I KNEW you would know!

Ok, riddle me this: how does the ECM know the circuit is open? I could tell it knows, because it turns on the low coolant light - close the switch, light goes right out. The only way I can see, is that there is a load resistance in the Fan Module and the ECM can sense the lack of load.

Or............ (and I haven't checked this with my scope) the pulses I saw before the mod were not all from the ECM. Perhaps the Fan Module sends pulses back along the same line to the ECM?

Not that it matters, but have you ever scoped the PWM circuit with the line between the two open?

EDIT: Just saw your last sentence. ROFL! I KNEW IT!
Are your modules for sale? Me first, everyone!