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If your '80-86 Ford truck has factory air, you can expect the following failure to happen at some unspecified time. Your windshield will fog over on the inside, and when you select "Defrost" and turn up the fan, practically all the air blows out the registers on the face of the dash, instead of from the defrost nozzles.
When (not if) this happens, expect to find the hinge broken for the door that controls airflow to the dash registers. The actuator for this door is spring-loaded to the closed position, so if there is any sort of vacuum or control problem, the door should automatically close. This is by design, so that with most possible failures, you still have defrost for safety reasons. However, for reasons unknown, Ford thought a plastic flex hinge would be perfect for this door. What could possibly go wrong? Oh, I don't know... What would happen if you flexed a thin piece of plastic back and forth every day for over 30 years?
If air is always blowing from the registers on the face of your dash, it's easy enough to check the hinge. Note I didn't see "easy to see the hinge" or anything like that. It's buried. But you can easily see the linkage from the vacuum actuator to the door. Open the glove box and look over the top left corner of the liner. You will see a small round canister on its side. That's the vacuum actuator. To the right of the actuator is the linkage to the door. With the engine running so you will have a vacuum supply, select Vent on the control panel and turn the fan to high. Vacuum is now supplied to the actuator, and the linkage will pull to the left to open the door. It's pretty easy to see the linkage move, once you find the best vantage point. Now select Floor and the vacuum is removed from the actuator. The linkage should move to the right. If the linkage moves, but you still get full airflow from the dash registers, the hinge has failed and the door isn't moving to the closed position.
Here's an illustration from the factory service manual, showing the vent door and how it operates. Note how when the door is closed (no vacuum at actuator, linkage extended), no airflow reaches the dash registers. When the door is open (vacuum at actuator, linkage retracted), all air is sent to the dash registers and all airflow to the defrestor and floor vents is blocked. The door seals against the plenum housing on one side to seal off the dash registers, and on the other side of the plenum to seal off the defroster and floor vents:
Well, you might say, "If Ford knew this plastic flex hinge would eventually fail, at least they made access easy, right?" Oh, stop it, you're killing me. Did you hear about the penguin who took his car to the mechanic? If you're going to tell jokes, so am I:
The panel door is buried under the dash. Reading through the factory manual, apparently the panel door was the first part down the assembly line, and the truck was built around it. Call me lazy, but there's no way I was going to completely tear apart the dash to get to this door. If you follow my suggested plan of attack, you can have the broken door and hinge removed in about an hour. Plan for a couple of hours to fabricate a durable replacement, and then maybe another hour to reinstall it and put everything back together again.
Courtesy of Gary Lewis, here's a picture showing the main heating/cooling plenum under the dash. This part is directly on the aft side of the firewall, tucked up out of sight. The only parts you can see from the seat are the floor duct and the bottom edge of the heater core area. The vent door and hinge are smack dab in the middle, but more on that later. Note the vent door actuator just to the left of the door. This is the actuator you can see, as mentioned earlier, by looking over the top left corner of the glove box liner:
If that was all you had to deal with, access wouldn't be so bad. However, there's a big duct that hangs down on the aft side of the plenum. This duct sends air to the dash registers, and completely blocks access to the door. Here's another view from the factory manual, showing the guilty party. You will need to shift that duct for access to the vent door on the main plenum. I've circled the 3 screws that need to be removed. Note how this duct (right register duct #19B680) meets up with another duct on the driver's side. We can leave the driver's side alone, fortunately:
Once again courtesy of Gary Lewis, here's a picture of the same duct. Note the two rectangular openings for the dash registers above the glove box. Also note the two screw tabs at the top and the metal bracket at the bottom. Access to the vent door is blocked by the portion of this duct that hangs down to the bottom of the dash.
Just for giggles, here's a view of the firewall side of that same duct, showing the large foam-bordered opening that mates to the vent door opening on the main plenum. Thanks again to Gary Lewis for taking these pictures:
My original plan was to completely remove this duct, but there's no way without getting in way too deep. I had hoped it could slide to the right and disengage from the left register duct, but that isn't feasible. The right register duct hits the door pillar before separating from the left duct. I was going to have to work around the right register duct.
