On my 1994 Camry the AC was selectable when using Defrost -- I liked it like that. My 1997 Camry and 2000 Sienna turns on the AC automatically when selecting Defrost -- I don't like that.
The air in my 03 Matrix runs in both of the last two settings... partial and full defrost I agree with the OP that it would be nice to have the option. My wife leaves it in defrost all the time which wastes gas unnecessarily.
No, that isn't the reason. The purpose of the a/c coming on when you want to defog (it isn't necessarily "frost"), is that the air blowing across the windshield interior must be dryer than the ambient air or the defogging won't work. The a/c remove moisture from warm, moist air.
When the temperature selector is in the hot position, the fact that the air coming out the vents is warmer than ambient reduces the relative humidity and it is not necessary for the a/c compressor to come on.
I don't know about the Sienna, but I would be surprised if it is designed to turn on the a/c when you are using the heater to defog the windows.
Wether it blows hot or cold in the defog (not de- "frost") position should depend on whether the temperature selector is in the cold or hot position. In the hot position (as you would use in the wintertime), I don't believe the a/c comes on with defog.
Sometimes the lights in your house are on needlessly because you left them on when you went out. Don't blame that on the design of the lighting system.
Why in the world would you select defog to get fresh air? Just leave the a/c off, select the dashboard vents, and turn on the fan.
You can have what you buy and pay for. The automobile manufacturer can not customize his product for each individual. Well, he could, but you couldn't afford to pay for it.
The difference is not that great. If you don't want a/c, don't buy it, or if the car already has it, don't turn it on. You are wrong to suggest that the a/c always comes on when you just want fresh air to blow from the HVAC system. An HVAC system has three modes: heating, ventilation, and air conditioning. Simply select the ventilation mode.
You really don't need to look under the hood to know that the air conditioning is on :) I tried it again yesterday in my 2000 Echo, ice cold in defrost position, luke warm in any other position, and the light does not come on unless you push the button. Same as my 2000 Sienna.
I do that as well. I select the the dash vents in that case, and direct the air out of my face.
I happen to agree with your view that the A/C should work independently of the output of the ventilation system. But, I'd like to put the lawyer's spin on this, if I may. A driver is an idiot, they're ALL idiots - except, of course, you and I -- and if they think they need the Defrost setting, then they will expect clear glass. If we simply blow air up on the glass, there is no assurance that the fog will be cleared, and the idiots can come back and sue us because they selected the proper outlet but the glass remained foggy. If we -- the car maker -- turn the A/C on when the Defroster is selected, then we can be assured that the fog will be cleared from the glass, and since the driver is an idiot, we don't care that he might want to have fresh air come out of the defroster vents so that it isn't blowing on his face. He is an idiot, so he can select the dash vents, then turn them away from his face. At least he can't sue us because the glass is fogged when the defroster was selected.
This makes perfect sense, but the people that are not idiots -- you and I -- can't control the life support systems as we would prefer. Thank the lawyers, not the engineers.
It could be worse. I have a friend with a Ford Escort, and the A/C works in any position of the selector, except the dash vent and the floor vent. If the dash/floor mix is selected, the A/C comes on. If the selector is anywhere except OFF, Dash, or Floor, the A/C comes on even if the fan is off.
While that is generally true, the A/C does come on when the Defrost is selected. The driver can not have Defrost and not have A/C, unless the car hasn't got A/C - which is very rare these days.
Sure it does Merritt...it won't blow cold but it will be 'conditioned' (dried)...air conditioning means only that it's dry, it won't necessarily be 'cold'.
I have discovered that in my 2004 Sienna van that when I put on the defrost that the air conditioning turns on.
When I got my first car with AC, '87 Corolla, someone told me to turn on the AC, temp to hot, and defrost...to de-fog everything. It works. Apparently they've now made it a feature, nothing wrong with that. If you need to defrost, and the AC comes on automatically, it will take only a few seconds.
I don't know if US cars now have separate heat and AC controls, but my sister has a....US-made something, the heat and AC are on the same control switch. I'm obviously no techno-expert, but I'm still doin' this in my '03 Corolla, and up here in New England, the car can fog up when you take a breath. I'm not driving down the road in my dad's car, wiping down the windshield with my sleeve.
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