1986 Mercedes Benz 190e

Engine Cooling problem
1986 Mercedes Benz 190e 4 cyl Two Wheel Drive Automatic

Hello, my boyfriend and i took my water pump out and put a new one in and there is a senser that appears to have gone missing in the day that the vehicle was sitting there. I was just wondering if you could possibly tell me what the sensor is and where it does to........




the location on the water pump where it was located is where the arrow is pointing too

also if my battery is completely hooked up (negative and possitive) my battery will drain within a matter of hours..... where would i begin to fix that