Forgive me if this has been asked and I can't find it. 

 

I am having trouble with my blend knob... Starting at around the 12 o'clock mark, one 'click' to the left and it will freeze you out, one 'click' back to the right and you get roasted out. Very Cold or Very Hot! no 'blend' at all... 2 different controls do this, really always has since I got the truck but I'm getting annoyed by it.

Head over to heater treaters and get the blend door shaft. I'm pretty sure the plastic shaft broke and it not controlling the door correctly. Fairly easy to install. Pull the carpet back and cut a little square out of the foam padding. Using a Phillips screwdriver bit and a 1/4" wrench and remove the screws. Follow the calibration process of the gears and install again. It will work right again.

5 hours ago, Mopar1973Man said:

Head over to heater treaters and get the blend door shaft. I'm pretty sure the plastic shaft broke and it not controlling the door correctly. Fairly easy to install. Pull the carpet back and cut a little square out of the foam padding. Using a Phillips screwdriver bit and a 1/4" wrench and remove the screws. Follow the calibration process of the gears and install again. It will work right again.

My 3500 and my brothers 1500 both had problems with heater blend, both were the actuator but both had just failed to work at all, shaft was ok on both

 

Seems odd though that such a small movement of the control moves the door such a big amount, I'd be removing it as you say and checking it's not broke then if it isn't plug it back not fitted  to see what is happening.

On my brothers some clown had removed it before and dropped what I presume to be the screw that is behind the actuator (cos that's a mule to get back in with big hands)  It was right under the carpet sitting there waiting to be found

The little kit that comes from heater treater has a 9V battery clip for testing the motor on the blend door unit. 

 

Most of the time the shaft breaks so the motor just keeps running push the blend door full hot, then one tick back it reverses and keeps turning making it cold. I had this exact issue last summer with a customer. The plastic shaft broke flat so it attempt to work but the motor kept turning so either full hot or full cold no middle ground. This is due no sense of limit (hitting a stop).

 

Temp fix I did was pull the blend door motor set the temp for the customer by just moving the blend by hand. 

 

Then when the heater treater came in installed and tested making sure it hit it limit and stop. Be aware there is timing marks inside the blend motor if it not in time it will behave very strange. Yeah I've skip that step once and had the timing of and it foul the limits would almost act backwards cold is hot and hot is cold as it attempted to adjust temperature.