Heat gauge

running the ac makes the engine work harder so if it is prone to overheating it will, the reason for it over hearing is what you need to find, weather it be somthing like what way2roll mentioned, or somthing else.  another comon cause of being close to that edge if everthing else is good, is running to hot of a spark plug.  so if you did the plugs and they didn't have the right one and you put a hotter one in it then that can be an issue also.  but realy anything that can make the engine wirk harder can contribute to that.  a idler pully thats starting to go, or a berring in an altanator and do on..