Asked in Cadillac DTS
What effect does the outside temperature sensor have on controlling the air conditioning?
As long as the a/c compressor is on (the amber light with snowflake symbol), air is cooled as it moves through the system. The outside air temperature together with the inside car temperature and the input from the sunlight load detector (in center of dash under windshield) determines how much, if any, "reheating" is done. The wider the variance between the set temperature and the actual in-car temperature, the higher/lower the fan speed with be when set on automatic, and the more or less hot coolant flows through the heater core to "reheat." Reheating is necessary because if the system simply disengaged the a/c compressor to maintain the temperature, the humidity would not be removed, and you would go from comfortable to too cold, then clammy/sticky, back to too cold. The A/c compressor does cycle off and on to maintain the condensor above the freezing point, so it won't run much if the outside temp is 35 or lower. Below about 30 it probably won't run at all. As long as your system is working properly it's best to keep the system on automatic (mode and fan speed) with the a/c compressor engaged. The fuel saved by avoiding use the a/c compressor is very minimal. The exception would be if you were in a very very dry climate with low humidity you could use the system without a/c compressor engaged unless you actually needed to cool the car. Although the new systems are digital, Cadillac actually invented this system and has used it in all air conditioned cars since 1964. It worked basically the same way, but the control was analog and used vacuum and electromechanical means to accomplish the same thing.