check your thermostat. it could be sticking closed. so the coolant is in the system its just stuck when the thermostat sticks closed. coolant doesnt normally burn off. a car that old is going to have leaks even small ones in the cooling system. check hose connections. the ac on will make the car work harder but it doesnt affect antifreeze levels. u can check the fans connections. could be a fan relay? with the ac on both fans should be running. if its off only the radiator fan would be running.