simplest thing could be that it is low on refrigerant. If you car has been "converted" to r134 this is a very likely scenario as r134 cannot be contained properly by the original hoses.

Check the static pressure of the refrigerant. this will tell you if there is any refrigerant in the system.

Short the wire leads to the pressure switch on the dryer. if everything else is good this should cause the compressor to kick on regardless of refrigerant status.

If the pressure is too low the pressure switch will prevent the compressor from engaging.

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Current fleet
2006 E320 CDI
1992 300D - 5speed manual swapped

former members
1984 300D "Blues Mobile"

1978 300CD "El Toro"