1.4.1 Diagnosis of Evaporative System with DM-TL: Notes

P1449 (DMTLmax), P1448 (DMTLmin), P1434 (DMTLsig), P1447 (DMTLnpl) 

The DM-TL (Diagnostic Module Tank Leakage) is used for monitoring the evaporative system for very small (> 0.02 in) and small (> 0.04 in) leaks.

It consists of an electrically driven air pump, a change-over valve and a 0.02 inch orifice for reference measurement - see Fig 1. The tightness of the tank system is obtained by comparing the actual air pump's motor current with that measured when the system is operated with a standardized reference orifice.

When the air pump is switched off the fuel tank breathes through the charcoal canister, the change-over valve and the air filter. For canister purging, the purge control valve is opened and fresh air flows via the air filter, the change-over valve and the charcoal canister into the intake manifold.

For a reference current measurement the air pump's motor is switched on Figure for a short period of time while the change-over and purge control valves are closed. The air pump's motor current is measured while fresh filtered air is being pumped through the 0.02 inch reference orifice.

The diagnosis is aborted when at some unusual operating conditions the motor current doesn't stabilize. To prevent a permanent disablement of the leak check due to a malfunction of the diagnostic module (DM-TL), the number of consecutive irregular current measurements is counted and a module error DMTL is set as soon as a calibrated value is reached.

In the Monitoring Mode the change-over valve is switched on while the purge control valve remains closed. The motor current drops to a zero-load level. Fresh air is now pumped through the charcoal canister into the tank. A small overpressure, indicated by an increase in the motor current, builds up if the evaporative system is tight.