My engine overheats, but only when the AC is on. I’ve changed the thermostat but that didn’t fix it
Sounds like the cooling system on your car is overwhelmed. A/C increases engine load and dumps hot air into the engine compartment, both of which would tend to increase cooling demands.
When it overheats, is it usually in stop+go driving? That would be a good indication that the electric fan is malfunctioning (since no make+model was given, I’m making an educated guess that you DON’T have a belt-powered, “always on” fan.) Open the hood just before overheat and see if the fan aft of the radiator is running. If not, check the associated fuse/relay.
Until you can fix the problem, you can forestall engine damage by shutting off the A/C when it starts to overheat. If that, alone, is insufficient, you can turn on the heater 
(because the heater core is, effectively, an auxillary radiator.)
Are the radiator cooling fans working the way they should? Are the radiator or the AC condenser fins clogged with debris, restricting airflow?
Thanks for the advice. FYI - it’s a 2001 Mazda B3000, 6 cyl, auto.
Has the radiator cap been pressure tested? A cap that can’t hold pressure will cause the radiator to boil over at lower temps. A new cap should be less than $10.
Ed B.
I’ll check the fans, but fairly certain they are working. The AC condenser fins I’m not sure of. That is another problem. The AC works for a while then only cool air. Mechanic said it was the condenser that needed replacement.
I have not had the radiator cap tested. That’s a good inexpensive option to try. Thanks.
In addition to the fans working, the fins being cleaned, and the radiator cap holding pressure,
the thermostat has to be opening fully,
the water pump has to be pumping fully (no eroded impellars),
the radiator has to be fully flowing inside (no plugged core tubes and no unauthorized coatings or buildup),
and the hoses all have to flow freely.
All of these items can be tested.
The T-stat can be tested with a pyrex vessel, a hot plate, and a cooking thermometr…or just changed.
The water pump can be flow tested.
The radiator can be checked for hot spots (clogged tubes) with a pyrometer and can be flow tested.
Hoses can be inspected or inexpensively replaced. With hoses it’s tough, because the lining can seperate from the outside and collapse.