Your fans are controled by a thermostatic sensor that measures the temperature of your coolant. If the fans come on like you say , when you turn on the A/C, then the fans are good, but the sensor is bad, or the wiring to the sensor is bad.When working properly, usually only one fan will operate when the engin is warm, the second comes on with the A/C for added cooling under the load.If it's been awhile, changing your engine thermostat is an inexpensive piece of preventive maintenence, well worth the hassle, but it's not why your fan isn't operating.