Chrysler Repair: radiator fans run in high speed at all tine, chrysler concord, coolant temp

Question
Hi I got a 1994 chrysler concord 3.3 l engine. The radiator fans run in high speed at all time. As soon as i start the engine the fans start running in high speed.
I did a self diagnosis on the ATC AND I GOT THE CODE #36 (ATC head communication failure) I dont know if that has anything to do with it. Now if i remember corectly the fans should only run in high speed when the AC is on, but they run in high speed regardless of the AC or heat beeing turned on or off.
Any help would be greatly apreciated.

Answer
Hi John,
You were clever to suspect the ATC and to check it for a fault code. The ATC can certainly cause the fans to come on if it is falsely saying you have asked for A/C. The signal to the fan relay is mediated by the engine controller, and that controller will also energize the relay if the coolant temp sensor says the coolant in hot enough to need the fans. The first thing though you might want to do is check the fan relay to be sure that it isn't stuck closed. I don't have the wiring diagram for your car, but the power distribution center for the '91 3.3L engine in the C and Y body Chrysler cars that I do have show it to be located in the lower right corner of 9 relays in the center. If you can find it in your center, I would remove the relay and measure for continuity across the terminals on opposite sides of the relay. In one measurement you should get a low ohm reading (the coil,near 0 ohms) and in the other direction you should get infinite ohms (the switch which should be open). If you get low ohms on both measurements then the relay is struck closed. If the relay checks o.k. then I would disconnect the ATC control unit (no doubt a connector on the back side), or more easily perhaps is to pull the fuse that powers the ATC, and see if that turns the fans off (but the fans should still come on when the coolant gets hot which may not happen in this wintertime condition!) If the ATC disablement solves the fans on all the time then you will either have to get into the control head and see why it thinks you have asked for A/C (maybe just the A/C push button switch is shorted, or it could be an circuit component failure) or get another ATC control unit at a wrecking yard. I believe the ATC control unit changed in '93 so you may not be able to find an identical unit from a car earlier than that (I know the code numbers did not include a #36 in the '89 model year).
A final possibility is that the coolant sensor has been disconnected or has failed, which would also cause the fans to be on all the time. So check that plug and also measure the resistance across the sensor; it should be 13,000 to 27,000 ohms or so at 70F. But the 36 code certainly makes one suspect the ATC. But it has to be proven out.
So those are my best ideas. It sounds like you already may have a "handle' on this one.
Roland