The aux fan on a 300SDL has two modes; it comes on through a resistor when there is appropriate pressure in the receiver/dryer. Jump the pins with the smaller connectors to test the fan. I think it's the brown wires and slim white insulators, the kind you can't just peel back to make contact. It also comes on at 105C based on the temp switch hear the upper radiator hose.

The fan should come on whether or not the AC compressor is working if the engine gets to 105C, and it should come on when the AC compressor is engaged. So it's a semantic thing to say that it is or isn't associated with the compressor.

My apologies if I caused the confusion.

To toknow's point, it might make more sense to test for a controller signal at the KL port of the Klima relay socket, upstream of the relays and interrupts. That would be the port between the blank port and an end port; row furthest from the windshield, second from right. The SDL gets a ground signal from the controller to engage the compressor.

The compressor rpm sensor bolts in to the back side of the compressor. I'm 99% sure it's not connected to the refrigerant circuit.

FWIW, my relay housing has the KAE brand on it.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD