My 2002 Eldorado engine temp gauge seems to rise quickly when ilding or driving at adjusting speeds. Can anyone tell me more about this?

 

I have a 2002 ETC with only 27K miles which I purchased new. The engine is overheating. I replaced the thermostat & rad cap. I also checked for blown head gasket with an Engine Block tester. No dioxide in water tank. Yesterday I found the cooling fans are not working unless I turn the A/C on. I'm trying to check the sensor switch but so far I have not found any website that tells me how to do that. You can check your fans to see if they come on when the engine is hot.