Evacuate any remaining R-12 refrigerant in the system. Pump the system out into a recovery tank or bag until the pressure has equalized with atmospheric pressure. Detach the recovery tank from the system carefully to prevent R-12 from leaking.
Remove the gas stems from the high and low service ports. Install new R-134A input and output stems to reduce the risk of refrigerant contamination.
Attach the nozzle of the PAG oil can to the input stem of the system. Load the system with the contents of the can. Listen for the sound of the oil filling the system. Detach the can from the input stem once empty.
Determine the amount of R-134A required. Locate the amount of R-12 recommended in your vehicle owner's manual. Multiply the amount of R-12 by .9 and subtract 1/4 lb.. Multiply pounds by 16 to convert to ounces when necessary. Round up to the nearest whole number. For example, if a system requires 32 oz. of R-12, then 32 times .9 equals 28.8 oz., minus 1.4 lb. (4 oz.) equals 24.8 oz., rounded to 25 oz. of R-134A required.
Load the system with the required amount of R-134A. Attach the nozzle of the refrigerant can to the input stem and secure tightly. Pump the contents of the can into the system. Disconnect the can from the input stem carefully.
Adjust the clutch cycling switch on the accumulator to adjust for the pressure difference between R-12 and R-134A. Turn the adjustment screw approximately one-quarter turn counterclockwise to decrease the pressure from 25 pounds per square inch (PSI) to 21 PSI.
Affix the label indicating that you converted the system from R-12 to R-134A.