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Air blowing out dash vents even when defrost or heat is selected?

Discussion in 'Heat & AC' started by mcarlo86, Jan 14, 2020.

  1. mcarlo86

    mcarlo86 Supporting Member Supporting Member

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    Has anyone had a situation where the air is always blowing out the dash vents no matter what position the heater control is on? I can put it on defrost and some air comes out the defrost vents, but most comes out the dash vents. Then if I put it on heat, some air will come out the floor and most comes out the dash vents. I'm assuming there is a door that must be stuck open behind the dash somewhere?
     
  2. thecantaloupeman

    thecantaloupeman Full Access Member

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    My truck does this too. IIRC it was a feature that allowed outside air to circulate in the cabin when the blower isn’t on. It’s always on at a very low speed.
     
  3. MisterB

    MisterB Full Access Member

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    The fan blower motor is always on when the key is in the on position. You can test it by turning the control all the way off, and then turn the key to on, and you'll hear the blower motor still spinning slowly.

    It's likely a feature to help circulate air in the cab so that exhaust fumes aren't as much of a problem

    Sent from my SM-N975U1 using Tapatalk
     
  4. bucket

    bucket Super Moderator Staff Member

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    Yes. Ever since I bought my '88, it has done this. Every mode works well, but air flow always comes out the vents too. I never have been able to figure out what was wrong. More recently, my '78 began doing this, but the other modes hardly had any airflow. In this case, the vacuum line to the reservoir bottle had deteriorated.
     
  5. chengny

    chengny Full Access Member

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    Check the condition of this part. It's called the "diverter Valve link connector". It breaks frequently- and when it does, the link is unable to move the diverter valve into the fully closed position.

    HPIM1580.jpg

    Result is undesired airflow up to the dash vents.
     
  6. Goldie Driver

    Goldie Driver Full Access Member

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    Is it normal to get some airflow out of the dash vents when set on Heat ?

    Blows hard out of the floor ducts, but a little comes out the vents, too.

    Not gonna change it- keeps my hands warm:), but I am curious.
     
  7. bucket

    bucket Super Moderator Staff Member

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    Normal? One might say yes, lol. But I don't think it's what GM intended. Most of my trucks have not done it.
     
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  8. Goldie Driver

    Goldie Driver Full Access Member

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    The 4Runner does that, too, but only out of the outside vents. That is by design, though, not age related vacuum actuator weakness. :p
     
  9. Reinovator

    Reinovator Member

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    I have had three trucks do this the white plastic piece was broken on on Two had to make one for them But they may be available aftermarket now and you can use home door insulation to help close it off for more air flow this worked on one of mine, if you wish.
     
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  10. Charlie7985

    Charlie7985 Junior Member

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    I had this same problem and it was from the actuator that controlled that door having a slight leak that kept it from closing the door all the way. You can test it by pulling the rubber piece where all of the vacuum lines connect to the back of the dash controls off. Then plug a hand held vacuum pump into each line one at a time, pump them up a little and see if they hold. You will have to pump it a few times to fully deflate the actuator before it will start to hold.
     
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