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81 HVAC system: windshield defrost ducts dont open

Discussion in 'The Garage' started by R72K5, Jul 4, 2005.

  1. R72K5

    R72K5 Banned

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    something ive been needing to fix before winter in the 81 and cant figure out and give up on it til i can find out something from someone

    and that is:

    what opens and shuts the air flow to the defroster ducts for the windshield on the 81 trucks with a/c ?

    is it one of those metal vacuum valves? there is one near the floor center outlets and another on on passenger side kick panel under the plastic panel piece, and maybe a third one up on top of the HVAC box inside the cab

    i have vacuum in one of the hoses on back of the HVAC control panel and
    with the panel in windshield position the vacuum valve near the floor ducts is getting vacuum- i guess it runs a door in the middle

    what do i need to be checking?

    it doesnt seem like im getting air out anywhere actually but the fan motor is just spinning away, has all the speeds


    thanks for any ideas on what to take apart/check

    the main heat door and cable works fine i know,
    i disconnected the cable from the control panel and i can pull and push it and i can hear the door inside the box near heater core slam shut and i can open it back up, all by hand with the cable, it seems fine


    so im at a loss right now,

    also how many vacuum hoses are supposed to be plugged into the back of the control panel ? i have 5 on mine, and there are two unused ports and no hoses for them, its an 81 regular cab pickup

    thanks
     
    Last edited: Jul 11, 2005
  2. scubahard

    scubahard Registered Member

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    I just replaced my heater core on my 1981 Pickup, so I was able to learn a lot about the duct system. Since we have the same model I hope this helps. Before my heater core went bad I had no defrost. If I remember correctly, the far left vac diaphragm controls a long rectangulat door. I think the door can be exposed if you remove the small main heater duct piece (this is located right in the miffle of the tuck, just behind the transfercase shifter). Everything seemed to work correct, but I wasn't getting any air out the top. Try this:

    1) Remove glove compartment.
    2) In the front of the heater core enclosure there is a duct that travels upward and connects to the long duct that goes over to the left passenger side vent. Remove this. This exposes a door that is controlled by a vacuum diaphragm. There is a little bracket that attaches the door to the actuating arm (the thing that opens and closes the door). My little bracket was broken, but not all the way off. So it was still able to open and close (so it sounded okay), but not all the way.
    3) Towards the upper left hand corner of the glove compartment cut out there is a section where parts of the duct system mate together. This is a terrible factory design because there is a huge gap where the two rectangular tubes meet each other. Use Duct tape to seal this gap.

    After I did all this I was able to have defrost again. Make sure you check the gap in the duct and seal it. Remove all the duct covers and try all the vent settings to see and make sure all the doors are sealing and opening. Good Luck.
     
  3. R72K5

    R72K5 Banned

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    cool good info

    my duct that runs up the middle in front of globebox lid assembly basically, it is loose not attaches ot the main box very well at all looks to be missing some screws

    hmmm

    my goal is to set air path to defrost permanently, unless i find the cause of loss of control of air flow in the system

    gona tear it all apart one of these days if i am ever off work


    thanks
     
  4. HarryH3

    HarryH3 1 ton status Author

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    Randy, open the main air duct just under the center of the dash. I bought a '77 Blazer years ago that would only blow air out onto your feet, even in A/C mode. :eek1: Inside the main air control door assembly I found an old BIC lighter that had fallen down into a defroster duct and worked its way down into the control. It was preventing the main door from rotating. Once I removed the lighter, the HVAC system worked perfectly. :cool1:
     
  5. R72K5

    R72K5 Banned

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    which one is the main one ? vertical, horizontal ? out on backside of box assembly out in open or ?

    thanks
     
  6. HarryH3

    HarryH3 1 ton status Author

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    Look under the dash, just above the transmission hump. There's a large, curved, hard plastic duct there that's held to the HVAC housing with a few hex-head screws. Behind it is the air control door.
     
