Fuel Tank: Testing and Inspection
Fuel Tank Leakage Diagnosis Module
NOTE: The diagnosis module for tank leaks (DMTL) is country dependent.
A system for leak diagnosis is compulsory in the following countries: USA, Canada, Korea.
The tank-leak diagnosis can be used to determine the leak-tightness of the tank-ventilation system.
Tank leakage diagnosis allows a difference to be made between:
- Minor leak, greater than 1 mm
- Superfine leak, 0.5 mm to 1 mm
Functional description
The diagnosis module for tank leaks is located on the activated carbon filter.
The leak test of the tank-ventilation system takes place after the drive cycle after stopping the engine. Various processes run in the after-run time of the engine management system:
The tank-leak diagnosis carried out within the framework of OBD 2 is based on the overpressure method. The tank-leak diagnosis indicates a leak where the diameter is greater than 0.5 mm and 1 mm. During the course of diagnosis, the tank-ventilation valve closes the connection to the intake pipe. At the start of the diagnosis, the leak diagnosis pump pumps via a defined leak of 0.5 mm diameter. The necessary current consumption is stored as an internal reference value. This is followed by the actual tank-leak diagnosis. The leak diagnosis pump pumps air into the tank-ventilation system. The current used is continuously measured. The comparison between the currently used current and the stored reference current indicates the type of leak (minor leak, superfine leak).
Structure and inner circuit
The diagnosis module for tank leaks consists of an electrically driven overpressure pump (leak diagnosis pump) as well as an electromagnetic changeover valve and reference bore hole. In addition, a heating element is integrated in the DMTL to remove any condensate that might be present.
Characteristic curves and target values
The tank-leak diagnosis is started under the following conditions:
- Engine off: engine management system in after-run (15 seconds delay)
- Cold start: coolant temperature less than ambient temperature plus 6.8 °C
- Duration of the current journey: greater than 10 minutes, total above 40 km/h
- Tank fill level: between 1/4 and 3/4
- Ambient temperature: between 1.5 and 38 °C
- Height above mean sea level: less than 2400 meters
- Filling of activated charcoal filter: less than limit value (virtually full)
- Battery voltage: between 10.9 and 14.5 Volts
The fill level of the activated carbon filter is determined in the preceding drive cycle.
The minor leak diagnosis is carried out after every drive cycle (after stopping the engine) after the start conditions have been met. The superfine leak diagnosis is carried out after every second time the start conditions have been met (after stopping the engine).
Observe the following target values for the diagnosis module for tank leaks:
Variable Value
Leak diagnosis pump, operating approx. 30 hPa
pressure
Leak diagnosis pump current 15 to 40 mA
consumption at rated voltage
Leak diagnosis pump resistance 118 Ohms
Heating element current approx. 0.3 A
consumption
Heating element rated voltage 14 V
Temperature range -40 to 80 °C
Diagnosis instructions
Failure of the component
If the diagnosis module for tank leaks fails, the following behavior is to be expected:
- Fault code memory entry in the engine control unit
- The emissions warning lamp lights up in the instrument cluster
No liability can be accepted for printing or other errors. Subject to changes of a technical nature