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Thread: air is stuck on defrost

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  1. #1
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    air is stuck on defrost

    i got a 00 sierra, and 1 day my heat decided to stay on defrost. no matter what i turn the knob on, the air still comes out of the defrost vents. i tried changing out the HVAC controls but that didnt do anything. please help. thanks.

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    Re: air is stuck on defrost

    bump, i hope someone has an idea. after doing some searching i read it could be an actuator? but what does it look like and where would it be? thanks.

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    Registered User 1988c1500's Avatar
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    Re: air is stuck on defrost

    To start with I'd go to a parts store website or Rock Auto and look up pictures of actuators. Keep looking and you should find pics of them all. Then look under your dash and see if you can find them. A lot of time the gears in one will strip or jam but you can see it trying to move or hear it clicking.

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    Re: air is stuck on defrost

    i found the actuator, is there just 1. i took it out opened it up, gears look fine, and hooked it up to the battery and watch the motor turn the gears, so i assume its fine??

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    Registered User 1988c1500's Avatar
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    Re: air is stuck on defrost

    On my '93 there was one to switch between vents and defrost, one to vent the cab when you turned the truck off, and I think one other one.

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    Re: air is stuck on defrost

    The blend door itself could be defective


    00 Sierra Z71 ECSB, Keys, Zone 1.5"" BL, 16x8 ProComp 6079s, 315/75/16 BFG A/Ts, MSD 8.5mm Wires , Vette Servo,40 Series, 6 on Hi, GT5 4:10s, 212k mi

    94 Chevrolet K2500 L/D ECSB, 5.7,4L60E/HD2, 9.5 14 Bolt, 205k mi

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    Re: air is stuck on defrost

    i cant find the blend door actuator, surely someone might know! lol

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    Registered User east302's Avatar
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    Re: air is stuck on defrost

    You should have three actuators that control the air doors:

    1. Temperature (hot/cold, diverts air to or away from heater core)
    2. Mode (floor/dash/defrost)
    3. Air inlet valve (Recirculation or fresh air at the blower)

    Here's a diagram for a 2000 with manual controls (versus the electronic ones sometimes used in Tahoes and Suburbans). The view is from the engine looking through the firewall--right is driver side, left is passenger.



    Part 3 on the diagram is the Air inlet valve actuator
    Part 14 = Mode actuator --which I what I think you're looking for, should be to the right of the gas pedal
    Part 19 = Temperature

    There should be a small notched cam or position indicator on the actuator. Watching that indicator, select floor, vent or defrost on the control panel and see if that cam turns.

    Hope this helps.
    Last edited by east302; 02-05-2011 at 11:44 AM.
    1998 Chevrolet Z71, 350 auto
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    When I die, I want to go peacefully in my sleep like my grandpa, not yelling and screaming like the passengers in his car.

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    Re: air is stuck on defrost

    Quote Originally Posted by east302 View Post
    You should have three actuators that control the air doors:

    1. Temperature (hot/cold, diverts air to or away from heater core)
    2. Mode (floor/dash/defrost)
    3. Air inlet valve (Recirculation or fresh air at the blower)

    Here's a diagram for a 2000 with manual controls (versus the electronic ones sometimes used in Tahoes and Suburbans). The view is from the engine looking through the firewall--right is driver side, left is passenger.



    Part 3 on the diagram is the Air inlet valve actuator
    Part 14 = Mode actuator --which I what I think you're looking for, should be to the right of the gas pedal
    Part 19 = Temperature

    There should be a small notched cam or position indicator on the actuator. Watching that indicator, select floor, vent or defrost on the control panel and see if that cam turns.

    Hope this helps.
    The diagram appears to be missing, anyone else have one they can post?

