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.
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.
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.
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.
HeaterTreater
