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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