Does A/C automatically run in defrost mode?

Discussion in 'Gen 3 Prius Main Forum' started by mainerinexile, Apr 10, 2010.

  1. mainerinexile

    mainerinexile No longer in exile!

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    I've not found an answer to this question on this forum or in the owners manual. In my previous Toyotas (two Tacomas, Matrix, Corolla), the A/C came on automatically in defrost mode, even though the A/C light didn't light up. It meant that my wife, who insists on using only defrost mode all winter, lost gas mileage to the A/C all winter. I wish Toyota didn't make these kinds of decisions for us, or if they insist, at least have the A/C light come on so that we know about it.

    Does the Prius III A/C come on automatically in defrost mode?
     
  2. chrisj428

    chrisj428 Active Member

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    The a/c will come on during defrost, providing the temperature is over approximately 40°F. This is done out of safety considerations in order to provide the maximum dehumidification possible so that the winshield may clear as quickly as possible.
     
  3. qbee42

    qbee42 My other car is a boat

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    If the A/C compressor is enabled, it will run as needed in any mode. The climate control system in the Prius is very sophisticated, so A/C operation is not a simple yes/no decision. The compressor will run only as needed, and only at the speed needed. It is electrically driven and variable speed. Temperature and humidity are used to decide when and how much it should run.

    I leave mine on auto and let the system decide when it should run, other than open window driving.

    Tom
     
  4. chrisj428

    chrisj428 Active Member

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    Tom,

    Is a humidity sensor part of the Prius' climate control system?
     
  5. alokeprasad

    alokeprasad Member

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    So, if the AC button us set to Off, the AC compressor will NEVER come on? regardless of defrost, auto climate control, or whatever?

    On the other hand, if the AC button is set to On, the compressor will be used as needed?

    Are you sure this is the same with Gen 3 vs earlier Priusii?
     
  6. qbee42

    qbee42 My other car is a boat

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    Yes, it is standard as far as I know.

    Correct. Think of the A/C button as an "enable" switch. When it is on, the A/C compressor *can* run, but will run only as much as needed. When off, the A/C compress will not run.

    I can't say for certain that the Gen III also works this way, but I can't imagine Toyota going backward on an important feature such as this. Any Gen III owners want to chime in?

    Tom
     
  7. Tideland Prius

    Tideland Prius Moderator of the North
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    Yes it will come on automatically. The A/C will help clear the windshield faster as it pumps out dry air.

    alokeprasad: Yes except it'll still be on in Defrost mode. Same as earlier Priuses
     
  8. bedrock8x

    bedrock8x Senior Member

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    The A/C compressor will turn on if the front defrost is enabled even if the A/C button is not pressed. I found this out one night after using the defrost on a cold night. After parking the car on the driveway for a while and found a puddle of water underneath the car. I thought something was leaking and checked over the engine compartment and found no leak. The second night found the same. Then the third night and decided not to use the defrost button and just turn up the heat, the no water puddle.

    The conclusion is the automatic climate control will use the A/C compressor when in defrost mode.
     
  9. jim256

    jim256 Member

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    I did find it in the manual--page 274 heading "Defogging the Windshield"
    >>>
    "The air conditioning system control operates automatically. Recirculated air mode will automatically switch to outside air mode."
    [FONT=Arial,Arial][FONT=Arial,Arial]
    [/FONT][/FONT]
     
  10. mainerinexile

    mainerinexile No longer in exile!

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    Thanks for all the quick comments. The lesson is, as I expected based on past experience with Toyotas, that in addition to the good advice to minimize use of AC to get better gas mileage, is to minimze use of defrost because it is causing the AC to run even in the winter.

    I'd like to add to the wish list for future Prius models (indeed all Toyotas) that Toyota would have the AC button light up whenever the AC is running, so that the driver is aware that s(he) is using AC when they don't realize it.
     
  11. paprius4030

    paprius4030 My first Prius

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    I understand where your coming from but consider this. Especially in light of all the recent braking and acceleration issues. If Toyota designed the defrost the way you would like it, thousands of OTHER people would be complaining and suing Toyota that their windshield fogged over and the defrost doesn't work. And god forbid they hit something because they kept driving after they couldn't see anything. So I can see why Toyota and other car makers make the A/C compressor run always when it's in defrost and above a certain temp.
     
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  12. mainerinexile

    mainerinexile No longer in exile!

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    Yeah, I understand your point, but all Totota has to do is to have the AC light come on whenever the AC is running, for whatever reason. Full automotive disclosure, to to speak.
     
  13. HTMLSpinnr

    HTMLSpinnr Super Moderator
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    I've found that there is no user override for A/C, and you also cannot override fresh/recirc while in defrost mode (not so good when following a truck w/ hot brakes or a car burning lots of oil).
     
  14. bedrock8x

    bedrock8x Senior Member

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    I also confirmed the A/C will run if the air control is set to outlet to windshield and kick out of the ECO mode in the TCH.

     
  15. Eoin

    Eoin Active Member

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    My 2005 Prius has no separate A/C button. You just leave it on automatic and set the temperature. I just got my new Prius and wonder why it has a separate A/C button? Is this so you can turn it off? I just want to set the temperature, as on the older Prius and leave it at that. So should I leave the A/C button on all the time? Thanks.
     
  16. Tideland Prius

    Tideland Prius Moderator of the North
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    Umm... the old Prius had a separate A/C button... it was in the CLIMATE screen.

    Some may want automatic temperature control without A/C. (early spring or late fall)
     
  17. Radarguy

    Radarguy New Member

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    If using the remote start both the front, and rear defrost will come on when its cold outside.
     
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