ac light on most times

With some cars (and it seems random to me), the AC compressor comes on any time you use the defroster. Although there are times when it's not necessary, there are idiots driving around with all windows completely fogged up. This feature helps those people. It helps smart people, too.

Since you mention *some cars* may I add that the Prius has two primary defroster settings. When the control knob is set to one of them the AC compressor isn't used. When it's set to the other the AC will run (I believe) at half utility. Sometimes this isn't enough to dehumidify or it's difficult to determine what the temperature setting should be for all the windows. When this happens I press the AC for full coverage and skip the defroster settings altogether. I'm sure their engineers took saturation point into consideration so I'm not too worried about running the AC at full throttle.

mark_

Sounds like an interesting system, and it makes sense, considering that we can often see the tach change when the compressor kicks in.

On a Prius (at least the 2nd generation ones) the AC compressor runs of electricity and not off of the gasoline motor. Tomes

This is not a flaw it is a feature. If you have it on, the defrost setting the AC does turn on automatically to dry out the air. This is why the light turns on whether the button is pressed in or not - the AC unit is indeed turning on. I do not know of any recent vehicle that has AC and does not do this. Tomes

When the HVAC system is set to defrost or bi-level, the AC light on the AC button will illuminate. If the ambient air temperature is above freezing, the AC compressor will come on to dehumidify the air blowing on the windshield.

Is this true on my 05 Corolla? On these OT posts. Is it school vacation and mommies letting their special children use the computer?

Yes.

On these OT posts. Is it school

I have no idea.