Leak Detector: Description and Operation

2048 - Design And Operating Principle Of The Tank Leakage Diagnostics Module (DM-TL)







The tank leakage diagnostics module is required in order to monitor the tank ventilation system for fine leaks (> 0.5 mm) and major leaks (> 1 mm).

It consists of an electrically operated vane-type pump (1) and a switching valve (2) with integrated reference opening (3). The tank system is monitored for leaks by comparing the current pump motor current with the measured value when the system operates with the standardized reference opening.

Tank Ventilation







When no leakage diagnosis is being carried out, the switching valve (2) remains in position (a). When the pump is not active, the fuel vapors are routed via the active charcoal filter (4), the open switching valve (2) and the clean air filter (5) into the outside environment.

In order to regenerate the active charcoal filter while the vehicle is in motion, the tank ventilation valve is operated in pulsed mode. The fresh air flows through the clean air filter (5), the open switching valve (2 in setting a), the active charcoal filter (4) and the tank ventilation valve into the intake pipe.

The active charcoal filter can be regenerated via a large flow cross-section. This results in only minor pressure release. This effect is especially favorable for high active charcoal filter regeneration rates. The clean air filter (5) protects the pump, reference opening and switching valve from soiling. Additionally, the regenerative air does not flow through the pump, which has a positive effect on the service life.

Leakage Diagnosis

When the conditions for diagnosis are met, tank leakage diagnosis is carried out after switching off the engine. In order to measure the reference current of the pump motor, the pump is switched on briefly while the switching valve (2 in setting a) and the tank ventilation valve are closed. The pump reference current (R) is measured while filtered fresh air is being pumped through the 0.5 mm orifice opening.

In diagnosis mode the switching valve (2 in setting b) is switched over, as a result of which the fresh air is now pumped into the tank system while the tank ventilation valve remains closed. At the same time, the tank system is sealed off from the surrounding environment. The motor current drops to a zero load level (M), following which a minimal level of overpressure builds up in the active charcoal filter and tank. This pressure is measured by reference to an increase in the motor current. In a non-leaking system, the current characteristic corresponds to the upper line (OK).

The Motronic control module measures the power consumption of the pump motor (lp). The system is monitored by comparing current characteristic (OK) with current characteristic (L) at a leakage of 0.5. A leakage the size of the reference opening (3) exists if the power consumption is equal in both cases. The comparison with the orifice renders the procedure independent of the tolerances of the excess pressure pump and of environmental influences.