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    Re: air is stuck on defrost

    Quote Originally Posted by mc87ss View Post
    The diagram appears to be missing, anyone else have one they can post?
    What year is your truck. Heater box change in '03 I think. Here's a link:

    Click evaporator for '99 - '02 picture. http://parts.arniebauer.com/p/Chevro...wd=&origin=pla
    [IMG][/IMG]

    1974 Jimmy 350
    1982 Sierra 402 BB (1971)
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    Re: air is stuck on defrost

    which actuator did you pull?
    the one I had to replace was under the dash on the drivers side(under the hvac controls) it was attached to a big plastic gear drive:
    http://www.partsgeek.com/lzcykzn-che...+Door+Actuator

    might find it cheaper on ebay
    Last edited by NEUMANNZZ; 07-10-2016 at 11:38 AM.
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    Re: air is stuck on defrost


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    Re: air is stuck on defrost

    Thanks for the info.
    I will check into it further and let yall know what i find.

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    Re: air is stuck on defrost

    Quote Originally Posted by NEUMANNZZ View Post
    which actuator did you pull?
    the one I had to replace was under the dash on the drivers side(under the hvac controls) it was attached to a big plastic gear drive:
    http://www.partsgeek.com/lzcykzn-che...+Door+Actuator

    might find it cheaper on ebay
    The one i removed was at the floor by the trans hump, it did the hot - cold door.
    The one you supplied the link to appears to be the one i am having issues with, where is it located?
    Can i get to it without removing the dash?

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    Re: air is stuck on defrost

    Quote Originally Posted by mc87ss View Post
    The one i removed was at the floor by the trans hump, it did the hot - cold door.
    The one you supplied the link to appears to be the one i am having issues with, where is it located?
    Can i get to it without removing the dash?


    Item #21 is the actuator for the mode doors. It is located on the driver's side of the heater box. The picture of the heater box in this photo is as you are looking through the firewall at the heater box.
    [IMG][/IMG]

    1974 Jimmy 350
    1982 Sierra 402 BB (1971)
    2002 Sierra 4.3L
    1998 3500HD 7.4L

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    Re: air is stuck on defrost

    remove the driverside floor duct(if equipped) to gain access under the dash. it will be a pita to remove. installation will be almost impossible. need to align the door levers to the proper positions to reinstall the plastic gear/actuator unit. the diagram above is useless.
    O2 GMC RCSB 4.8L V8 AUTO 3.73's
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    Re: air is stuck on defrost

    Quote Originally Posted by NEUMANNZZ View Post
    the diagram above is useless.
    Sorry, it's a parts diagram to locate the correct part, not a repair diagram.
    [IMG][/IMG]

    1974 Jimmy 350
    1982 Sierra 402 BB (1971)
    2002 Sierra 4.3L
    1998 3500HD 7.4L

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    Re: air is stuck on defrost

    Quote Originally Posted by turbo4whl View Post
    Sorry, it's a parts diagram to locate the correct part, not a repair diagram.
    Not useless, served its purpose to help me locate the right one.
    Thanks for taking the time to post it.
    I guess the one on the top of the unit (#9) is the outside air ' recirculate servo. Mine was unplugged.
    I have a new set of controls on the way, i will see if that is the issue before i go any further.
    Last edited by mc87ss; 07-17-2016 at 05:33 PM.

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    Re: air is stuck on defrost

    I still have not checked the servo yet but i did get my new control unit. Hooked it up and it did not change the vent issue.
    Another issue i was having, my fan would not work on high. I tried a new Blower Motor Resistor but it still didn't fix that issue.
    Any other ideas?

  20. #20
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    Re: air is stuck on defrost

    The Video I have shows what you need to do..
    Its the actuator on the drivers side.. easy to get to
    And what ever you do.. DO NOT Power it up with it not attached.. it will get out of sync and wont work.
    here is the video it took me like 10 mins to fix this issue.. The controls are very seldom the problem.
    the actuator is the problem here. and the Fan High Speed is the Fan Speed Resistor


    Watch this video.. Actuator
    https://www.youtube.com/watch?v=Ow0OOfIFtlU

    Watch this Video Fan Speed Resistor
    https://www.youtube.com/watch?v=M2dCKwPYb-Y

    This two videos should fix both of your issues.

    PS:
    Just seen you did the Resistor..
    Then I would look at the Fan Motor it self.
    But that being said.. did you use a NEW Resistor or used?
    sometimes those can get damaged just putting them in or bad right from the box.
    Last edited by ldelaney; 07-21-2016 at 10:48 AM.

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