The fan runs as slow speed when the A/C is on, and will run at high (or at least HIGHER) speed if the engine gets too warm. The needle will be just past halfway when the engine is getting "too hot".

Also:

1) The FCM, under command of the ECM, is a variable speed controller. But it's the ECM that tells the FCM what speed to run. Running at slow speed when the A/C is on is a default, to insure that SOME air is moving over the condenser when the car is stopped.

2) A faulty sensor telling the ECM that the motor is too hot will make the fan run at high speed.

ALSO, if communication breaks down between the ECM and FCM, the fan will run in "emergency mode" which is high speed. When the communication breaks down, the ECM will also see the breakdown and light the light on the left side of the dash - it looks like a radiator.

If you have no light, this would indicate that communication between the FCM and ECM is probably ok AND that the FCM is likely ok (since the ECM is "talking" to the ECM enough so that the ECM is not lighting the fault light.) I'd lean towards the problem being a bad temperature sensor or bad ECM itself - especially since you indicated the FCM has been replaced.



I hope this information helps.