Auxilliary Fan does not kick in

[mbworld]

The auxilliary fan on a 1996 S420 does not kick in.
At idle the engine temp rises to 120 degrees but when running the temp is 80 degrees.

I tested the following.

1. I checked the fan separately and it functions in both directions.

2. I replaced temp. sensor top of the engine manifold.

3 Before replacing the temp. sensor I hooked up the car to a SNAP-ON diagnostic tool and it read negative 47 degrees. I did not check the reading after that.

4. I replaced the RELAYs just to be sure.

Is there anything that I am missing?

Thanks.