Hi there,
I am wondering if your compressor was even turning - the 90psi means that the ambient temperature was somewhere around 84 degrees. Was that the approximate temperature when you checked the pressure? If it was, the compressor wasn't running. Usually a system charged with R-12 will show around 200 - 240psi on the high side, R-134 system will show 250 - 300psi on the high side. On a properly charged R-12 system the low side is usually regulated at around 34 - 36 psi so the evaporator won't freeze up. On a converted system you will see a higher pressure, around 40 - 45 psi.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C