Functional description, see BMW Service Technology:
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Fuel supply system regulated to requirements with variable system pressure is handled by 2 fuel pumps switched in parallel. Depending on requirements, the DME issues a PWM signal with various cycle ratios to the fuel pump final stage. Depending on the cycle ratio, the fuel pump final stage activates one fuel pump or both fuel pumps.
The first fuel pump covers the fuel requirement up to approx. 60 % of the maximum fuel requirement. Here, the fuel pump is operated with pressure control. The DME activates the pump via the fuel pump final stage with a variable cycle ratio.
With a higher requirement (as of approx. 5000 rpm full load), the first pump is activated with 100 % and the second pump added with 100 %. As of a cycle ratio of 98 %, the fuel pump final stage recognises that the second pump has to be engaged. In this case, the fuel delivery pressure is limited by the mechanical pressure regulator in the fuel tank to 6.0 bar.
The activation signal of the DME to the fuel pump final stage is a rectangular signal with a frequency of approx. 500 Hz.
The activation signal of the fuel pump final stage to the fuel pumps is a rectangular signal with a frequency of approx. 5 Hz.
In order to be able to operate the first fuel pump with pressure control, a fuel-pressure sensor is used in the fuel feed.
If faults are detected in the fuel supply, the fuel pumps are run in emergency operation, i.e. both pumps are operated with 100 % activation.
Important! If the DME fully activates the fuel pump final stage, despite the activation at 100 % edges can be detected in the signal. This is necessary to enable diagnosis of the activation line.
The fuel system is airtight, i.e. the pressure is maintained after stopping the engine. After one hour, at least 1 bar must be maintained.