89 300SE Heater Seems To Stick
 

When the heater comes on, it seems to stay on and almost run you out of the car at times. Sometimes it doesn't and works perfect. It seems there is a valve or damper which is sticking in the heat mode. Adjustment of the temp control does not make a difference when it sticks.

Does the system use a heater control valve or a damper to direct air over the heater coils and what do you think is sticking. I would like to know how the system works so I can trouble shoot.
Thanks in advance