service Valve as possible... while either a single or bi-directional Filter-Drier (for HPs) is installed in the Attic Air Handler... as close as possible upstream of the Orifice Tube or TVX at the Evaporator Coil to ensure only cleaned and desiccated Refrigerant is allowed to enter the Evaporator Coils:
If I pulled a vacuum on a system with this device installed after filling up the system with Dry Nitrogen for about an hour or so... the Moisture Indicator "DOT" would change colour from Yellow to Pale Green... and be Good To Go.
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