Fuel Pump Control Unit: Description and Operation

GF47.40-P-2001MAF Component Description For Fuel Pump Control Unit
- ENGINE 156.985 in MODEL 212.077 /277





View of right door sill in the rear compartment
N118 Fuel pump control unit

Location
The fuel pump control unit is located under the trim of the right rear door sill.

Task
The fuel pump control unit is designated internally as FSCM DECOS (Fuel System Control Module DECOS).

The fuel pump control unit actuates the fuel pump (M3) as required (between 3.0 to 4.5 bar).

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
- Terminal 15
- Circuit 30
- Tml. 31
- "Fuel pump ON" ground signal
- "Specified fuel pressure" pulse width modulated signal (PWM signal)

Direct output signals
- AC signal for fuel pump (phase A)
- AC signal for fuel pump (phase B)
- AC signal for fuel pump (phase C)

CAN input signals
- Diagnosis requests
- Data for variant coding

CAN output signals
- Fuel supply status
- Self-diagnosis data
- Requested diagnostic data

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

Function
The function is described at the following points:
- Fuel pump control
- Emergency running
- Tank-draining function
- Diagnosis

Fuel pump control
The fuel supply is controlled and monitored by the ME-SFI control unit (N3/10) and fuel pump control unit.
The fuel pump is switched on when the "fuel pump ON" direct ground signal is received by the fuel system control unit from the ME-SFI [ME] control unit.

The ME-SFI control unit registers the current fuel pressure using the fuel pressure sensor (B4/15). To control the fuel pump, the ME-SFI control unit evaluates the fuel pressure and load requirement and sends the "specified fuel pressure" PWM signal to the fuel pump control unit (with a duty cycle of 10 to 90 %). This evaluates the PWM signal and actuates the fuel pump with a phase-shifted alternating current as required, so that the actual fuel pressure corresponds to the specified value.
Depending on the fuel requirement, fuel pressure is varied between 3.0 and 4.5 bar.

Emergency running
- If the "fuel pump ON" ground signal fails or is implausible, the fuel pump is shut off.
- If the "specified fuel pressure" signal fails or is implausible, the fuel pumps are shut off.
- If the "Circuit 15" signal fails or is implausible, the fuel pump control unit (and therefore the fuel pump) is switched off.

Tank-draining function
The fuel tank can be emptied over the fuel pump 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 4.5 bar

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

Diagnosis
The fuel pump control unit conducts an own and component check. Recognized errors are stored by the fuel pump control unit and sent to the Xentry diagnostics via the diagnostic CAN (CAN D) to the ME-SFI control unit over the drive train CAN (CAN C).

Variants
The vehicle and engine variants are already stored in the fuel pump control unit. If a fuel pump control unit is replaced, it must be given the variant coding.