I have a 94 Honda Civic–it overheats when the a/c is on, but only after about 20 minutes. It’s a puzzler, though. Coolant is full, as is overflow tank. Water pump is only 1 year old, less than 10K miles. No coolant being lost. A/C works fine. BOTH fans DO run properly. New thermostat. Temp gauge works. Car does NOT overheat under any other circumstances and needle does not rise above halfway until car has run for about 20 minutes, even though it reaches normal operating temperature quickly, as it should. I’m puzzled!!
Define “overheat.” Does it boil over and lose coolant, or does the gauge just read higher than normal?
Using the AC puts additional heat on the radiator, since the AC condenser is in front of it. Perhaps the flow through your radiator is marginal, and the additional heat from the condenser is enough to overwhelm it.
You said it does not lose coolant, so it’s not really “overheating,” it’s just running hotter than normal. I’d wonder about the radiator, especially considering the car’s age.
Define what exactly makes you think it is overheating?
I am thinking radiator as well. Is it possible that someone installed a fan blowing in the wrong direction?