I know it's an old thread - but... I'm having an overheat issue right now. It only seems to run hot only when ac is on. I thought it might be inefficient aftermarket electric fans but fans should not be the issue if you are at highway speeds. All that air rushing in should be enough to cool the engine with no fans running (maybe not with turbo engines?) - but I'm thinking maybe a problem with the a/c system - particularly the condenser getting too hot because of a problem there. I will attempt to get a better fan - I initially was suspecting the main radiator fan because it is no longer a clutch fan - but without the a/c on - the temperature is fine. Now I'm considering an inefficient ac condenser fan or condenser problem. Also - just realized that if the oem condenser fan is two speed... and I replace with a one speed - which speed will the controller give the single speed fan? low or high? If it was high all the time I wouldn't mind - but if it starts off at low and cannot switch to high - there could be my problem... HELP!