Ford Blend Door Fixes

It seems the dreaded blend door problem manifests itself throughout the Ford product line. Google searches locate references to it on trucks, Expeditions, Lincoln Navigators and even some of the automobile lines. Most likely the cause is failure of the ABS plastic in the top door mount due to prolonged exposure to heat. The plastic becomes brittle and eventually shatters.

The symptom is loss of control over the the temperature of the air exiting through the various outlets of the manual heat/AC system. When it's broken, you either get full heat or full cold with no control in between. The Max AC still works okay as it uses different ducting, but normal AC gives you full heat. Or more rarely, the door can get stuck in the cool position and you get no heat. So I created the Last Resort Blend Door Fix as explained below.

NEWS FLASH! Recent Developments

An alternative to the below described Last Resort Blend Door Fix, there is now also a product called the HeaterTreater that specifically addresses the broken blend door. This didn't exist when I did it the hard way. It is a steel door kit that uses the same method to access the plenum box. There is no dashboard removal required, is good for the life of the car, and takes about an hour to install. For those who would rather have a garage shop perform the installation, HeaterTreater has a network of shop affiliates around the country. They also make blend door repair kits for many other vehicles including Ford, Jeep, Dodge, and Chevy. Check them out at

Heater Treater for Blend DoorsHeater Treater Kit


If you really want to fabricate it yourself -- this is not for the faint of heart. This permanent fix requires some extensive modifications to your vehicle. Don't try it if you are not mechanically inclined and don't blame me if it doesn't work or something gets seriously screwed up! YOU ARE ON YOUR OWN. This page assumes you have seen and understand the quick fix found a this site.

First, turn the ignition on, turn on the heater and then rotate the temp control to full heat. This is done to get the actuator servo in the right position.

Then, open the glove box door and squeeze the tabs towards each other so they clear the frame and the door drops all the way down. You need not remove the door.

Carefully pry the actuator servo off the top of the plenum with a large flat blade screwdriver. Turn the ignition back on and rotate the heat control back and forth to test the motor. The heater or AC has to be on for it to work. If the shaft doesn't turn, you need a new actuator. Rotate it to the full heat position again. Disconnect the wiring plug and get it out of the way.

Cut a major access hole in the front of the plenum. Use a hobbyists tool like a Dremel Moto Tool or equivalent. I got one at a local auto parts store for about half the price of a Dremel. You will need the optional flexible extension shaft and some extra grinding/cutting wheels. Take a deep breath and start cutting. Create a hole similar to the one shown below. Be careful around the AC and heat components. A slip here can cost a bundle!

As you can see, the hole is quite large. It needs to be. You cannot remove the blend door through a smaller hole, and you need to remove it completely from the the plenum. I actually had to enlarge the hole twice after making the orginal cut. (we'll come back the seam noted on this photo.)

When I could get my hand in through the hole, I found the blend door had fallen completely out of place because the entire top of the shaft was gone. When the door is intact it is retained by the lower end of the shaft fitting into a socket on the bottom of the plenum and a depressed area on top around the actuator servo's shaft access hole.

Here's a view of the door along with a broken piece from the top of the shaft. The remaining pieces are lost forever.

The bottom of the door is on the left, the top on the right. The top always breaks off. Note the purpose of the foam is to seal the door when in the full open or full closed positions. Be careful with it or you will have leakage and inexact control over the output temp.

Obtain a section of 9/16 thin wall brass tubing. You can find it at most hobby shops or at ACE Hardware. I had to order one from the local ACE as they didn't ordinarily carry that size. Be sure it's 9/16. It comes in 1/32" increments, but one size smaller or one size larger won't work.

Using the grinder, carefully slit the tubing up the side so it fits snugly over the existing shaft for the blend door and the slit ends such that you have a complete circle near the top--do NOT slit the tube all the way to the top!
You may have to grind some material away from the top area of the door so the tube seats firmly on the top of the remaining shaft. The length of the tube is not critical, just make it long enough so it gets a good grip on the door's shaft.

Note that the tube protrudes 1/4" above the top of the door. It should be a tight fit and not easiy move when it's in place. While you have the grinder in hand, shorten the bottom of the hinge post by 1/4"and round the edges like it was before you shortened it. It makes no difference in function, but it is impossible to reinstall the door if you don't.

The following three photos show the insertion of a plastic (nylon) spacer, also found at ACE hardware. This spacer is important because it holds the output shaft of the actuator servo in place, keeping it from flopping around and possibly causing it to come loose from the blend door once it's installed. Slot the spacer so the pin(screw) in the actuator servo has something to grip. The spacer fit a little loosely so I wrapped one turn of duct tape around it to hold it place and center it. It cannot possibly fall out so duct tape is fine.