  7. shane74

    shane74 1/2 ton status

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    There is a little plastic clip that screws to the flapper on the main control box. There is an arm that goes from the actuator to the clip. If it's broken, nothing will work right. it's a $1.50 part from the dealer. Sounds like that is what is broken. They go bad and when they do, all you will get is air out the bottom no matter what position the selector is on.

    I just went through this s few months ago. You'll know it when you see it. It's right inside the main cover of the heater box. You might have to remove the glove box to get to it all, but it's a very easy repair. I did it in less than 1/2 hour. Now, everything works perfect again. :wink1:
     
  8. R72K5

    R72K5 Banned

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    ok

    out of the blue was given a day off work today-

    so.,,,,,

    i took the center part cover off

    that clip that screws to the flapper door is busted, yep,

    and i found a good one in my attic parts stash still attached to a flapper door,

    however- up on top of middle of box there is a rod that connects this door shaft to another door shaft, the other one is closer to pass side of truck, it is a funky wierd all-metal door

    the threaded end of the rod is layhing up on top of the box, not connected

    the shaft for this wierd door has a round bright white plastic clip connector

    but no way to connect the rod to it

    something is missing

    what is this metal door for and how does this link rod connect to the clip up there ?

    i think the door is to control air between a/c and heat

    also how do i rmeove the broken clip from the flapper door in center other than rmeoving the screw thats threaded into the door? do you remove the nail looking pin or what ?

    how ?

    thanks
     
    Last edited: Jul 11, 2005
  9. R72K5

    R72K5 Banned

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    heres the threaded rod link on top of box that connects the two door shafts, its huge pic i have NO clue how to resize pics... thats like rocket science or something,, :confused: i need the part that attaches to the threaded end of this rod, anyone have one of them ? i have no idea what it is or looks like,
    thanks

    [​IMG]
     
  10. R72K5

    R72K5 Banned

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    and here is the door and plastic clip with that damn nail pin that i found in attic, the clip in the truck is broken and the nail pin is not coming out:



    [​IMG]
     
  11. R72K5

    R72K5 Banned

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    heres the clip thats supposed to somehow hold the threaded end of the rod- i do see soem threads in the top split apart part of the clip, shouldnt there be a nut or something else to keep the rod in the split of this clip= its not just gonna stay there all by itself magically i know, :confused:


    [​IMG]
     
  12. R72K5

    R72K5 Banned

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    ok i went back out into the truck for round II just now

    i was lookign around and i notice that all the air path ducting and such closest to the dash metal away from the firewall seems to be for all the dash and/or a/c vents


    i dont need all this stuff, i dont want it, sinc ei just need/want windshield defrost and floor vent

    so id be fine with removing all of it- from the flapper door(with that damn busted clip) and beyond(rear-wards of truck), right ?



    i was already missing the under-column long duct piece, and the round dashpad flex ducts, the dash pad i dont care about its a non matching junk one and i have a non a/c dash pad strip i can put on it some day or whatever, i dont really care,



    however, i found a new problem- where the upper defroster split apart duct connects to top of the main firewall box- that screw there on front where the defrost duct overlaps the main box top opening doesnt hold them together anymore- the ear/hole is broken- and i cant get the duct back together onto the main box top,
    the main firewall box seems to need to be picked up higher- as you cant pull the defrost duct down as it is held to the underside of the dash top metal by the flexible metal strips like the wiring harness and vacuum lines do

    i can pull the box unit upwards but it sags back downwards when i let go of it,
    maybe i can just wedge some blocks of wood or soemthing under it
    i dunno
    i need to get the defroster duct back onto the top of the box unit somehow


    thanks
     
    Last edited: Jul 11, 2005
  13. R72K5

    R72K5 Banned

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    ok im fiddling around with this round rod clip and the threraded rod and well it seems that the threaded end just kinda sits in the split of the clip and thats it

    somehow it popped off, dunno
     

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