About a month ago I realized that my a/c went out while i was driving on the highway and over about a week the problem got worse. Dumb me didnt think to look at the temp gauge and realized that the a/c went out when the car started to overheat. We looked at the problem and it seemed that it only overheated when the car was driving for a while 3000 rpms, and checked some things. We were told to check the fan clutch and watching the fan, it sped up when the rpms rose, and it all looked normal. I just heard that the fan shouldnt be spinning all the time only when the engine is revving(not idling) but im not sure about that because it spins all the time. Anyways, we checked the thermostat by separating the upper radiator hose form the radiator and pouring boiling water into the hose with the car hot and running, and the water went in and water/antifreeze came out of where the upper radiator hole. We came to find out the upper radiator hose was old and flimsy and when the engine was revved the upper radiator hose would close up due to the vacuum. Replaced the upper and checked the lower and it was fine. Problem went away briefly but reappeared a couple days later and gradually got worse again. I have now come to realize the engine only overheats when the a/c is on. When the vent is on it runs at normal temp and when its overheating with the a/c and i switch it to vent the temp drops to normal within a minute or two sometimes very quickly. Dont have any idea what the problem and no one seems to know either.

 

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What condition is the rad in? Also check to see if the condenser and rad are not plugged up with bugs and debris. Take a hose and blast out the fins of the rad and condenser. Even a light coat of dust on the fins reduce cooling drastically.

 

You obviously have insufficient cooling. That means some part of the cooling system is operating at less than optimum efficiency. It could be in a couple of places. The radiator might be partially clogged internally or externally. It can be flow tested. The water pump might have damaged impellers and not pushing a sufficient volume through the system. The thermostat might be stuck partially open. That would restrict the flow so that it was just enough to keep you cool under ordinary conditions, but not allow enough coolant through the system to keep it cool while all that extra heat is produced by the condenser and increased engine load. It might even be that the fan is not pushing enough air for various reasons.

The thermostat is the cheapest place to start and has a higher failure rate than either the water pump or radiator. I'd try replacing it first. I had that precice problem with thermostats in my old 4.0 '92 Ranger. They failed one a year for 3 years until I flushed the coolant. They always stuck in the partially open state. The old coolant looked pristine, but evidentally something was in it that coroded or stuck the 'stat.