Hey guys

I have quick AUX fan question.

So the aux fan in my '06 3.0D is not spinning when the AC is on or when the coolant hits 85c. However I dont think it is the fan itself, as it DOES spin half a turn when the car is started.

So i figured I would call on the vast wealth of knowledge and experience here before wasting hours chasing the issue. given the symptoms, what would be the most common cause of the fan not running when it is supposed to, even though it has power and does spin when the car is started?