AC Condenser fan question?

I have a 1999 rodeo 4x2 LS 3.2L. The AC condenser fan doesn't run all the time and I have to tap it with a long extension to get it started. I replaced the fan, but the new fan doesn't run all the time either. Does anyone know what controls the fans turning on and off. My AC works great when I'm driving, but sitting still it doesn't work very well without that fan working. I put power to the fan and it works, but I just don't know what the trigger is to activate it. Any help would be greatly appreciated.