I have a 1991 190e 1.8, but I'll try to help you as far as checking the circuit.Should be the same principle.

You will probably see wires going from the solenoid to a sensor on the engine and then back to the fuse box.

The sensor basically acts as a switch.It closes at a predetermined temperature (107 deg??? - I forgot) and in turn, provides current to the solenoid.

In order to test if the solenoid is working, remove the wires at the sensor (one goes to the solenoid and the other supplies +12V) and short them together,

The fan should work by shorting these 2 wires.Then you know you have a working solenoid.

You now need to test the sensor.

You let the car idle till it reaches approx. 105 to 110 deg C.If the sensor is OK, the fan should kick in.

If the sensor is fried, the fan will not kick in at all.

From what I have experienced with W201 190s and W124 200s, running temp of 90 deg C is good.

In traffic jams, I have seen some Mercs go up to 110 deg C.

But don't ever let it go near the redline (120 deg C).

I'm am not sure if the 190D and 190e are similar in these aspects but I hope you find the info above helpful for what it's worth.

Good luck.