Chrysler Repair: ATC defaults to defrost: : 95 LHS, 1995 chrysler lhs, diagnostic capability

Question
A 1995 Chrysler LHS with the ATC defaulting to defrost.  Hot air constantly blows out of the defroster (on high). No way to shut it off. Air conditioning would be good, as we are in the middle of a heat wave in Montana.  Mechanic told her to replace the ATC unit @ $500.  A large investment, but heat is intolerable.  Could it be anything else?  The A/C did resume working for a few days, but defaulted back to defrost again.  Thank you for any advice you may have.

Answer
Hi Susanne,
If the blower only works on high speed, that is a sign that the blower motor's solid state speed controller unit has a problem. It is located in the duct near the blower motor and has a plug and a couple of screws that hold it in place. It can be replaced, or possibly serviced by someone with electronics experience.
The air diversion and temperature of the air is controlled by actuator motors on several doors of the unit. The doors are controlled by the ATC head unit. But before concluding that the head unit is the problem it would be worthwhile doing a self-diagnostic test which is a feature of the head unit. Here is how I previously answered a similar question:
"Before concluding a new panel is needed a test should be done to see if the computer has recognized any problems and described them in the form of two digit fault codes stored in its memory. Did someone do such a inquiry? If so, what numbers did they get? If not you can do the readout yourself:
The unit does have a self-diagnostic capability. The engine has to be running and the car not in motion to do the test. Simultaneously press the floor, mix, and defrost buttons, (the 1st,2nd and 4th on the top row) and see if the display unit begins to blink. Release the buttons when it has started to do that and then after it blinks for a while longer watch for two digit numbers to appear on the readout. Those are the fault codes. If the first number appears and stays in place, push on the first button in the bottom row (panel) to continue the readout. At the end of the readout the unit will return to normal settings.
Then write back with the number(s) and also tell me more about exactly what is wrong with the function of the unit.*
If the head unit doesn't respond at all, then check fuse #6 under the dash. There is also a 12V supply line to the control that is directly connected to the battery on a black/pink wire on the green plug on the rear of the control unit that should be checked to verify that you are getting that power to the control unit.
I would do a thorough readout before buying a new control unit. It may be a part in the air distribution system itself rather than the computer that is the problem. And you also might find a working used unit at a wrecking yard.
Roland
*The code numbers range from 23-36. You can also get a decoder list at www.allpar.com/fix/codes_climate.html "

Once you get any codes it can then direct you toward which part(s) might be the cause of the fault. Also, there is a Chrysler diagnostic readout box (DRB II) that can be plugged in under the dash and in the hands of a knowledgeable tech do further testing to focus in on exactly what is wrong. It that too hasn't been done then see if there is a Chrysler dealer with the device (and a cartridge that cover the model year of your car) and a person who knows how to operate it to check out your system.
If you get a code let me know what it is and I can check my '94 shop manual and possibly copy some pages to you by xerox and postal mail that will get to the bottom of the problem.
Roland