Fuel System Control Unit, Basic Function - GF47.40-P-9890A

Engine all (4xWD)

Engine all (CAR)

Fig 1: Fuel System Control Unit Components Location - Engine All (Car)

Courtesy of MERCEDES-BENZ USA

Example illustration: fuel system control unit

Overview

This document contains information on:

General
Function
- Controlling the fuel pump
- Fuel system control unit limp-home mode
- Fill level evaluation of fuel tank
- Tank drainage service

General

The fuel system control unit regulates the fuel pump according to need and transmits information about the current fuel pressure to the combustion engine control unit. Information on the fill level is shared with the instrument cluster.

Function

Controlling the fuel pump

The fuel pump is switched on when the "fuel pump ON" signal is received by the fuel system control unit. The fuel system control unit also receives the signal "fuel specified pressure" from the combustion engine control unit.

Fuel system control unit detects the current fuel pressure via a voltage signal from the fuel pressure sensor and transmits this information to the combustion engine control unit.

To regulate the fuel pump, the fuel system control unit performs the following tasks:

Evaluation of the current fuel pressure
Checking of the fuel pressure against the fuel specified pressure
Actuation of the fuel pump by means of a pulse width modulated signal, so that the actual value corresponds to the specified value.
In this way, fuel delivery is variably regulated depending on fuel requirements.

Fuel system control unit limp-home mode

If the "fuel pump ON" ground signal is missing or implausible, the fuel pump is switched off.
If the "specified fuel pressure" signal is missing or is implausible, the fuel pump is switched off.
If the "circuit 15" signal is missing or implausible, the fuel system control unit and thus also the fuel pump are switched off.
Various limp-home functions ensure that the fuel supply is also assured in the event that the internal control system fails (e.g. failure of the processor).

Fill level evaluation of fuel tank

The fuel system control unit reads in values from the fuel tank fuel level indicator fill level sensors on the left and right to compute and evaluate the fill level of the fuel tank.

The following information is calculated from the measured electrical resistance of the fill level sensor and sent to the combustion engine control unit with a corresponding signal:

Nominal volume of the fuel tank
The current fill level in percent
The reserve quantity in the fuel tank

The fill level is made available to the instrument cluster.

The signals from the fill level sensors are also checked for plausibility.

In the event of implausible measurement values, a fault message is sent to the combustion engine control unit.

Tank drainage service

The fuel tank can be emptied without the engine running via the XENTRY Diagnosis COM kit.

For adaptation to different vehicle, engine and fuel tank variants and fuel pumps, different parameter sets are stored in the fuel system control unit.

For in-service date and exchange of the FSCU the respective parameters must be selected over the variant coding.

Component descriptions
Component description for fuel pump control unit	Engine 176, 177, 256, 279, 656 in model 223	GF47.40-P-2001A
	Engine 139, 254, 654 in model 206	GF47.40-P-2001C