Just to add to what mach4 has said, spinning the pulley is easy because the clutch must be engaged before you will be able to spin the compressor. You would likely not be able to spin a good compressor by hand.

I have converted a couple of R-12 systems to 134a, and while I was pleased with the results, it requires an understanding of the system and it's operation. The old oil is not compatible with 134a so the old oil must be removed and new PAG oil added, a new drier is typically used, and new green O-rings used.

Since yours was not being used when you bought it, there is likely a problem with it of some sort. I believe it would be better to take it to someone who can diagnose it and sort of watch, and ask questions, before you work on it yourself.

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'97 E 300 D