Cooling fans won't come on as temperatures rises.

Bahamaspete

Hi guys, having a problem with my cars cooling system fans. I've noticed that they won't come on as temperatures rises.
If I operate the a/c the fans come on but they won't at high temperature.
As the car idles I can hear a dip in the rpms, a sound you usually hear on most cars as the fans come on, but the fans won't come on.
I checked the fuses and I've checked the temperature at the lower and upper rad hoses, the thermostat seems to be open as temp between the two hoses slightly differ at long idle.