Inforamtion on this is available in the archives. Using the search function will reveal quite a few hours of reading.

The compressor relay is black in color, located next to the fuel pump relay (green), close to the ECU and battery, atleast on my 94.

You can remove the relay and manualy activate the compressor.

MB manual has 6 or 7 checks for this relay.

alldatadiy.com has the manual online.

It could be as simple as loose connection at compressor power input, bad relay or even the push button unit.

You can also manualy activate the low pressure switch to see if the compressor comes on.

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Zafar
94 E320 58000 Miles