Purging System With Leak Test Function - GF47.30-P-3016MRH
Engine 274.9 in model 205.047
with code 494 (US version)
Engine 274.9 in model 253.954
with code 494 (US version)
Schematic diagram of purge system
Purge system function requirements with leak test, general
- Engine switched off
- Circuit 15 OFF
- Min. 10 min operating time after a cold start
- Ambient temperature between 4 and 35°C
- Air pressure >780 hPa (which means that no test takes place above an altitude of about 2500 meters)
- Fuel level in the fuel tank < 85%
- Tank cap closed
- Low loading of activated charcoal canister
- No fault at involved actuators or sensors
- On-board electrical system voltage > 11 V
Purge system with leak test, general
The leak test takes place during the computer run-on of the ME-SFI [ME] control unit (N3/10) which can take up to 20 minutes. The leak test of the purge system takes place in two stages and must recognize the following leak:
- Leak equal to or greater than 1 mm dia.
- Leak equal to or greater than 0.5 mm dia.
Purge system with leak test, function sequence
The leaktightness of the fuel tank is measured from the ME-SFI [ME] control unit after a cold start. To do this the fuel tank inner pressure measured by the fuel tank pressure sensor 1 is compared with the atmospheric pressure.
If the difference is greater than 10 hPa one can assume that the fuel tank is sealed. A pressure test for a "small spray cabinet" is carried out after switching off the vehicle and switching off the ignition (circuit 15 OFF). The "small spray cabinet" includes the activated charcoal canister, its aeration and vent lines as well as the fuel tank leakage diagnostic module.
If the difference is less than 10 hPa, a pressure test for a "large spray cabinet", consisting of the small spray cabinet and the fuel tank is carried out.
In order to perform the pressure test the ME-SFI [ME] control unit compares the power consumption of the fuel tank diagnostic module during generation of an overpressure over a reference leak with the power consumption during generation of an overpressure in the respective spray cabinet.
In order to avoid false measurements caused by condensation or ice formation, the channel that acts as a reference leak is heated by the fuel tank diagnostic module heating element (A101r1).
Sequence of the test for the reference leak measurement:
- The vehicle parked and ignition switched off.
- The purging switchover valve is closed (switchover valve not actuated = purging inhibited).
- The ME-SFI [ME] control unit enables diagnosis.
- The fuel tank diagnostic module is actuated by the ME-SFI [ME] control unit. The fuel tank diagnostic module solenoid valve (A101y1) is switched to the internal reference leak.
The fuel tank diagnostic module pump (A101m1) pumps air through the reference leak and measures the current that is required by the pump.
Sequence of the test for the "small spray cabinet":
- The fuel tank shutoff valve is de-energized and closed and closes off the aeration and ventilation of the fuel tank.
- The solenoid valve in the diagnostic module switches over and the pump generates a reference pressure in the activated charcoal canister. During this operation, the current is also measured.
- If, during measurement, the value of the current for the reference leak is exceeded, the activated charcoal canister and its ventilation system are leaktight, otherwise there is a leak present.
The magnitude of the leak is recognized by the time the pump has to operate before a defined reference current is reached.
Sequence of the test for the "large spray cabinet":
- The fuel tank system shutoff valve is actuated by the ME-SFI [ME] control unit and it opens off the aeration and ventilation of the fuel tank.
- The solenoid valve in the diagnostic module switches over and the pump pumps a reference pressure into the fuel tank via the activated charcoal canister. During this operation, the current is also measured.
- If the value of the current taken for the reference leak is exceeded during measurement then the activated charcoal canister, the aeration and ventilation system as well as the fuel tank and their aeration and ventilation system are leaktight.
If the value of the current measured is less then a leak has been determined. The magnitude of the leak is recognized by the time the pump has to operate before a defined reference current is reached.
Any leaks detected, are stored in the fault memory of the ME-SFI control unit. After double occurrence of a fault, the engine diagnosis indicator lamp (A1e58) in the instrument cluster illuminates (A1).
The pressure buildup depends on the fuel level in the fuel tank. This is determined by the left and right fuel tank fuel level indicator fill level sensors (B4/1, B4/2) and used by the ME-SFI [ME] control unit during the evaluation.
| Electrical function schematic for system purging with leak test | PE47.30-P-2051-97FBH | ||
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