Hi all,I am new to this sight,and I have just purchased a 2200 pickup.The a/c doesn't work,the compressor turns on and off.So I am trying to retrofit the system to put in freon.But I cannot tell which is the low side that I need to put the freon in.Any input would be appreciated.Thanks

 

Low side is always on the larger pipe and marked with an S for suction.

 

The low side fitting is 1/4 inch. The high side fitting is 3/16, and needs a Ford-style adapter. If there's no refrigerant, then it leaked out somehow and the leak needs to be fixed. Some use R134a for finding the leak with an electronic sniffer detector. However, I'd keep the system R-12 because it already has that mineral oil inside it, and that would need to be completely flushed out too use with R134a. R-12 is plentiful, even on Ebay, and you can legally purchase it after a $25 online exam and certificate. So R-12 ends up being less expensive, and works better, in the long run.