Blend air flap actuator motor problem

[Ron in SC]

Vehicle is 1999 ML 320.

Problem:
Intermittently I can not turn heat completely off with temperature control knob.


I removed lower dash so I could get a look at the blend air flap actuator shaft and the linkages that attach to it. Right now the linkage moves thru its full range of motion with the heat shutting completly off as it should.

What else might be the source of the problem I'm experiencing? I did have another complete heating and A/C switch unit so I swapped them out and that did not make any difference.

In the photo what is circled in red is the shaft and in yellow is the linkage.