Component description for fuel system control unit

P47.40-2112-06

View of rear bench seat

N118 Fuel pump control unit

Location

The FSCU is located in the crossmember under the rear bench seat.

Task

The FSCU carries the internal designation FSCM-EC (Fuel System Control Module for Electronically Commutated fuel pumps).

The fuel pump control unit regulates the fuel pump (M3) according to need and transmits information via the current fuel pressure to the ME-SFI [ME] control unit (N3/10).

Input and output signals

The following input signals are evaluated by the fuel pump control unit and appropriate output signals are emitted:

 Direct input signals

 Direct output signals

 CAN input signals

 CAN output signals

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Direct input signals

- Circuit 15

- Circuit 30

- Circuit 31

- Ground signal "fuel pump ON"

- Fuel pressure sensor sensor signal (B4/7)

Direct output signals

- Fuel pressure sensor power supply (5 V)

- Fuel pressure sensor ground

- Ground (fuel pump)

- AC signal for fuel pump (phase U)

- AC signal for fuel pump (phase V)

- AC signal for fuel pump (phase W)

CAN input signals

- Fuel pump ON

- Specified fuel pressure

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CAN output signals

- Fuel pressure

- Diagnostic data

Design

The fuel system control unit consists of a fuel housing with two plugs and internal measuring and control electronics.

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Function

The function description is subdivided into the following points:

 Fuel pump regulation

 Fuel system limp-home mode control unit

 Diagnosis

 Tank-draining function

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Fuel pump regulation

Switching on of the fuel pump takes place if a signal "fuel pump ON" is received by the fuel system control unit.

This signal is sent redundantly from the ME-SFI [ME] control unit as a CAN signal via the drive train CAN (CAN C) and as a ground signal.

The fuel pump control unit also receives the CAN signal "nominal pressure of the fuel" from the ME-SFI [ME] control unit.

The fuel pump control unit detects the current fuel pressure by means of a voltage signal from the fuel pressure sensor and transmits this information via the drive train CAN to the ME control unit.

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The fuel system control unit evaluates the current fuel pressure, compares it with the specified fuel pressure and actuates the fuel pump appropriately by means of these phase-offset AC voltage signals in such a way that the actual value corresponds to the specified value.

The fuel pump pressure is regulated dependent on the fuel temperature and the engine rpm between about 4.5 and a maximum of 6.7 bar.

Natural gas operation

In natural gas operation a fixed fuel priming pressure is prescribed to the FSCU by the ME-SFI control unit. The FSCU reacts by regulating the fuel pump within a prescribed range.

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Fuel system control unit limp-home mode

- If the signal "nominal pressure of the fuel" is missing a substitute value is created and the fuel pump is appropriately actuated by the fuel pump control unit.

- If the "fuel specified pressure" signal and the signal from the fuel pressure sensor are missing, the fuel pump will be controlled as per a standard specified pressure stored in the FSCU.

- Over the direct signals "fuel pump ON" and "circuit 15", the limp-home function ensures that the fuel supply is also secured for failure of the internal control system.

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Diagnosis

The fuel system control unit conducts a self-test and a component test.

Recognized errors are stored by the fuel pump control unit and sent to the Xentry diagnostics via the diagnostic CAN to the ME-SFI control unit via the drive train CAN.

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If failure of the CAN system occurs, or a faulty fuel priming pressure or a faulty value for the fuel pressure sensor is detected by the ME-SFI [ME] control unit for natural gas operation, then switch over takes place to gasoline operation.

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Tank-draining function

The fuel tank can be emptied over the fuel system control unit without the engine running.

To do this, the tank draining function is selected using Xentry Diagnostics, and the following parameters are entered:

- Parameter 01 for "fuel pump ON"

- Time "0-99999" seconds

During draining, Xentry Diagnostics must remain connected and the ignition must remain switched on.

Variants

The vehicle and engine variants are already stored in the fuel system control unit. After replacing the fuel system control unit, variant coding must be conducted.