the coolant switch/sensor on my car (1990 2.6 190E 6cyl) is in the top center between the valve cover(left) and the air filter assy(right), its right there in the the block. Visually, It should have a BLACK top(plug) with(2) wires coming out and a BLUE body which is the sensor. slide the plug UP a little (car running) w/o fully taking it off, then place a screwdriver between the contacts (shorting) if the high speed AUX FANS do not come on as they should, the coolant sensor is BAD (cheap fix) If there is a clicking noise to your upper right and NO fans come on (as in my case), the AUX FAN RELAY is bad (another cheap fix). NOTE:If you don't have the relay with the fuse in it you should replace it anyway, the one in there is out of date!

I guess in some instances BOTH could be bad, But for me that is where I started and it was great advise I got right from HERE.The folks here saved me a lot of money by doing just what I am with you. I found my answers by SEARCHING KEY WORDS, they will help 90% of the time! not doing so could get you no answers to really common questions(it happend to me once )

At any rate check your relay in the FUSE BOX in the upper right hand section of your 190E (under the hood) in my car there are 2 fuses boxes(one had ceramic fuses(upper) the other relays(lower). While standing in the grill/radiator area and reaching HIGH right the fuse box closest to you is the one contaning the relays.

If you go to the link I placed above there is a picture of what your looking for. Here it is exactly as shown on the COOLHARNESS SITE

http://pages.prodigy.net/jforgione/MB_CTS.html

Hope I helped a bit!!!