Now is the time to remove the glove box door and liner. To remove the glove box door, remove the three fasteners under the bottom of the dash. Keep the hinge attached to the door, unlike the way I did it. Also remove the five screws for the glove box liner. A ratcheting right angle Phillips screwdriver makes easy work of the screws across the bottom.
If you look closely at the following picture through the glove box opening, you'll see a white piece of sheet metal between the main plenum and the right register duct. The hinge failed during the middle of winter and it was too cold to do this repair. I slid a piece of 7"W x 7-3/4"H sheet metal between the duct and plenum to block off the airflow to the dash registers so I could have my defrost again. The foam rubber seal held the sheet metal perfectly:
While not mandatory, I popped out the two dash registers to allow me to help persuade the duct. I didn't see it mentioned anywhere in the manual, but the protruding bosses at each end are different sizes. You can only install the registers one way:
Here's the factory manual explaining how to remove the registers:
Above the right outboard register by the passenger door, remove only the screw shown. The other fasteners can be left alone:
Above the right center register, at the left side of the glove box, remove only the screw shown:
Underneath the left side of the glove box, remove the hidden screw for the bracket on the right register duct. Also remove the bolt that secures the firewall brace to the bottom edge of the dash:
At the right side of the glove box opening, remove the single bolt securing the dash to the door pillar area. The bottom edge of the dash is now free, and can be pulled back in a later step to allow more room to work:
I forgot to get a picture, but also remove the cover for the heater core. This is at the right side of the main plenum, above the passenger's feet. See the first picture above for the location of the heater core. With this cover removed, you can sneak your hand inside the main plenum for access to the vent door.
Let's take a close-up look at that pesky hinge. Once again, many thanks to Gary Lewis for graciously supplying these pics. I've added a yellow line showing the approximate position of the hinge inside the main plenum. The hinge is secured inside the main plenum by two hex head screws at the top and two rivets at the bottom:
Access to the bottom rivet isn't too bad. Center punch the rivet and take your time, starting with a small bit and working up. You don't need to drill completely through it. You only need to drill deep enough to grab the hole with a punch and snap off the head. The rivets are 5/32" diameter. I used a #22 drill bit and snapped off the head with a 5/32" punch. Don't let the rivet get hot or it will spin. If that happens, secure it with a dab of super glue and drill a little deeper to break the shank free from the head:
The upper rivet is tougher to reach. I hooked a light duty motorcycle tiedown strap to the upper hinge area for the passenger door, and pulled the duct to the right for clearance. The duct flexed just enough to let me drill out the upper rivet. Don't leave the strap under tension for too long, as you don't want the duct to warp like that. Note how the head had already been drilled off the lower rivet:
With both rivet heads drilled out, it's time to tackle the remaining two fasteners on the vent door hinge. These are the 5/16" hex head screws above the rivets. For access, loop the tiedown strap around the bottom of the register duct, and connect the other end to the metal loop for the passenger's shoulder belt. Gently pull aft on the bottom of the duct, along with the bottom edge of the dash, and cinch up the strap as you go. The whole dash and duct can easily swing out several inches at the bottom. Not ideal, and working around the strap is awkward, but you'll have just enough room to snake your hand up there for access to those two bolts. Undo the top bolt first, as the remnant of the hinge will swing around as you loosen the last fastener. This view is from the passenger footwell, looking towards the center of the truck. (If you are eagle-eyed, you'd notice this photo actually shows the new hinge installed):
You will need a ratcheting 5/16" box end wrench. There's no room for a socket, and repeatedly repositioning a fixed wrench would take too long:
You will breathe a sigh of relief after freeing the broken hinge piece, but unfortunately you are not quite in the clear. The linkage from the actuator is attached to an arm on the vent door, inside the plenum. The linkage is secured by a plastic push pin that snaps in place. Try as I might, I couldn't get that pin to release. Working by Braille, I reached in from the heater core area with a hacksaw blade and cut off the head of the pin. Then I was able to pull that pin down with long needlenose pliers to release the door from the linkage. In retrospect, it probably would have been easier to push down on the pin shank with a small screwdriver. I'm amazed how well this view turned out. This is inside the plenum, looking towards the center from the heater core area. The wavy pattern at the right of the picture is the heater core:
With the pin out of the way, remove the door and hinge piece from the plenum. Release the tension on the strap so the duct doesn't take a set. Here's what the door and integral hinge look like, seeing daylight for the first time in 33 years:
Now it's time to get fabricating. You're done wrestling under the dash for a while, so take a deep relaxing breath. This shot shows the profile of the original plastic hinge, and the aluminum piano hinge I used as a replacement. It's very important to locate the new hinge pin centerline in the same location as the old. If different, the door may not operate properly. Remember, the door seals in both directions, so the geometry has to be correct.
