I have a 1985 300TD.
This vaccum switch or controller sits a top the ALDA valve and is right in front the oil filter housing. The vaccum hose you see goes thru the firewall above the steering and is not connected.
The Alda valve itself seems to be connected aNd the car seems to have good power. Also if I suck thru the valve does not hold any vaccum, it seems to just suck air thru it.