Ok. So this is what I have so far. Mind you I had a thing or two backwards.:
2013 SL. Testing done between -20°F and 6°F. The following conditions, so far, need to be met:
- The ambient temperature needs to be very cold. I'm not sure how cold. I do know that if it's cold enough for the heat pump to not come on it's cold enough for this to work.
- The actual cabin temperature can't be too much colder than the climate control "thermostat" temperature. This means the cabin must be preheated somehow, or the heater left on for quite a while. Also the climate control "thermostat" can't be set too high.
- Use the Auto button. If the previous conditions are met, then when you use the Auto button you will see it automatically adjusts the vents to defrost and feet, as well as adjusts the fan speed automatically.
- Turn on recirculation or partial recirculation. After you get the defrost and feet option from using the Auto feature you can now turn on recirculation and still have feet and defrost. You can also adjust your fan speed and "thermostat" to whatever you want because it will be out of Auto mode once you touch the recirculation button. I do believe you have to also turn full recirculation one once to switch from Auto to manual mode before pressing it for a couple seconds to get partial recirculation.
What seems to be happening is this. Normally when Auto is used it selects the vents to be used depending on the cabin temperature, namely; dash vents when A/C is on, bi-level (feet and dash vents) when it needs to slightly warm the cabin air, and feet vents when the cabin is much cooler than the desired temperature.
But when the ambient outside temperature is extremely cold it does this; defrost and feet vents when it needs to slightly warm the cabin air, and feet only vents when the cabin is much cooler than the desired temperature.
Basically, at very cold ambient temperatures the Auto function replaces defrost where it would normally use dash vents along with the feet vents. And for some reason when it does that it also allows you to chose recirculating air.
2013 SL, 40,000 miles, 12 bars.