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How is it doing it?

With the rising price of gas I have been trying to make sure my driving habits (ie tire inflation, air filter)are not lowering my efficiency. When a small mystery arose. I own a 2004 Nissan Sentra. while getting ready to drive home from work I turned the fan on to close my windows (for highway driving). I realized several minutes later that it was on defrost. When I changed the setting to panel I felt cold air coming out. The AC controll button is not depressed, the temperature setting is set to cold, not recirculating and it is a hot (90 deg) day out. The air stays cool for the twenty minutes it takes to get home and when I pull into the driveway there is a small trail of condensation drops. Is the Ac being used? Am I actually saving money?

When you turn on your defroster, your AC compressor comes on to help dry the air. Yes, it will cause condensation on the windshield if you don’t also have the heater’s temperature turned up.

You only save money when you have the system on VENT with fresh outside air being brought into the cabin.

When the defrost setting is selected, the A/C compressor automatically is turned on. Thus, when you pushed the defrost button, you activated the A/C. If you don’t believe me, read the section of your Owner’s Manual for the HVAC system.

As to why the air would stay cold for 20 minutes after switching to the vent function without the A/C button being depressed, I cannot say, but at least I answered half of your question.

As to saving money, there are a number of studies indicating that opening the car’s windows at highway speed will increase the wind resistance sufficiently to add enough “drag” to reduce your gas mileage more than the use of the A/C would do. At highway speed, use your A/C on a hot day, rather than opening the windows.

Why have A/C and not use it? The cost in MPG is going to be negligible. I don’t notice any difference in MPG running my AC and I keep track with every tank full.

No pain, no gain?!