As an after-thought.
To accurately test the gauge, simply purchase a couple of 1% tol. resistors [ R.Shack-couple of $- May have to do some combinations to get correct ohms] of the proper ohms by MB Docs temp/resistance chart and hook them up instead of the sensor . Turn the key on and see what the gauge says. That will tell all.
Guessing at temps and varying resistance values of the sensor/thermistor is hard to nail it.
As Dale has mentioned , there is a change as of 8/94 in the other ETCs on the E320s 124s. They have combined the FI-ECU temp unit with the aux fan one into a four prong unit.
So for those interested in the Jim F/IBenz aux fan mod., the new aux fan pins on the 4 prong will be 2&3. This is where the mod resistor will parellel. This ETC has nothing to do with the above gauge sensor circuit.