Emission Control Systems: Description and Operation

Diagnostic Module For Tank Leaks (DMTL)

The diagnostic module for tank leaks (DMTL) serves to determine leaks in the fuel system. The monitoring of the tightness of the fuel system is a statutory requirement as part of the on-board diagnosis.

Brief component description
The diagnostic module for tank leaks consists of the following components:
- Electrical vane-type compressor: Generation of the required excess pressure in the fuel tank for tank leak diagnosis
- Electromagnetic changeover valve: Setting of different operating conditions
- Module housing with integrated reference leak of 0.5 mm
- Heating: Heating of the vane-type compressor to remove condensate
The function of the diagnostic module for tank leak detection is controlled with the DME.

Function of the tank leak diagnosis
The leak test of the fuel system is run regularly after stopping the engine. The following processes are thereby running during the running-after phase of the DME as long as the starting conditions have been met:
- Starting situation: During normal engine operation, the changeover valve in the diagnosis module is in the so called "Regeneration" position. Fuel vapors are stored in the carbon canister and fed into the engine depending on the actuation of the tank vent valve (please refer to "Fuel tank ventilation").
- Checking starting 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 fault entries in the DME for the diagnostic module for tank leaks or the tank ventilation system
- Fuel tank level exceeds 15% and below 85%
- Ambient temperature between 4.5 °C and 35.3 °C
With a positive result, the tank leak diagnosis is started with the reference leak measurement.
- Reference leak measurement: The tank vent valve is always closed after the engine is switched off. The changeover valve of the diagnosis module remains in the "Regeneration" position. The electric vane-type compressor pumps fresh air from the surrounding area through a defined leak of 0.5 mm diameter. The subsequently required pump flow is stored as reference value. This is followed by the actual tank leak check.
- Tank leak check: The tank vent valve remains closed. The changeover valve of the diagnosis module switches to the "Diagnosis" position. The vane-type compressor pumps fresh air from the environment into the fuel tank, slowly raising the internal tank pressure. At the start of the tank leak check, the internal tank pressure corresponds with the ambient pressure. Therefore, the pump flow is very low. The pump flow increases with increasing internal tank pressure. The DME evaluates the pump flow.
- Evaluation of the pump flow: The DME evaluates the rise of the pump flow in a defined interval. When the pump flow exceeds the reference flow during this interval, the fuel system will be deemed to be OK. Tank leak diagnosis is ended. In contrast, when the pump flow does not reach the reference flow value, the fuel system will be deemed to be not OK. The diagnosis allows a difference to be made between:
- Minor leak
- Micro-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 switched back to the "Regeneration" position. The after-running period of the DME is available for other functions.
This function can also be actively triggered with the DIS tester. In this case, the processes take place as described above.

Diagnosis of module components
The components of the diagnostic module for tank leak tests are diagnosed through the DME. If a relevant fault occurs during engine operation, the corresponding fault will be entered in the fault memory for the DME. The following faults can thereby be diagnosed:
- Activation of electromagnetic changeover valve
- Activation of pump motor
- Module fault
- Activation of heating, diagnostic module for tank leak testing
The diagnostic program provides the option to initiate the system test through the DIS tester.