Heater issues

I am having a problem with the heater controls. The control that is giving me trouble is the temp control. When you start bringing the temp control up to the hot on the dial, the door slams open or shut and blows hot air. But when you turn the knob towards the cold side the door will never open or close and it will keep blowing hot air. The only way I can get it to blow cold is to shut the system down for about 30 seconds and then trun back on. And I have actually had the door slam shut when it has been on the cold setting. I have tested the switch with an ohms reader and it seems to be working. I have also taken one from another truck and still does not work. I know the heater core has a small leak which is getting replaced this weekend. Any ideas on what to check since I will have the dash ripped apart for the heater core?

I found the issue it was a plastic bushing that connects the motor to the blend door was cracked:iceslolan.

Good work. What year truck are you working on?

It a 2000. And if anyone every has this problem, there is a guy on ebay that makes the new bushing out of metal, instead of plastic. And besides if you call the dealership for this piece, you have to buy the whole heater box assembly, which will cost over $200.