I always thought a negative superheat (evap outlet cooler than inlet) on a fixed orifice tube/accumulator system (say Ford or later GM; NOT mercedes) indicated too much refrigerant (way too much); but that on a TXV/receiver dryer system (our Mercedes'), a negative superheat meant liquid refrigerant had a HIGH probability of "slugging" the compressor since it has no accumulator on the low side to "catch" the excess refrigerant and was to be avoided like the plague. A positive superheat of 4+ºF is the minimum I've heard, so that in low load situations, you did not have the possibility of liquid refrigerant returning to the suction side of the compressor. FWIW

jimandsuzy,

Four Seasons (not that I like the company) official retrofit document calls for 85% of the R12 weight to be used in an R134a conversion. Some techs like a lower amount. The amount of a retrofit refrigerant is directly related to its molecular weight and, oddly enough, R134a has a M.W. that is ~84% of R134a. I think you are overcharged if there is 48 oz. in there now.