My automatic AC system went kaput a month ago in my 94 850 T-Wagon. No response from the control panel (and it was stuck in full defrost... not good in south Louisiana). I had to pull the fuse for the high blower speed to get it to stop. That was MOST of the time. Occasionally, the defrost would stop and cool air would dribble out. Rarely, when the ignition was turned on, the REC and AC OFF lights would come on and burn steadily no matter what position the switch was in. They did not blink, indicating a logged fault code. I went to the diagnostic unit to check for codes, but it gave no response at all...nothing. I checked the diagnostic unit for other systems and it worked fine. I decided that the head unit must be DOA.<P>I was advised by my local dealer and my personal mechanic that my suspicions were probably right. Then they gave me the price for a new one....geeze louise! (I just spent $4500 on the durn thing...head job, front struts, new radiator, new power steering pump....and now more??? I already have a smoking turbo, which is in line next to be replaced! Not wanting to buy one new without being sure, I found a wrecked 850 at a junk yard and popped its nasty, rusty, algae covered unit in my dash. It WORKED! (sort of). It turned off the defrost and I got cold air blowing out the vents in manual mode. Auto mode did not seem to work, so I asked for a 50% discount and got the unit for $75.00. On the way home I realized that the AUT function did seem to work...except for the fact that the blower would shut off the second the car rolled to a stop at lights and stopsigns. It would not come back on unless you switch to a manual fan speed, then back to AUT fan when the call was rolling again.<P>I gutted the electronics from both units, cleaned up the circuit boards and connectors from the replacement unit and installed them in my car's original housing and faceplate. The only difference I could see in the units were 1)that my original three female plugs on the unit were green, whereas the junk yard plugs are yellow, and 2) the faceplate printing on the junk yard unit was in a slightly different font (REC and AUX) and the fan speed increase graphic was different. <P>The diagnostic unit now works...it spits out almost all the codes listed for the ac system. I reset it and the codes came back. I then reset it as if new damper motors had been installed...it took the 9-9-9 code but the codes came back. <P>My feeling is that the odds that all the damper motors position sensors and temp sensors went out at once would be astronomical. Maybe a ground fault somewhere in the harness? I think the junk yard unit was from a '95....would there be a difference between the units in the signal inputs? I am stumped...any advice would be welcome. <P>BTW...just went out to the car...My kid locked it with the window down and I pulled the lock up and opened the door, setting off the alarm. The alarm usually sets off the horn and lights...but the horn did not sound and there was a really nasty, low raspy alarm sound coming from the dash...the lights flashed fine. Weird! The sound stopped after a minute...<BR>what is up with this car???