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To make vacuum.
Diesels have no manifold vacuum as gas engines do. That is because they have no throttle plate (atleast not since the 76 240D). Vacuum is used to add power assist to brakes and control numerous actuators.
There is a pump driven by the motor for brakes, AC, and vacuum shut-off. The SD (depending on year) also has an electric vacuum/pressure pump for the door locks.
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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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