The HVAC uses the PSE pump for vacuum but there is also supposed to be a line from engine vacuum. There are 2 check valves in the lines that meet at the kick plate on passenger side. One from engine and one from PSE pump.

HVAC vacuum control block similar to a W124 but has about 15 circuits. It has a dedicated data bus connection to the HVAC controller (not on the Body CAN). But I think it asks for vacuum from the PSE pump over a CAN message.

Also before giving the car over go ahead and change out the crank position sensor, with a dealer part. I discovered that the engine will just randomly die whenever the sensor goes out. Nice job on MB’s (well really Bosch/Siemens part, they designed the engine management system on this vehicle) to have it just die if a sensor fails with no warning. Not like it absolutely needs the crank position to know when to fire, as it still has a mechanical IP.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech