Purging system with leak test function - GF47.30-P-3016MMW

ENGINE 276.9 in MODEL 212.095 as of model year 2014 with CODE 494 (US version) 

Function requirements for system purging with leak test 

• Circuit 87M (engine timing ON)
• Engine running
• Ambient temperature 4 and 35°C
• Air pressure >780 hPa (which means that no test takes place above an altitude of about 2500 meters)
• Battery voltage 11 - 14.5 V
• Fuel tank fill level 15%...85%
• No fault in LH or RH fuel level indicator sensor (B4/1, B4/2)
• No fault on purging switchover valve (Y58/1) (e.g. switchover valve not closed correctly)
• Fuel tank cap closed

Purge system with leak test, general points 

The legislator requires that no fuel vapors reaches the open air.

Perform leak test for fuel system in two stages and the following leaks must be detected:

• Leak equal to or greater than 1 mm dia.
• Leak equal to or greater than Ø 0.5 mm

Function sequence for purge system with leak test 

The leaktightness of the fuel tank is checked by the ME-SFI [ME] control unit (N3/10) through comparing the power consumption of the fuel tank diagnostic module pump (A101m1).

The respective power consumption is measured during generation of an overpressure in the fuel tank without and over the reference leak.

The purging switchover valve must be closed (switchover valve not actuated = purging inhibited).

Test sequence:

• Vehicle parked and ignition switched off.
• ME-SFI control unit enables diagnosis
• The fuel tank diagnostic module solenoid valve (A101y1) closes the aeration and ventilation of the activated charcoal canister.
• The fuel tank diagnostic module pump pumps air through an internal reference leak and measures the current strength, which is needed for this.
• The fuel tank diagnostic module solenoid valve switches over and the fuel tank diagnostic module pump pumps a reference pressure into the fuel tank.

  During this operation, the current is also measured.

• If, during measurement, the value of the current for the reference leak is exceeded, the fuel tank and its vent system are leaktight, otherwise a leak is present.
• The magnitude of the leak is recognized by the time the fuel tank diagnostic module pump operates before reaching a defined reference current.

If the following conditions arise the leak test is broken off:

• Battery voltage fluctuations
• Fuel tank cap is opened (e.g. for refueling)
• Maximum diagnostic time is exceeded (maximum diagnostic time is longer than the computer run-on of the ME-SFI [ME] control unit
• Fluctuations in the reference current (moisture)
• Dropping of the pump current (moisture)

Recognized leaks are stored in the fault memory of the ME-SFI [ME] control unit. In addition, the fuel reserve warning lamp (A1e4) in the instrument cluster (A1) comes on and a display message appears in the multifunction display (A1p13), when the vehicle ignition is switched on.

The pressure buildup depends on the fuel level in the fuel tank. This is determined by the fuel level indicator sensor, on the left or right and taken account of from ME-SFI [ME] control unit during the evaluation.

Fig 1: Fuel Tank Diagnostic Module Pump Circuit Diagram

Courtesy of MERCEDES-BENZ USA