Well this was fun to troubleshoot!
We've been attempting to fix the blend doors in our salvaged 2002 Jeep Grand Cherokee. Common failure point, and my partner through she'd replace the actuators (little motors and gear drives that move the blend doors) while we had the dash apart.
Installed new actuators. Vent selection actuator (floor, face, defrost, etc) worked fine, but the other three didn't work. I ran the calibration test, which should have each motor run through its full range of motion. However, all three turned in one direction, continued to turn while jumping teeth, and then stopped and threw error codes.
These are two-wire, so there's no hall effect sensor or similar. Also no sensors on the doors/blend box to tell the actuator when to stop. So I figured the HVAC computer had to be sensing current draw (electric motors will draw more current under load). When draw spikes, you've hit the physical limit of travel.
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