Well, mostly. The FCM gets a pulsed 12v signal for the PTCM (ECM ) that tells it to supply X number of volts to the fan.
The shorted time between pulses, the faster the fan runs.
No pulses, no fan.

 

Ok, here is how it all works:

The ECM sends widely spaced pulses to the FCM, the FCM responds with a pulse each time it gets one form the ECM. If the pulse from the ECM is very short, it means: "Do not run the fan, but are you there?"
The FCM sends a quick pulse back that means: "I am here".

As the engine warms or the A/C is turned on, the ECM sends a wider pulse. The wider the pulse, the faster the ECM wants the fan to run.

Now, if communications break down between the FCM and ECM, this happens:

• The ECM does not get that pulse back from the FCM, it lights the "General Coolant Alarm Light" which looks like a little radiator. Commonly referred to as the "Low Coolant Light".
• The FCM is not getting pulses, so it has no idea how fast to run the fan, so it goes into emergency mode and runs the fan at 100% to protect the engine from overheating.

Which is what you are describing. The communications bus between the two is one bi-directional wire. You can measure the voltage on the wire but that tells you nothing. Grab an oscilloscope and see what you have - just know that NO WHERE in any documentation will you find the specifications of the width of the pulses. At one time, I tried to compare:

• Cold engine - fan not turning
• Cold engine with A/C on - fan running at minimal speed
• Hot engine - fan running around 50% speed

Problem was, I could not find a set of triggering parameters that I could see the difference between the pulse from the ECM and the response from the FCM.

Back to your issue:
It almost sounds like the damn wire (or a connection at a connector at either end or a junction somewhere) has gone intermittent. I say this because it "comes and goes" which means that, essentially, both modules work at times. Of course, iit could be fractured solder on the circuit board(s) in one or other modules. The EMC is, I think, something you can take apart, but the FCM is hard to take apart. I have not seen this in any other car, but I have seen issues with wiring harnesses. Several SRT roadsters have had issues with the "duovalve" in the heater that I and Bob Davis, on separate occasions, traced to the harness.

Perhaps you have a connector at the ECM that is corroded/bad. If you change the FCM and it still does this, look at the ECM. As a last resort, you could run a new wire between the modules. Finding tools that will release the pins in the connectors, and finding the connector pins, may be a challenge.

 

 

 

 

 

Stuck closed thermostat maybe ?