What do you mean changed the AC system to r-134???
YOu just drained the R-12 out and put R 134 in? If thats what you did you can't. the system is ment to run on r 12. YOu can buy the stuff that is ment for an R 12 system. I don't know if this has anything to do with you over heating unless the motor has to work that much harder with the 134 in it.
Your AC is working now... which will cause you engine to get hotter. Your AC was probably not working very well before, hence your need to recharge it. When the AC is working, it is moving more heat from the cabin to the radiator area (assumung that is where you have the thing mounted).
Looks like you might want to look into a bigger radiator or something else to keep you cool.
R134a conversions will work in a system originaly designed for R12,but will eventualy cause compressor front seal failure(as Ford is now finding out).Also certain precautions should be taken for safty.Mainly a high presure cut out switch installed in the high presure line(discharge).R134a is not as efficiant as R12 an therefor has to run at a higher presure.In a R12 system high side presure usualy runs 175-275psi.a typical R134a system will run 225-350 and at higher temp 450psi+.Systems designed for 134a use a larger condenser for more heat transfer also.It is very possable that your overheating problem is caused by the conversion,with the added load on the engine from the compressor and the extra heat from the condenser.However I would check base engine causes first.
Why not just run the car without the a/c and see if it still overheats. Running the a/c WILL cause your engine to run hotter, especially in these older cars.
I just had my a/c compressor (65 Tbird) rebuilt by Classic Air out of FL, and they claim these old systems will run R134 just fine -- just not as cold as R12. Obviously you need to put in a new dryer, flush the system and if money exists, replace existing hoses with barrier free hoses. Without barrier free hoses you will likely have a slow leak.
You also need to make sure you use eshter oil (POE), not PAG, and again flush your system to get the old mineral oil out.
You private mailed me a couple of weeks ago and wanted to know if I had some R-12 for sale. You have your private messages turned off in your provfile, and I didn't know how to respond to your private message - computer wouldn't let me. You need to go in there and change it.
Yes I have freon in both small cans and a 30 pound cylinder.