The aluminum piano hinge I used in not available in most hardware stores. Each leg is 1.5" wide (3" open width), and has no holes. Don't try to make do with narrower stuff with predrilled holes. (Edit) I ordered my hinge from Amazon, but it's no longer available there. You can order the right stuff from the following links. McMaster-Carr is probably best, as you can order by the foot and save on oversized shipping costs:
I'm measuring the offset to the centerline of the original hinge in this picture. For reasons unknown, the offset was not consistent. It measured .307" in the middle, but the offset was reduced to approximately .250" at each end. All I could figure was this was to compensate for curvature in the plenum face. However, the plenum seems straight enough to me, so I decided to split the difference and make a consistent offset of .278":
Bending the raw hinge stock in a press:
Okay, fine, I'll admit I'm just showing off at this point. I don't want to listen to your whining if you don't have a press:
If your shop doesn't have a press (I don't judge...), you can easily bend the hinge by clamping it between two blocks in a vise. However you bend the hinge, leave the pin installed so you don't accidently crush the loops:
Here's a mockup of the hinge half for the door. The hinge is still extra long and is being positioned side-to-side relative to the the original centerline:
This is a closer view of the same thing. The next step is to trim the edge of the door and then slightly bend this hinge half to raise the new hinge centerline relative to the face of the door.
I noticed the door was slightly warped. I made my best guesstimate to straighten it, using wide-jaw Visegrips as a weight overnight:
Here's the finished hinge, perfectly matching the geometry of the original. I've replacing the two rivets at the bottom with hex-head screws, just like the original ones at the top. The second screw from the bottom (formerly the top rivet) needs to be about 1/2" long maximum, to provide clearance for the control arm on the door. Note the plastic guide pins for aligning the original hinge inside the plenum. These weren't needed on the replacement, as the four fasteners will work just fine for that:
The next question was what to do about the foam seals on the door. The original had a piece of foam wrapped around both sides, but it was in poor condition. Some type of foam is needed on both sides for good sealing, so I went with foam tape instead. The original foam measured .200" thick. I used slightly thinner .188" foam around the edges, the closest I could find. Across the hinge, I used .125" foam to compensate for the extra thickness of the metal:
Here's the other side of the door, showing the arm that attaches to the actuator linkage. To replace the pin that was previously cut away, I used a small 3/16" diameter cleavis pin. Note how I ground the end to a taper for easier installation. It would be virtually impossible to get a cotter pin in the pin, so I used some sticky dum-dum putty (not shown) under the head of the new pin to hold it in place. Any type of non-hardening putty would work. If you can find a pin that snaps in place like the original, have at it, just be aware you'd probably have to cut it for removal. Also note the backing washers used with the rivets that secure the hinge.
The installation of the door is actually anti-climatic. Pull the duct aft again and tighten the strap to make room for access. With the hinge now attached to the door, clearance is a little tighter to snake it in place. The only way it would fit was through the gap to the left of the heater core. It also helps to retract the vent door actuator, to keep the linkage out of the way. To do this, start the engine and select Vent. On the exterior of the plenum, gently clamp a pair of small needlenose Visegrips on the exposed portion of the actuator linkage. When you shut off the engine, the linkage will stay retracted and out of the way.
To make it easier to maneuver the door, I added a loop of stiff foil tape on the face of the door to act as a little handle. Peel it off when done. Reaching in from the heater core, you have barely just enough room for your hand on the other side of the door. Maneuver it in place and start the fasteners. Don't forget the second fastener from the bottom, formerly the top rivet, must be no longer than 1/2" for clearance with the door's control arm. With the fasteners secured, make sure the door has full freedom of travel in each direction and adjust as necessary. There's no need to overtorque the fasteners, as you don't want to induce any binding.
