The thermostat does not control the fan, it only controls the flow of coolant.

The Fan Control Module (FCM) is not a relay, it is a control module and it provides Pulse Width Modulated 12 volts to the fan motor.
It also produces pulses in response to the pulses sent to it by the ECM. Your coolant light is on because the ECM is not getting pulses back from the FCM. This is because your FCM is bad!

How it all works: The ECM actually determines the fan speed needed in response to the temperature sense the ECM is getting from the coolant temperature sensor and tells the FCM what speed to run the fan at. When you turn the A/C on, the ECM tells the FCM to at least run at slow speed so you have air movement over the condensor when stopped. If the engine temperature is high, the fan will run at higher speed, no matter if the A/C is on or not.

Since the FCM is not turning the fan on, you have no air movement over the condenser when stopped so you have no A/C when stopped.

Consider:
1) You have proven the fan motor is good.
2) The ECM is lighting the "Coolant Light" (many things can cause that light to come on, but ONE is that the ECM has lost communication with the Fan Control Module).


I am not there, but it certainly seems certain that the Fan Control Module is bad. I think you are safe in ordering a new FCM. It is POSSIBLE that the ECM is bad, but if it was so screwed up that it can't talk to the FCM, why is it still smart enough to light the light and run the car?

Get ya a new FCM.

Yer welcome.