1997 Explorer - No heat control

My Explorer has full heat all the time. No control of air box damper using the heater control. I'm suspecting a loss of vacuum, bad vacuum switch or
vacuum motor, etc. Before I dig in....does anybody have any suggestions for common causes of this problem?
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By saying "no control of air box damper" do you mean that it won't switch between heat, defrost, vent, etc. or that you can't control the temperature?
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Here's a follow-up to my own posting. By doing some more internet research, I found that this is a very common problem in '97-ish Explorers & Aerostar's, etc. The constant heat (or cold) is caused by a broken drive mechanism of the air mix damper door. I found a few websites that discuss this problem which indicate that the repair could cost upwards of $1000. However, a simple fix was outlined on several websites that show you how to do a modification to the damper door servo drive by drilling a hole and installing a drive pin (common 3 penny finish nail cut to size) on the servo shaft. I tried this today and it works fine. Time will tell.

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