yo!
It's most likely the other door (recirc) next to da Blend Door as shown in
this Diagram in 80-86 & 92-96; "...
80-91 similar, except 87-early 88 w/factory air; The 80-86 vacuum tank is a plastic ball on the R wheelwell..."
Source: by Steve83 at
http://www.supermotors.net/registry/media/741015
And as Steve83 wrote; vacuum line (to the recirculate motor) is the reason. The white plastic is far more susceptible to UV light than any other color, and the gap along the edge of the hood allows it to degrade.
http://www.supermotors.net/getfile/767412/fullsize/recircline.jpg
some backgrnd info & a tip for ya;
More Heat; "...the '87-'91 trucks with factory A/C have a recirculation door in their HVAC system, it's there to cut off outside air and keep running only in-cab air through the blower. It's vacuum controlled, and only cuts outside air when you put the control lever to "off" or "max A/C". The problem I see with this is when you have 15F ambient temperature with some -20 windchill, heating up that air good could be a challenge for a truck with an aging heater core or tired blower fan. Last year just to see how much of difference it actually makes I zip-tied my recirculation door closed, thus running inside air through the HVAC system all winter long - things got real warm real quick, noticeable improvement even when truck is stationary. For the warmer days tho I want the recirculation door open so I can get fresh air in the cab as I drive... So I devised and implemented a solution in my '90 truck - I now have a manually controlled recirculation door that I can open and close to my liking, which means I can get tons of heat in the winter even with some nasty ambient temperatures..."
Source: by M.L.S.C
Parts needed:
- one constant-duty vacuum solenoid (I used an emissions solenoid from an '89 Ford Crown Victoria, grab the harness plug for that and a few inches of the wires):
http://img193.imageshack.us/img193/2937/01fordvacuumsolenoid.jpg
- one on/off electric switch (I used a sealed toggle switch from Lowes, it comes with wires already attached to it)
- several feet of small-diameter vacuum hose (I think I used 5/32" ID)
- some lengths of wire, wire cutters/strippers, and heat shrink.
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Step one of the installation is to decide where you want the solenoid - I chose to have it on top of the air box, right next to the recirculation door vacuum servo:
http://img23.imageshack.us/img23/7443/03recirculationdoorvacu.jpg
http://img40.imageshack.us/img40/5471/04vacuumsolenoidinstall.jpg
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Step two is to decide where you want the control switch - I put mine in the HVAC control panel, on the passenger side of the two sliding control levers there is plenty of space for a small toggle or rocker switch. Just so ya'll are warned, if you go with the Lowes sealed toggle switch, make sure you take some measurements before drilling for it, or you'll never be able to fit it behind that black plastic panel:
http://img40.imageshack.us/img40/476/05vacuumsolenoidcontrol.jpg
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Step three (that's the fun one) is wiring: basically the solenoid needs 12V key-on power and a good ground to work, there are several ways you can do this but I chose to power it through the engine bay and ground it inside the cab with the toggle switch. Then on top of the air box I drilled a small hole right next to the rubber grommet for the big harness passing through there:
http://img41.imageshack.us/img41/5412/06vacuumsolenoidwiring.jpg
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then ran one wire through there, around the heater core box inside the cab, under the dash, and to ne terminal of the switch on the HVAC control panel - one end of the wire gets hooked up to the toggle switch, the other end to the solenoid (splice into original solenoid plug's other wire, this one is either green or yellowish in color depending on which of the two identical solenoids you took from the donor car). Finally the toggle switch needs a ground - run a short piece of wire from its other (still free) terminal to a good ground under the dash. Electrical work is done!
Step four is plumbing the solenoid - pull the factory vacuum line off the recirculation door servo and cap it off (cut it short if you wish, I did), it will not be used anymore. Look at the vacuum solenoid, and you'll notice one of the ports has a small filter-looking thing in it - this is the port that needs to be hooked up to the vacuum servo for the door, use some rubber hose or plastic line with rubber fittings for this:
http://img24.imageshack.us/img24/1407/02fordvacuumsolenoidpor.jpg
The other port (that has nothing inside it) is the one that gets hooked up to the vacuum tree - pull a rubber cap off one f the small ports of the vacuum tree, run a piece of hose from there to the open port of the vacuum solenoid. You're done!
Now when you fire up the truck the recirculation door will be under your command, feel free to open and close it as you please - when you close the toggle switch the solenoid gets power, lets vacuum to the servo, and the door closes killing outside air flow, when you disengage the switch the solenoid closes off the vacuum tree and vents the servo and the door returns to open position and you have fresh air. Enjoy!
===http://img32.imageshack.us/img32/4063/07alldone.jpg
Notes:
1) it was suggested to me that '87 trucks have different HVAC control panels that have a separate control lever for the fresh-air door and also a separate button for engaging the A/C compressor (fr trucks equipped with A/C) - I did check in a junkyard and it is true, the '87 trucks have that different control panel, and it can be had with or without the A/C button, but since my setup works rather good I never got the motivation to pull my HVAC controls out and see if an '87 panel will plug directly in the electrical harness and the vacuum lines.
2) the '92-'96 trucks utilize the same recirculation door setup as the '87-'91, so it should be possible to do this mod to one of these truck as well, however they have different HVAC control panel with turn knobs instead of slide levers, so I don't know how much space there is behind their face panel for a manual switch...."
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