Ford Explorer 02+ Blend Door Failure Analysis and HeaterTreater Fix

We were a little surprised to get multiple questions on the 02+ Ford Explorer after we introduced our 92-02 Ford Explorer HeaterTreater blend door fix product. We have design fixes for multiple automobiles from different makers and generally the blend door failures are a wear out mechanism from over stressing a plastic blend door with an overly strong DC motor. Usually the failure takes 4-6yrs to occur and our target market has generally been 2003 and earlier models. The later models have the same fail pattern and we are willing to wait.....

Ford significantly changed the design of the plenum box in 2002 and introduced dual control to the HVAC system. The new design still uses the same DC motor connected directly to the blend door, so our opinion was that over time the application would fall into our laps, but it was viewed as future business. We were surprised by request for a HeaterTreater solution for the new box but pushed it up the development list because Ford did sell a few of these automobiles and it is a good market for us.

We assumed that maybe Ford used a cheaper plastic for the doors to save money and it is a larger door, which will have more leverage against the motor and be more prone to breakage, so we proceeded on. The older model Explorer had the blend door on a vertical axis and access from under the passenger side dash was fairly easy to work with. The newer mode has the blend door on a horizontal axis and the axis runs parallel to the dash panel, making access somewhat difficult, but possible. Note: we has some early feedback from customers that access to the actuator motor and plenum box was too difficult so we developed a more elaborate process to pull the driver's half of the decorative covering of the dash panel to get better access. It takes more time, but is a better basic process for most people. The process is shown in a video in the TECH section to get an overview of the work involved.

We built a bench setup with a box removed from an Explorer and duplicated the blend door in metal and verified that it would fit an operate without binding. We connected an actuator motor and it all seemed to be working just fine. Again we were a little surprised that the Ford OEM door didn't look like it would cause problems...the design and plastic molding process looked OK compared to Chevy and Chrysler. One of the final checkouts we do is to operate the blend door with the motor casing open such that the gears are somewhat floating with attachment at the bottom, but no constraints on the top. The gears in the motor are all planar to the plenum box and if the blend door alignment is perfectly orthogonal to the motor, the gears will operate reasonably well. If the blend door is not at a perfect 90 degree alignment to the gears, the gears will tilt slightly and this is the root cause of the common "clicking" noise found on Ford products.

Well, our door "clicked" and the gears went out of alignment. Tool it all apart, checked all the measurement and tweaked everything, put it back in with the same problem again. After a couple of cycles of this, we got frustrated and purchased a new OEM door and ran our tests on the new Ford door. IT HAD THE SAME PROBLEM. It took a while, but here's what we found.

Fig 1. shows the problem that we found. Feel free to see if you can spot it before reading on.

Narrow Channel

Fig. 1

The picture is looking into the plenum box at the point where the axle for the blend door protrudes through the box. This is the passenger side of the box, but the driver's side looks the same. What you probably also noticed is that the spacing between the hole and the internal cross brace of the box is not unitform and is small at the top, virtually zero clearance. Fig 2. shows the end of two different doors from an Explorer...pre-05, and post-05. The one on the right is the older version and you can see a band that is about 1/8" wide with a 3/8" extension that fits through the hole. The one on the left has the same band but if you look closely you can see that it is very thin, about 1/32". The end of the axle protrudes through the hole in fig 1. and the little band on the axle contacts the inside of the box and provides lateral stability to keep the door from sliding back and forth. The problem is that the lateral stability band has to have some thickenss to do it's job and the plenum box does not have space for this band. When the door turns, the band rubs agains the top and back of the plenum box cross brace. The band is fairly thin, but since it's close to the edge of the box and the motor, even a small deflection causes a significant change in the angle of the the door to the motor from the desired 90 degrees.

Explorer Axle Band

Fig. 2

When the door turns, the band contacts the inside of the door channel and deflects the axle out of alignment. Since the channel is not uniform, the deflection is different dependent on the position of the door. The angle changes as the door swings through it's normal movement, stressing the motor our of alignment and providing stress on the door that will cause it to break prematurely. The door on the left has a thinner band and will probably wear off over time, but the diameter of the band is the same and it suffers from the same alignment problem as the first door. It probably provides a little less stress than the original, but is still a problem.

One interesting note, and we don't know this for a fact. We disassembled three systems on an 07 and 08 and noticed that the screws holding the actuator motor to the plenum box were loose. With a plastic box, these didn't "rattle" loose and had to either be intentional or an assembly mistake at the factory. It's pure speculation, but we think that Ford intentionally left the screws loose to provide a degree of freedom for motor movement to lessen the stress on the door. It's long been postulated that engineers operate with a screw loose...this may be the proof.

The HeaterTreater Solution

The problem with designing a replacement door is that you have to have some sort of lateral control of the door and you would like to have retaining rings on both ends of the axle with zero depth to avoid rubbing against the axle channel...no possible. Our solution was to provide the lateral control on one side of the box only, the side opposite the motor so that any variation would have minimal impact on the alignment of the motor to the axle. Fig 3. shows the HeaterTreater design. For lateral control, we have an external retaining ring set into a groove cut around the perifery of the axle. The groove keeps the spring loaded retaining ring from moving laterally and cuts down the height of the ring, but not to zero. To accomplish this, we sharpen the edge of the retaining ring to almost a knife edge. When you insert the door, the retaining ring custs into the plastic channel and allows the axle to fit exactly into the channel with no cantilever movement as it rotates. The "blade" cuts a groove into the plastic with one or two swings of the door through its full range of motion and perfect orthogonal alignment of the door to the motor is achieved. The retaining ring provides lateral stability against the outside edge of the box, and some additional stability against movement toward or away from the motor with the groove cut into the axle channel. This is probably enough lateral control, but we couldn't be completely certain, so we added a nylon washer on the passenger side outside of the box. The plenum box wall provides both X and Y stability with the retaining ring against the inside of the box and the nylon washer against the outside of the box. There is no retaining mechanism on the motor side of the axle to guarantee absolute perfect alignment to the motor with no rubbing of the retaining ring with the axle channel.

External Retaining Ring

Fig. 3

We were able to operate this setup with the open motor and verify that the alignment was good and that there was no deflection of the motor gears causing aligment problems and the eventual "clicking" of gears not meshing properly. We have run the solution through multiple tests including extended operation, and the metal replacement provides the strength to handle the force of the motor without rubbing against the axle channel. The HeaterTreater blend door solves the problems with the wear out mechanism of plastic doors that breaks and solves the alignment problems that are a design problem with the OEM doors. The solution is better than the original and will outlast the Explorer.

We started the evaluation in confusion and ended with a good understanding of the design issues with the system and the solution. We spent considerable time with the evaluation and sincerely hope that you will find our work valuable for solving a common problem on the Explorer.

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