AC light?

Your AC was on. It was dehumidifying probably due to high humidity in the car. Defrost will do the same. You have the option to turn off auto or turn off the AC.

 

When my Toyota dealer had an open house several months ago, this was the first question I asked: why does the AC light come on when the heat is supposed to be on?

The technician's answer: all the light is showing is that the AC is ready to come on if the car should need it. It's sort of like an "armed" light. It's not actually ON blowing cold air.

 

As I understand it, the light indicates that the A/C is available as required by the climate control system. It does not indicate that the compressor is running. Whenever I push auto on my V three, the A/C light comes on. According to the manual, this is a setting that can be changed by the dealer so that the default after pushing auto would be A/C is not available to the climate control system and the A/C light is off.

 

Yes, thanks for the clarification.
I too was wondering this.

 

Interesting - I guess this makes sense. But I've also read in other car manuals (including the v's, if I recall correctly) that the AC often comes on solely to dehumidify the air. In fact, whenever you turn on the front defrost in most cars, the AC turns on also. In other words, AC doesn't ONLY exist to make air colder - it's also good at removing humidity. In fact, a traditional dehumidifier is functionally very similar to an AC unit, except it doesn't exchange outside air. It's still a set of coils that are chilled by a compressor, and as air is pushed across the coils by a fan, water condenses (then usually freezes) on the coils. Then the ice melts, and the water (that used to be in the air) drips into a collection pail. AC units work the same way, except they have a stronger fan, and the air that's blown across the coils is then blown into the room you want to cool. Warm air is either vented to the outside or blown back across the coils, but waste heat from the compressor is definitely dissipated outside, by all the thin aluminum sheets that are too easy to bend. Make sense?
The details don't really matter - my point is the same system works to cool air and remove moisture from air. Modern cars take advantage of this and turn on AC in both circumstances - when air needs to be cooled, and when air needs to be dehumidified (so you windows don't fog up).
It could be that the Prius lights up the "AC" indicator even when the compressor isn't running, but it's definitely true that it runs during the winter in order to dehumidify the air.
Andy

 

Maybe we've become too accustomed to thinking of air conditioning as air cooling, when it's not necessarily the same thing. If the temperature or humidity is being adjusted, the air is still being 'conditioned', even if it isn't being cooled. In that sense, 'AC' is usually on year round.

Plus, it's pretty handy in the Winter to defog the windows.

 

I understand the Prius is also one of the few cars to include a humidity sensor as part of their climate control.

 

My (old) Lexus RX300 also turns on the A/C light whenever Auto is pressed. I can feel the A/C compressor some on every time, no matter the temperature or humidity. I saw the same light come on with my (new) Prius v 5 with ATP and assumed it was doing the same, but it is hard to tell when the compressor is on.

After a few days I figured out that there are a lot of options available only when the car is stopped, I was able to change the auto-A/C behavior. I am not in my car, so in general terms, go to the Climate Control options under Setup and disable turning on A/C when selecting Auto. I will keep mine in this mode for winter and change it back to auto-on when it gets warmer.

 

I looked for this option on my Prius v 5 without the ATP package. It's not there. It has to be set by the dealer, according the the manual. Not sure it matters, as long as the a/c isn't running all the time.

 

But again, it doesn't really matter since the compressor is NOT running in the Prius -- it's only in "ready" mode -- when you're heating the interior. This fact came directly from the head of service at my Toyota dealer.

 

The whole reason it is called AUTO A/C is because it does everything automatically. Including turning on and off the compressor. Every car I have had with auto a/c has been this way. I play it safe and turn it off. That way there is no chance of the compressor running.

 

That's right. It's called "Auto" so that you don't have to do anything manually... like turning it off.