Does the A/C compressor run/engage under certain conditions (like defrost) even if A/C has not been on for a week?

We got a puddle of about a 1/2 liter of (confirmed) clear water under the car tonight, and it continues to drip quickly from the tube outlet (passenger foot area under car), and this is without using A/C for the past week (it is cool fall here now). The dripping also is directly related to whether the climate fan is on or off.

Is it normal for the front rad fan was cycling on/off continuously immediately upon cold start? It seems to be directly related to whether the climate controls were set to both defrost or partial defrost... if climate controls are set to "face" or "feet" the front rad fan shuts off.

Just want to confirm that this behavior is normal, and that the coolant being low is unrelated. Thanks guys!

1 Answer 1


It is normal behavior in most vehicles for the A/C compressor to kick on when the controls are set to defrost or partial-defrost. It works this way to throw dry (dehumidified) air at the windshield to get it to clear faster. If the weather outside were below freezing and you pumped humid air at it (even your the humidity in your breath would do it) onto the windshield, it would freeze there. You would start to have a layer of ice on the inside of your windshield, which would be a total bummer to get off. By pumping dehumidified air on to it, it clears any fog from it, making it easier to see and doing it faster than waiting for the vehicle to heat up. The radiator fan is set to engage when the A/C pump has kicked on, so that is normal as well.

Since the A/C is running, that means you'd have condensation coming from the A/C drain tube. Which means it would not have anything to do with low coolant, especially considering the liquid you found is clear water. If it were coolant leaking into the A/C box, you'd get that sickly sweet smell of the anti-freeze hitting you in the face like a sledge hammer. I don't know why you'd have 1/2 liter of condensation draining into a puddle, unless you left it set in one place while running for an extended period of time and/or it is very humid where you are at. Water coming out is very much normal, though.

To me it doesn't sound as if you have any issues. Refill the coolant with your anti-freeze mix and keep an eye on it. I've had vehicles where some will disappear for no apparent reason with absolutely nothing being wrong with the car. Doesn't happen often, but it does happen.

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