Jimm 8/28/2012The overheating condition may be related to; leaking coolant, coolant mixture is incorrect (not at recommended 50/50 water and anti-freeze), faulty thermostat, faulty / clogged radiator hoses, clogged / restricted radiator, faulty water pump, clogged / restricted cooling passages in the engine, radiator cooling fans faulty, faulty radiator fan clutch (if equipped), faulty radiator cooling fan temperature swich, fan belt stretched or slipping.
HouseCallAuto 8/28/2012When the AC is turned on it causes the coolling fans to run continuously, so that tells me that your cooling fan opertion is what is causing your running hot problem. If you had a cooling system component that was bad like a thermostat or a radiator, then turning on the AC would not reduce the temperature, simple deductive reasoning. All that being said, you have to find out why the cooling fan will not apparenmtly turn on while the car sits idling in park while the temperature gauge rises. I would replace the Engine Coolant Temperature (ECT) Sensor first because it is a cheap part that is easily replaced and if that does not solve it I would then replace the Radiator Cooling Fan Resistor.