Why does a Nissan 240 SX start overheating when the air conditioner is on?

This is symptom of a clogged radiator. Clean the exterior fins on the radiator. Sounds like it is time for a complete cooling system flush. Replace the thermostat and any defective hoses at this time. Re read the question. This is about the AC cooling flow, not the engine cooling flow. Have updated the AC system to r134, now AC over heats - not engine. the question seemed straightforward. the question doesn't say the ac is overheating. the question says the "Nissan 240sx" is overheating which traditionally means the engine is overheating. but i digress.... so, you converted to 134a (which i did to my 90 240sx also, and it works amazingly well) and you don't get ANY cooling? or it starts out cooling each time but fades? or it cooled at first but doesn't cool anymore? lots of possibilities. if you have any free water in the system, it will freeze in the expansion valve after the system runs a while and block refrigerant flow which stops cooling. if you don't have enough refrigerant (a leak) it will cool at first, then fade or cool right after the conversion and than stop cooling altogether. you (like i) probably didn't change out the suction pressure switch which is calibrated for r12. it takes much less r134a in a system to keep that switch made so it doesn't do much for you for low refrigerant pressure. I am having the same issue and have done all the usual suspects, changed my thermostst, radiator cap, water pump and a rad flush etc. I now suspect the electric fan relay, this turns on the radiators electric fan whenever you turn on your a/c. If it does not, I expect the engine cooling system would overheat from the additional thermal load. Good luck.