Fuel Delivery and Air Induction: Description and Operation
GF47.40-P-2001MGA Component Description For Fuel System Control Unit
ENGINES 271.861 in MODEL 172.4
Phantom view of fuel tank
N118 Fuel pump control unit
Location
The FSCU is located above on the closing plate of the fuel tank.
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
Direct input signals
- Circuit 15
- Circuit 30
- Circuit 31
- Ground signal "fuel pump ON"
- Fuel pressure sensor, sensor signal (B4/7)
Direct output signals
- Power supply for fuel pressure sensor (5 V)
- Fuel pressure sensor mass
- 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
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.
Function
The function description is subdivided into the following points:
^ Fuel pump regulation
^ Fuel system limp-home mode control unit
^ Diagnosis
^ Tank-draining function
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 redundant 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.
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.
Depending on the fuel requirement, fuel pressure is varied from about 3.8 bar.
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 signal "nominal pressure of the fuel" and the signal from fuel pressure sensor is missing, the fuel pump is actuated by an established permanent pulse width modulation signal (PWM signal).
- If both signals "Fuel pump ON" are missing, the fuel pump is actuated upon recognition of "circuit 15".
- 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.
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.
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
- Specified fuel pressure 3.8 bar
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.
