Is the car overheating at idle or while driving and just how hot is it getting?
The main fan should be spinning as fast as it can at around 95-100C. There will be a noticeable difference in airflow, especially when the engine is revved if it is working properly. If not, the fan clutch needs to be replaced.
As far as the auxiliary fan, it should come on at low speed with the air conditioner on when the air co pressure gets to a certain limit. The fan should come on high when the coolant temp hits 107C.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record