Why dose my 94 Chevy overheat if I run the ac ?
Check for blocked air flow draw through the radiator, dirt, bugs, etc. AC cooling fan operation, make sure it is running the right direction.
Even if thee cooling system is proper & clean, it could be near to becoming defective. Eg., a deteriorating pump impeller, old thermostat, bad radiator cap. The AC adds load to a running engine and that alone could cause overheating. Have the cooling system checked out by a mechanic; he'll probably find at least one of the problems I listed.