The following is a quote from the Dodge service manual:

Each blend door actuator is connected to the heater-
A/C control module through the vehicle electrical
system by a dedicated two-wire take out and connector
of the HVAC wire harness. The blend door actuator
can move the blend air door in two directions.
When the heater-A/C control module pulls the voltage
on one side of the motor connection high and the
other connection low, the blend air door will move in
one direction. When the module reverses the polarity
of the voltage to the motor, the blend air door moves
in the opposite direction. When the module makes
the voltage to both connections high or both connections
low, the blend air door stops and will not move.
These same motor connections also provide a feedback
signal to the heater-A/C control module. This
feedback signal allows the module to monitor the
operation and relative positions of the blend door
actuator and the blend air door.

I hav problems with my hvac mode doors but they are not broken yet, my recirculating door is broken.
With servo motors, when it is given a command for a position ( floor, vent..ect..) it will move/rotate untill the control module gets a feed back signal and will stop in the correct position. A servo motor has a lot of torque for their size. If the feed back signal is not recieved, the sevro motor will keep rotating until it gets against something that can stop it. (broken stops on the actuator)
I have been reading the problems that other people are having and noticed that some people have replaced the doors more than once.
i have no proof that this is a program problem and/or a electrical connection problem with the feed back signal. I have a little experience with servos, not much, but i have an understanding.

In our truck's hvac system the servos should turn no more than 180 degrees of rotation in one direction ( 360 degrees is one complete turn of the servo).
For example let's say that the vent only position is 90 degreesof the servo rotation. The mode selector switch is moved to the vent only position. Then when the servo moves to it's known 90 degree position it stops.
A three-wire DC servo motor incorporates a DC motor, a geartrain, limit stops beyond which the shaft cannot turn, a potentiometer for position feedback, and an integrated circuit for position control.Of the three wires protruding from the motor, one is for power, one is for ground, and one is a control input where a pulse-width signals to what position the motor should be. As long as the coded signal exists on the input line, the servo will maintain the angular position of the shaft. As the coded signal changes, the angular position of the shaft changes.
I may be out in left field on this if so i am sorry.

Sorry for the long post

 

bringing this back up.

This actually makes a lot of sense. I just started having the blend door issue yesterday (except it is blowing hot air out of the vents all the time, regardless of the **** position).

It seems to me that using stops to limit the servo movement was a bad idea. Perhaps a way to minimize the failure rate is to NOT turn the ***** all the way to the end of their range. This might prevent the servos from going 'all the way' and stripping and/or breaking the doors.

thoughts?

 

I don't know if this helps or not but I turn the ***** very slowly in hoping the motors will turn equally in speed. So far I have yet to replace a door (knock on wood).

MikeyB

 

I turn the ***** with my fingers crossed, and never on Friday the 13 th, no blend door problems yet. ;-)))

 

and never make any changes with the blower on, at least not on high speed,

67K miles so far and no issues......knocking on wood.

 

My door does not work properly if I move the controls fast or if the blower is on high. I now turn the blower to low, then move the ***** quite slowly to the desired position, then bring the blower up to the desired speed. If it still gets squirrelly, I turn all off, select the position I want, then turn the blower back on to low until all settles in, then up the speed. This normally takes care of it.
Bob

 

This is pretty sad, we all hold our breath every time we move a **** wondering if it will cost us big bucks to get the air where we want it. Who designed these things? I hope they are proud of themselves. They want a bail-out so they can keep building this way?