Diagnostic module, tank leak - DMTL

The tank-leak diagnostic module (DMTL) serves to identify leaks in the fuel system. Monitoring the fuel system for leaks is required by law within the framework of On-Board Diagnosis.

Brief description of components

The tank-leak diagnostic module consists of the following components:

• Electric vane pump: For establishing the required overpressure in the tank for tank-leak diagnosis
  
• Changeover valve: For setting the various operating states
  
• Module housing with integrated reference leak of 0.5 mm
  
• Heater: For heating the vane pump to eliminate condensate
  

The function of the tank-leak diagnostic module is controlled by the Digital Motor Electronics (DME).

Function of tank-leak diagnosis

The fuel system is regularly checked for leaks after the engine is switched off. In this case, the following processes take place in the DME run-on phase:

• Starting situation: During normal engine operation, the changeover valve in the diagnostic module is in the so-called regenerating position. The fuel vapours are stored in the carbon canister and fed to the engine as a function of activation of the tank-venting valve (see also Tank ventilation).
  
• Checking start conditions: The necessary start conditions are checked after the engine is switched off:
  
  • Engine OFF
    
  • Battery voltage between 11.5 V and 14.5 V
    
  • No entries in the DME fault memory for the tank-leak diagnostic module and the tank-venting system
    
  • Tank fill level greater than 10 % and less than 90 %
    
  • Ambient temperature between -7 °C and 35 °C
    
  In the event of a positive result, tank-leak diagnosis is started with the reference-leak measurement
  
• Reference-leak measurement: The tank-venting valve is always closed after the engine is switched off. The changeover valve of the diagnostic module remains in the regenerating position. The electric vane pump pumps fresh air from the surroundings over a reference leak of 0.5 mm in diameter. The pump flow required for this is stored as the reference value. This is followed by the actual tank-leak check.
  
• Tank-leak check: The tank-venting valve remains closed. The changeover valve of the diagnostic module is switched into the diagnosis position. The vane pump pumps pumps fresh air from the surroundings into the tank, during which the internal tank pressure increases slowly. At the start of the tank-leak check, the internal tank pressure corresponds to the ambient pressure. The pump flow is therefore low. It increases as the internal tank pressure rises. The pump flow is evaluated by the DME.
  
• Evaluation of pump flow: The DME evaluates the increase in the pump flow in a defined time interval. If the pump flow exceeds the reference flow within this time interval, the fuel system is evaluated as OK. Tank-leak diagnosis is terminated. If on the other hand the pump flow fails to reach the reference flow, the fuel system is evaluated as not OK. Diagnosis permits distinctions between:
  
  • Major leak / missing tank cap
    
  • Minor leak
    
  • Minimal leak
    
  The relevant fault is entered in the DME fault memory. Tank-leak diagnosis is then terminated.
  
• End of tank-leak diagnosis: The changeover valve is returned to the regenerating position. The DME run-on phase is available for further functions.
  

This function can also be initiated actively by means of the DIS Tester. In this case, the processes take place as described above.

Diagnosis of module components

The components of the tank-leak diagnostic module are diagnosed by the DME. If a relevant fault occurs during engine operation, a corresponding fault is stored in the DME fault memory. The following faults can be diagnosed:

• Activation of solenoid valve
  
• Activation of pump motor
  
• Module fault
  
• Activation of heater for tank-leak diagnostic module
  

The diagnostic program offers the option of initiating the system test by means of the DIS Tester. In addition, the system components can be checked individually.