Connecting the actuator linkage is all done by feel. Release the Visegrips and let the linkage extend. Reclamp the Visegrips to provide a handle to move the linkage as needed, and drop the pin in place. Don't forget the dab of dum-dum putty to keep the pin from working loose, although gravity will help, too.
Test the door operation with the engine running, and make sure all is good. Don't be alarmed as the actuator is very slow to open the vent door. It closes quickly, but is slow in the opposite direction as vacuum has to build up to overcome the spring-loaded automatic closing feature.
After releasing the strap, I noticed the register duct seemed slightly warped relative to the face of the plenum. To ensure a good seal, I added a sheet metal screw through the edge of the duct into the plenum. This screw is well clear of the door, but make sure if you decide to do the same thing. Please note the duct still has a large gap away from the plenum, closed by a thick foam seal. Don't overtighten the extra screw, as that is not necessary:
After that, reinstall all the rest of the fasteners, the glove box door and liner, etc.
Hope this helps, and many thanks again to Gary Lewis for his pictures. More details here:
The Panther-chassis cars of the early 1980's (Town Car, Mark VI, LTD, Grand Marquis) had the same issue with the flexing plastic as a hinge. Our 1983 broke around 2003. On the early years cars, could pull the whole plenum out from under the dash by loosening up all the dash attachment bolts on the right side, no need to drop the steering column like the later years to get the big box out. This was the same procedure as to replace a heater core on those.
I too used a length of piano hinge, on those had to get the hinge angle just right so the door would seal closed and not get hung up. Worked great. Never would have thought that a piece of plastic with a V-groove molded into it so it would bend there would be used as a hinge in a car climate control system!
That sort of hinge idea is usually seen on cheap plastic storage containers and the like!
Excellent write-up! Just did this on mine as a preventative measure while the dash was apart and could be performed on the benchtop.
Just a couple things I found (helped being able to look at it easily and dry fit it a few times):
I didn't have to bend the second half of the hinge - pulling the pin and flipping it over lined the center line up
I used 3/16 camper shell foam all around. I searched but wasn't able to find the thinner kind locally so tried the 3/16 on the hinge also and it sealed it evenly. A few factors could play into this but just wanted to mention it in case others can't find it locally either.
I appreciate your detailed post! I ordered the replacement door from Bronco Graveyard....everything else I can (probably) do. The fabrication is too much detail for me to do correctly.
Hopefully I come out on the other side with a defroster.
Hey Charlie - I got the project done over the weekend. The outcome is about 80%. The good news is I have a defroster and footwell heat now. The wife is happier. The bad news is I could not get the hinge properly mounted. I just couldn't get my fat hand into the space without breaking the plenum. I ended up removing the dash top for a bit more access. I decided to use foil furnace tape to simply seal the new door in place. I can change between blend, defrost and floor. In my unheated garage in Syracuse, NY...this was a good compromise. Down the road, I'll take the whole thing apart and put it back 100% correct. I have some pics I'll send in if you'd like to add but you're instructions were SPOT ON!
If you want to get a new door, the following 3D printed version is a much better option than the original design. Follow the link for ordering instructions.
I'm guessing he never saw my guide, though. His replacement instructions have you do a major teardown of the dash for access. I'm lazy and proud of it, and the guide you are currently viewing shows a much easier method with far less disassembly.
Many thanks to Dave G. (FuzzFace2) for sharing the link.
If you want to get a new door, the following 3D printed version is a much better option than the original design. Follow the link for ordering instructions.
I'm guessing he never saw my guide, though. His replacement instructions have you do a major teardown of the dash for access. I'm lazy and proud of it, and the guide you are currently viewing shows a much easier method with far less disassembly.
Many thanks to Dave G. (FuzzFace2) for sharing the link.
Hey, thanks for noticing and appreciating the design! Yes, the steps on Gary's site are a bit involved but I have an abridged version of the instructions on my Square site. In my own experience, I had EVERYTHING torn apart when I fixed my own door. It was much easier fixing the door than putting everything back together!
If you have any questions or comments about the door, please let me know! Happy to help fix people's trucks in a niche sort of way!
Jeff's Bronco Graveyard has been selling an original style replacement door (with flex plastic hinge), with a link in post #6 above. They now offer their own upgraded 3D printed version, item #372810:
