No problem. On my E320, that nipple connects to the 3 prong vacuum supply connector behind the brake booster (135). I should know since I just replaced the rubber boot connecting a white vacuum hose to that nipple a few weeks ago. See vacuum line at bottom of pic below.



The right-most of those 3 prongs connects to the transmission upshift retard switchover valve that has a green hose going to the transmission retard switch. It retards 2nd to 3rd gear upshifts at coolant temps below 50 C.

The other two lines go to the vacuum reservoir for door/boot/gas locks and HVAC flaps.




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