Ok, so this means the FCM is communicating with the PTCM.

Either the "new' FCM is bad OR the PTCM is bad.

But there is one OTHER possible answer: The PTCM is getting bad data from a sensor and it turning the fan on to try to cool the engine. I am reminded that, somewhere, I read that the sending unit for the gauge is NOT the sensor the PTCM uses to determine engine temperature. If THAT sensor is bad - there you go!

100/80 - am I right about this? (I'm almost 100% certain I am.) I have an ultragauge on my car and it's "coolant temperature" indication does not quite follow the gauge. I'm thinking the gauge is way too 'damped' or I am seeing the evidence for two sensors.

(You can put a scope probe on the PWM line and look at the length of the pulses and know right away what the PTCM is telling the FCM. Lotsa luck finding a guy at a dealership that has an oscilloscope and knows how to use it.)