Engine Fan will not engage

Complete noob question here, Have a 2000 Fld120 with Detroit 60, and the truck will over heat since the fan will not come on.
I removed what I think is the frame clutch sensor, and the fan will kick on fine. In other words, the fan works fine, its just not receiving the signal from a certain sensor?

Any suggestions on what I need to replace?

 

My problem is I don't know the name of that sensor to order it. Its located on the passenger side of the engine, and seems to be bolted on to the engine itself.

I also, didn't mention that the alarms inside go off, when its disconnected, even though the fan is now turning.

I guess I can try taking it out and asking for replacement of this here doohickey

 

Mechanic hooked the fan up to the thermostat, so it will auto turn on when thermostat is activated. Also, had him install a manual fan switch. He stated the signal came from ecm,cheaper than replacing the ecm.

Works fine, ready to roll again.