hello the fan will run all the time is the switch in the cab to override is in the on position-----Air conditioner sensor or circuit is malfunctioning--------Fan clutch actuator or circuit is malfunctioning---Engine electrical ground is malfunctioning-----Intake manifold temperature or coolant temperature is above the threshold to activate the cooling fan.-------or the fan clutch is locked up or seized hope this helpsDR.Cummins40408.7869824884

sure it could be the ecm. The calibration (software version) is out of date or only if someone was in there and changed a setting for the fan. But the ecm controls the fan by receiving inputs from the other components i listed.with out these inputs working correctly the fan will either not come on, go off, or be erratic. look-------The air conditioning pressure switch circuit signals the system that the air conditioner head pressure is high and the engine fan must be engaged. The air conditioning pressure circuit consists of the air conditioning pressure switch signal pin and switch return pin. This circuit is considered “fail safe”, meaning when the circuit is open, the engine fan will be engaged by the ECM.---------The Signature and ISX control system can control the fan clutch activation. The ECM energizes the air valve solenoid which in turn controls the fan clutch..--------