Ford Repair: 97 exporer heater on all the time, failure mechanism, air blows

Question
Hello,

We have a 97 explorer 4dr xlt V8, AWD, auto transmission and manual heating control.  This weekend it developed a problem with the heater.  Hot air blows from the vents regardless of where the temp control (rotating selector knob) is set - red or blue zones.  If the air flow selector is set to AC then the air will cool.  A qucik phone consult with the local dealer says the doors in the heater housing are probably broken and a complete housing/core is required at nearly $1000.  Operating the flow selection switch seems to generate appropriate sounds for the vent doors opening and closing.  When moved to "off" the vacuum actuated heater hose shutoff operates properly although it never actualtes otherwise.  1) is the temp selector an electronic sensor or analog mechanical adjuster? - or in ther words, how is the heat temperature actually regulated?  Is there a "typical" failure mechanism or part or is the dealer correct in the costly estimate.  What can I do to check / verify before I take it in?

Thanks,

Todd  

Answer
Hello and thanks for the question...I'll try to help.
 I wish I could tell you the dealship is pulling your leg, but they aren't.... They will have to replace the Evaporator case, which also holds the heater core. This process is about 4-6 hour process plus the cost of the part. There really isn't anything for you to check out, before bringing it in.
 The Temp is controlled by a blend door (which is what has failed), controlled my a small electric motor on top of the case.
Hope this helps,
Erik