Functional description, see BMW Service Technology:
TIS -> Document -> SI Technology -> Enter SBT Number
SBT number:
The partial vacuum created in the intake pipe is initiated via idle actuator 2 in the vacuum system. In some operating modes, especially with a cold engine, when the cat. heating function is active there is not enough partial vacuum available for brake power assistance.
The missing partial vacuum is created by an electrical vacuum pump that is activated if required by the DME. The vacuum pump is designed as a vane-type pump. The prevailing partial vacuum is measured by a partial vacuum sensor.
With the vehicle stationary, the vacuum pump is switched on at -250 mbar and switched off at -600 mbar. When the vehicle is moving, the operating points are speed-dependent.