Engine coolant is starting to overheat. Doesn't matter if the A/C is turned on or not. Normally the elect. cooling fan runs more frequently when it is turned on. Doesn't cause temperature to rise when at highway speed...just when idling at a long traffic signal. Radiator coolant is o.k. Coolant level in reserve tank is o.k. No visible leaks pooling under vehicle. Coolant is 50-50 strength. No leakage coming from hoses.