Sorry to resurrect an old post, but I'm seeing a similar cooling problem on my girlfriend's 1988 190e 2.3. I was planning on replacing the thermal switch, and when I looked under the hood, I had one red sensor and one blue sensor, both 2-prong, just like in the pic posted originally by Machnumber2.
I looked in the Performance Products catalog, and they list both sensors. I was just hoping someone could confirm Matt's reply - that the red sensor is the 100 deg. fan switch, and the blue one is for aux. fans.... The catalog doesn't specify which is which, I'll probably just replace both but wanted to be sure before I ordered anything!
I apologize if the answer may be buried in an old post - I found this one by searching, but can't play for too long with the boss always over my shoulder!
Thanks in advance,