I'm replacing a melted relay and the wiring *was* activating the fan upon temperature threshold *or* sensor wire disconnect. I don't have the original connection diagram.
Arron Cake shows clear diagrams on http://www.aaroncake.net/rx-7/efaninstall.htm, but he connects the fan motor positive to the relay pin 87 and the fan motor negative to the battery negative. My fan motor positive is connected to the battery positive and the fan motor negative was connected to the relay (not sure which pin).

It was a nice safety to see the fan go on when the sensor wire disconnected, but *maybe* this caused an electrical problem.

My question is: is it possible to wire the relay safely so that the fan goes on when the sensor wire is disconnected and which wire gets connected to what relay pin?

 

something like this???... I assume the sensor(signal) is grounded until the set temp is reached when the circuit is opened and the coil in the relay is released allowing the fan grounding circuit to close which turns the fan on... at the same time if the signal wire was disconnected from the sensor it would generate the same effect as if the sensor reached its preset temprature

 

Your diagram and explanation make sense, but with the sensor-ground being the cool temperature default, wont that create a battery draining circuit between pin 86 and pin 85 when the car in not running?

(Also, I assume that a standard 4 pin relay is being used).