My 2011 GT Mustang had the clicking failure for a long time. It would go CLUNK - CLUNK - CLUNK about four times every time I started the vehicle.
As far as I could tell the AC worked fine, except for the objectionable noise. There was obviously something wrong, maybe the dual zone control was not operating properly, but this was almost always a one-passenger vehicle.
This was just analyzed and repaired by me.
There are three blend door motors on my dual-zone HVAC system. Parts Listing
The glove box was removed. With me under the dash, the vehicle was started, to make the sound. I felt the strongest vibration from the passenger side front blend motor, so that is the one I yanked.
Of course Ford made one of the screws almost impossible to remove. The other two came out no problemo using a 8mm socket w/ 6" extension. The difficult screw required a combination wrench an 1/6 rotation turns to get it out. (That screw replacement required a magnet tool to get it started. My bet is that the dealership NEVER replaces that screw.)
Once the blend door motor was out I opened the motor to verify the failure. One tooth was gone on the final gear. Of course, the highest torque / lowest speed / most gear stress failed on that plastic gear.
The dealership parts department confirmed they have LOTS of this failure. Also a service guy started my car one time for another problem and announced I had the blend door motor failure. The listings for this part on eBay have sold thousands of these parts.
The new blend door motor was disassembled and appears to be identical to me, with NO design improvement to prevent this failure.
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