Below shows the spacer in place with a the slot properly aligned. Note that there is no scientific way to determine the alignment because the shape of the original top of the shaft was lost when it shattered. I eyeballed it a little less than 120° from a line drawn along the top of the blend door. (about the 1:00 position if you look at the door from the top.)

After pressing the spacer firmly in place, grind slots in the brass tubing to match those in the spacer. Not shown here, but grind the top of the spacer so it is flush with the top of the brass tubing, which, again, ends 1/4" above the top of the blend door.

Now drill a hole through the actuator servo such that it bisects the curved end of the shaft. The actuator was in the full heat position when it was disconnected so the hole lines up with the longest side of the actuator. Insert a 1/8" screw through the hole and clip the head off. File the ends and then test it in the slot in the brass tubing/plastic spacer now forming the top of th blend door. It should protrude from the slot, but not so much that it won't fit through the hole in the top of the plenum

Now it's time to start putting it back together. The bad news is that with the blend door returned to its orginal height, including the restored shaft, it's too long and won't fit back into place. So now you need to cut a clearance hole in the bottom of the plenum so you can tip the door into place. (See below)

I got a lot of cuts on my knuckles at this point and said some really bad words, but eventually it almost fits.
1) Insert the door assembly into the plenum with the brass tube down
2) Manipulate the door so that the bottom hinge pin exits the hole in the bottom of the plenum and stand the door upright.
3) Say a few bad words until you get the bottom hinge pin into the socket at the bottom of the plenum. It finally fits, trust me.
4) Guess what! The top of the door won't slide into to place under the hole in the top of the plenum where the actuator servo shaft enters. DON'T FORCE IT or you will damage the top of the soft brass tubing and the end of the actuator servo won't fit into the slot.

Here's where you scroll back up the page to the photo of the hole cut through the plenum to where it says to 'Note Seam." Insert a very large screwdriver or pry bar into that seam and apply pressure such that the plenum distorts and the top and bottom of the plenum separate by about a half inch at that seam. Say the magic words and the door will pop into place. If all is correct, the door will stay in position, with the top of the new shaft very slightly entering the hole for the shaft of the actuator servo. It's important that it does so the door doesn't fall away during assembly.

(Note that the first time I tried it and couldn't get it into place, I shortened the top of the 1/4" overhang from the top of the door to the top of the shaft and then it went in, but wouldn't stay there. (Not a good thing, I had to do it over)

In this photo of the bottom view of the plenum, you can see the clearance hole and that the blend door is installed and visible.

For convenience, turn on the ignition and, with the heater/AC on, turn the temp control to the center. The actuator will rotate to a position that will make it easier to install it into the top of the plenum. Then gingerly insert the actuator servo into place while manipulating the blend door through the hole in the front of the plenum. See the photo below that shows that the screw through the actuator shaft has engaged the slot in the top of the brass tubing & spacer. Rotate the temperature dial and you will feel the blend door move from side to side and seal against the housing on each side to provide either full heat, full cool or anywhere in between. Neat, huh?

I then covered up the holes in the plenum with duct tape for a couple of days to test the system. It works as good as it was engineered to work (not real great, actually, but that's a Ford issue) and it will now outlast the car, as it should.

The last step is purchase a fiberglass kit and repair the plenum. I bought a kit at Pep Boys for $9 that included glass cloth, resin and hardener. Protect the carpet (it's real messy), cut the glass cloth into strips and have at it. I used duct tape in some areas to hold the pieces in place until the resin dried and then removed the tape after it cured and applied another layer of resin and cloth.

CAUTION: Scroll back up to see the photo of the clearance hole cut into the bottom of the plenum. When you are repairing the bottom, do NOT slop resin inside and glue the blend door so it won't move. Turn the temp control so it is out of the way as shown in the photo. Do NOT allow any excess material to protrude into the hole that might impede the rotation of the door. I actually cut a scrap of plastic so it was about 3/4" larger than the piece I originally cut out from the plenum and epoxied the two together so it created somewhat of a flange around the hole and then carefuly glued the assembly into place. After it cured, I added a layer of fiberglass over the whole piece.

In most areas there are two layers of cloth/resin combo. It's not very pretty, but it works great. The whole project cost about $15 plus the cost of the Moto-Tool, but, hey, I wanted one of those anyway. It took a couple of hours.

by brokenford