Just run 12v to the compressor to get it to engage. As far as your ac being empty, but you want to make sure everything works.....don't know how you plan on doing that with an empty system. If it's been empty for a while you might as well replace all the o-rings, expansion valve, the high/low pressure switches, and receiver drier. You can get all those parts for under $100 from Pelican. If you are unfamiliar with working on an ac unit, I would suggest reading up on it first and watch a few youtube videos on how to vacuum down the system, check for leaks, etc., etc.

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1990 190E 3.0L