Cooling fan Troubleshoot

my cooling fans only comes on if i turn the a/c on, why their not coming on automatic i checked the relays. what the problem please help..

Check and replace the temp sensor located on the radiator, somewhere below the radiator cap. On some of the engine sizes especially the 3.8 the sensor is located on the thermostat housing. This is what normally activates the fans. It has probably gone bad and not turning them on when needed.