Ford/mercury diagnostics help
 

FIL has a 2000 merc grand marquis and has a problem with the Electronic Automatic Temperature Control. When put on 65 it blows cold, when put on 68 or higher blows hot. So it's either full cold or full hot, no in between. This leads me to believe that the blend air door is working, just not being controlled properly? I ran the eatc self diagnostics and the unit has no fault codes so I'm leaning towards the control head itself. Anybody have any exp. with these?

Not in a grand marquis, but I recently had an explorer eat my lunch with a similar problem. ( not same symptoms as yours though) . The components location is what makes it so tough to diagnose. Blend door actuator failure is pretty common on this era ford vehicles though.

On GM cars there are small servo motors that move the doors. The plastic gears strip on the motors and the doors go full open or full closed.
Another crappy engineering design on the planned obsolescence list.........
Sorry i cant be of any real assistance.

Had this same problem with my grandpa's Grand Marquis. I took the control unit out and cleaned up the electrical connections, re-seated them and no more problems.

FORD has a simple procedure for this: Trade it in at a dealership and get a new(er) vehicle. At over 10 years since its manufacture date it has exceeded its expiration date.