OBTW, the cooling fan on my 07 5.9L Cummins would run (be engaged) even when the engine was not even warm -- IF you had the A/C on and the vehicle was traveling at slow speeds, like when you take off from a light. When the vehicle speed was high enough to get enough air flow through the cooling stack, the fan cut out.
As stated, the fan clutch in your truck is controlled by the ECM/PCM, not viscous fluid controlled solely by temperature like most mechanical fans. The ECM will engage the fan to add extra cooling to the engine, to the trans, the intercooler, and to the radiator if it deems it is required.
If the engine is hot and you stop moving, and put the truck in Park, the ECM will engage the fan clutch and ramp up the idle to ~1000 RPM instead of 750 RPM (normal idle speed) to better cool the engine. It's a smart system.
