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Switchover vacuum valve of a Mercedes-Benz 300 D
The vacuum is needed to control the automatic transmission. On this particular model the vacuum controls both the shift points and the quality of the shift (hard or soft). When you don't press the accelerator, full vacuum (-0.75 bar) from the pump is fed to the transmission. When you press the accelerator the switchover valve connects the transmission to the output of the vacuum control valve on the diesel pump. The vacuum goes to -0.4 bar, the transmission shifts from 2nd to 1st gear. If you press the accelerator further, vacuum goes to 0 bar at full throttle. Less vacuum means later and harder shifts. Of course all these vacuum parts can fail after 30 years and then the shifts become hard and/or erratic.