This happened all at once. Cars been fine. Woke up one morning and it will drive fine with air conditioning on for about 10 miles the start to get hot. (The gage will strt to creep up from middle) I will turn on the heater full blast and the gage needle will go back to the middle. I can drive the car with the heater on full and it will run fine. The gage where it always is. If I turn on air conditioning it will start to heat up. The fans are working ( I have pulled over to check) radiator full of fluid. I have changed the thermostat and the relay also the fuse. Grateful for any help, it's the middle of August.