My 2002 S-500 drove me crazy with that. Yours might operate the same as mine, here is what I learned...

If the CCU senses something wrong, it will lock out the compressor and turn on the A/C OFf light. In my case, the light would not reset until the shop used the diagnostic computer to clear the codes. It did not display a code, but clearing them reset the CCU. They said my firmware of CCU has an error condition that doesn't display properly. The newer version CCU presents the code properly.

In my case, it was an intermittent Evaporator Temperature Sensor. I think low R134 will show the light as well.

Good luck! These things have become exponentially more complicated than they were just a few years ago!

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02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)