This is kind of a wierd one. The heater temperature controller works fine on my '97 XLT until you turn it all the way to the stop (right) on the "hot side", at which point it starts blowing cold air. If you back it off to the left just slightly, it goes to full hot again. Not a huge deal, but it's kind of a pain. What gives?

 

I don't have an answer to your question, but I do have a similar problem (See my thread re: Vaccum leak). I haven't had time to look for my leak, but based on your post, I think I'll try backing off the heat control and see if mine does the same thing. If it does, it would have to be a leak at the **** end of the system. Do you know if the 97 is similar design to the 98? There is yet a third thread concerning the panel/floor/defroster not working right. I know there is a problem with the blend door failing, but these all look like the vacume tubing or a "T" connector. Maybe becoming brittle and cracking?

UPDATE 2/13/03

Well, I tried backing the **** off a little and it didn't make any difference. My problem is getting worse, requiring higher and higher RPMs to maintain enough vaccum to keep the blend door closed.

 

Sounds like you definitely have a leak somehwere. Unfortunately, I've never delved into the HVAC on these beasts, so I haven't got a clue where you should start looking. Let me know if you find anything out as our problems might be symptomatic of a common vacuum problem. I'll do the same. Good luck.

 

<sheepish grin>

Well, I posted at flatratetech.com and got two answers real quick. Seems the blend door is electric, not vacuum. I was advised to check my coolant level. Sure enough, it was low. Added some water, heater working fine. Now in the process of trying to find Ford "compatable" coolant, as I don't want to do a complete exchange right now. So far, all I can find is DEX (GM) compatable.

You might try posting your problem over there. Lots of Ford techs who are very free with their advice.