what would cause car to over heat?

Hello, The A/C should be able to run any time the vehicle is not climbing extremely steep grades, and never overheat the engine. If you are getting overheating issues, the most common causes are thermostat, leaks, water pump, radiator, or blockage. Diagnosing overheating counts on knowing how to get the car to overheat, how to get it to cool off, and making sure there is adequate coolant/no leaks. Another thing most people miss is the radiator cap. If it is leaky, the vehicle will always run a little warm, if not overheat. How do you know the A/C is running? Are you watching the clutch fail to disengage the compressor, or are you just hearing the A/C fan running?