If you are using R12, I wouldn't bother with a parallel flow condenser. On my 84 SD, I installed one planning on using R134a. I can't recall my exact results now I believe I was getting in the mid 40s. I changed it out for R12. Slight improvement. Not enough to justify switching to a PFC IMO.

Only reason I switched was that my old condenser had a leak so I needed a new one anyways and the costs with new fittings on the PFC was comparable. Also I was planning on using R134a. Since you don't need a new condenser and are using R12 I'd stick with the regular one.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD