One interesting vacuum interface occurs on cars with the "REST" function on their AC controls.

The rest function allows one to drive somewhere, shut the cars motor off and sit in the car with the climate control using the engine heat for heating the cabin as prescribed on the controls; 72deg, etc.

To do this function the auxilary water pump keeps moving coolant from the warm motor to the heater core and the VACUUM is used to control duct function as commanded. This wouldn't last long from a car sized reservoir so the PSE module (vacuum pump, etc), pulls down the system while checked from the engine vacuum system which is the source when running and open when not.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician