DHB
Regarding the small round switch on the passenger side, my expertise does not lead me to explain it but my experience does. After installing my "specialized metal gasket" I removed the excess vacuum lines on my 240D, including the one running from this small round switch back to the thermo area. On a two mile run afterward the temp gauge behaved strangely so I stopped, retrieved an excess vacuum line from the trunk and returned it to the round switch and thermo area, attached a blocked piece of vacuum line to the other nipple in the thermo area and my temp gauge is reading accurately again.

Again, experience, not expertise.

My .02

__________________
Rick
Avid Benz watcher

Previously owned 126's: 2 1981 300SD's, 1987 300SDL, 1991 350SDL,
Previously owned 123's: 1978 300D, 1983 240D, 1985 300D
Previously owned 124: 1994 320TE

Currently:
2004 Toyota Sequoia SR5 206k
2010 Ford Edge 122k
Always looking for the next MB diesel!