Why the AC light comes on when calling for heat?

Discussion in 'Gen 4 Prius Main Forum' started by RoadNoise, Feb 4, 2019.

  1. RoadNoise

    RoadNoise Active Member

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    It always bugs me when people driving ICE cars leave the A/C compressor running in the middle of winter...on a dry day. An unnecessary drag on efficiency (or are newer ICE cars now smarter than that)?

    Always On
    seems to be the default A/C setting with Prius. Unless dehumidification is needed along with the heat, I generally turn it off. Does this help with Prius efficiency or am I fooling myself? Trying to outsmart a Prius may not be advised.
     
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  2. Salamander_King

    Salamander_King Senior Member

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    PRIME use a heat pump for heating which is a type of AC, but for regular Prius not so. In any case, Toyota calls a climate system AC, either for cooling or heating don't they?
     
  3. bisco

    bisco cookie crumbler

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    Are you in auto or manual? Mine doesn’t come on in manual, but maybe they changed gen 4
     
  4. Tideland Prius

    Tideland Prius Moderator of the North
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    If you’re in AUTO, then it doesn’t matter when the light is on. It will automatically cycle the compressor on/off as needed. I’ve had it on in -30°C temps. It’s not gonna run at those temps anyway even if the light is on.

    However, if you’re in manual mode, that’s different; the A/C compressor will run.
     
  5. RoadNoise

    RoadNoise Active Member

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    Sorry meant to include "auto" in original post. Interesting. Always thought the A/C light meant the compressor is running but must remember this is a Prius!
     
  6. alanclarkeau

    alanclarkeau Senior Member

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    If humidity is high, it'll run the A/C to dry the air - which is often the case here - very useful for stopping the windows fogging. My last 3 cars turned the A/C on if I put the heater on. You can turn it off if you like and put it on if the windows start fogging up.

    There are a lot of sensors doing different things to make you comfortable - this may not be all of them.:
    upload_2019-2-5_14-9-10.png
     
  7. FuelMiser

    FuelMiser Active Member

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    So you've discovered that pushing AUTO will always turn on the A/C light. Then you have the option to turn it off if you like and the rest of the AUTO functions--fan speed and vents--still operate normally to maintain temperature. As far as the A/C compressor degrading your MPG, in the Prius, it does not run off the ICE. It is an electric motor, so it is using traction battery for power. You're not using gas to power the A/C directly, as in traditional cars. Even so, personally I click it off when I do not think it is necessary for comfort.
     
  8. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    I've seen exactly the same behavior in pretty much every other car. A/C light just means that the compressor is "armed" and will run if the thermostat demands it.

    I have to think back to the 1980s to recall a car that used a dash light to explicitly indicate when the compressor was actually running, and back then people complained that it wouldn't stay on in hot weather. It didn't stay on because enough pressure had been stored to sustain the evaporator flow, so the compressor paused until the pressure switch tripped again.
     
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  9. sam spade 2

    sam spade 2 Senior Member

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    It doesn't mean that in ANY car.
    It only means that it is enabled and available if needed.
     
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