Originally posted by pesuazo
Another possibility is the relay sticking in the closed position.
GregS and Arthur Dalton have great suggestions (although I think is the opposite, the sensor drops its resistance as temperature rises, hence the sensor could be electrically "closed" at all times).
Unplug the temperature sensor, if the fan is still on, then the relay is likely at fault.
As you say, The sensor is , in fact , a Negative Coefficient Thermistor. [ resistance drops as temp increases] . And the circuit design ask for a closed circuit at all times... when the resistance gets to spec/temp, [ in this case around 185 ohms--don't hold me to that], a switching transistor in the control panel gets triggered to complete ground to the fan relay.. coil side]... BUT, the circuit design also has a default setting that , if any OPENS are detected in the sensor circuit, the control goes into default and triggers the relay for fan ON protection. [ this is a safety feature that protects the engine from overheating in the event that the temp sensor/circuit has broken/opened, unknown to the operator...eliminating the possibility of engine failure due to overheat protection via fan motor operation]
That is why a TECH fan test for coolant temp fan electrical integrity is to remove the connector of the temp sensor and look for fan operation...OPEN-CIRCUIT activation by default..
So, yes, stuck relay contactors can cause continual fan, but more likely , the control has detected an open in the sensor circuit.. be it open sensor or bad connections....