Can you tell if a car has been converted to use R-134a freon?
There are two ways. The old R-12 A/C's have a "screw thread" style service fitting for adding refrigerant. These resemble a tire valve. R-134A systems use a "quick connect" type fitting with no external threads. These fittings are on stock R-134A and R-12 systems that have been retrofitted to R-134A. The purpose is to prevent someone from charging R-12 into an R-134A system and vice-versa. Also, retrofitted A/C's should have a label saying "Retrofitted to R-134A," or something to that effect. The label should also say the freon charge weight and type of oil in the system, usually a polyol ester oil (POE or ester) in retrofitted systems and a polyalkylene glycol (PAG) in stock R-134A systems. Look for both the "quick-connect" style fittings and retrofit labels on the vehicle in question.