You will first need to connect a set of gauges to your high and low side and inspect your system pressures. Are you familiar with the system operation? You will only get A/C function when you have selected a mode other than 'EC'. Have someone select the middle button (solid bottom arrow) and physically watch to see if the compressor clutch engages. I find the clutches are pretty quiet and hard to hear engage so a physical inspection will confirm if the compressor engages.

If it does then you still have some pressure remaining. You may have a leak that has allowed you refrigerant to leak. If it does not engage then assume that your system is likely empty. Do you know if the car is on R12 or a converted system to R134A?

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1989 300CE