Diagnostic Monitoring of Tank Leakage (DMTL) - NAS Only
DIAGNOSTIC MONITORING OF TANK LEAKAGE (DMTL) - NAS ONLY
The DMTL system is a legislative requirement for NAS vehicles. The DMTL system periodically checks the EVAP system and the fuel tank for leaks when the ignition is switched off.
The DMTL system comprises the previously described components of the EVAP system with the following additional components; a DMTL pump and a DMTL filter.
DMTL Pump
The DMTL pump is located at the top of the fuel filler pipe, on a bracket near to the filler neck. The pump is connected by a pipe to the atmospheric vent of the charcoal canister and incorporates a Positive Temperature Co-efficient (PTC) heating element, a normally open valve and a reference orifice. The DMTL pump is only operated when the ignition is switched off and is controlled by the ECM. The ECM also monitors the electric air pump operation and the normally open valve for faults.
The DMTL filter protects the pump from dust being drawn into the system when the pump is being operated. The filter is located below the DMTL pump and is connected by a hose.
DMTL Operation
To check the fuel tank and the EVAP system for leaks, the ECM operates the DMTL pump and monitors the current draw. Initially, the ECM establishes a reference current by pumping air through the reference orifice and back to atmosphere. Once the reference current is determined, the ECM closes the normally open valve which seals the EVAP system. The purge valve remains de-energized and is therefore closed. The output from the air pump is diverted from the reference orifice and into the EVAP system.
DMTL System Inactive
In its inactive state, the DMTL pump motor and the change over valve solenoid are not energized. When the ECM energizes the purge valve, filtered fresh air enters the evaporative system through the sprung open change over valve of the DMTL pump. The filtered air enters the system compensating for engine vacuum drawing on the hydrocarbon vapors stored in the charcoal canister.
DMTL System Active
Phase 1 - Reference Measurement
When the ECM activates the DMTL system, it first activates only the DMTL pump motor. This pumps air through a 0.5mm reference orifice which causes the electric motor to draw a specific amperage value. This value equates to the size of the reference orifice.
When the change over valve solenoid is energized, the normally open valve is closed, sealing the EVAP system from atmosphere. Providing there are no leaks, the air pump will begin to pressurize the EVAP system and the load and current draw on the pump increases. By monitoring the rate and level of the current increase, the ECM can determine if there is a leak in the EVAP system.
During normal vehicle operation, the ECM energizes the heating element in the pump to prevent condensation formation and possible incorrect current readings.
Leaks are classified as:
^ Minor - equivalent to a hole diameter of 0.5 to 1.0 mm (0.02 to 0.04 in)
^ Major - equivalent to hole diameter of 1.0 mm (0.04 in) or greater.
The ECM performs a check for major leaks each time the ignition is switched off, providing the following conditions are met:
^ The vehicle speed is zero
^ The engine speed is zero
^ The pressure altitude (70 kPa (10.15 lbf/in2) derived from engine load calculations) is below 3047 m (10000 feet)
^ The ambient temperature is between 0 and 40 °C (32 and 104 °F)
^ The charcoal canister load factor is 2 or less (where the load factor is a measure, between -1 and +30, of the fuel vapor stored in the charcoal canister. Where -1 is 0% fuel vapor, 0 is stoichiometric fuel vapor level and +30 is 100% saturated with fuel vapor.
^ The fuel tank level is valid and between 15 and 85% of nominal capacity
^ The engine running time during the previous cycle was more than 10 minutes
^ The battery voltage is between 10 and 15 volts
^ The last engine off time was more than 180 minutes
^ No errors are detected with the EVAP components, the ambient air temperature and the fuel level
^ High range must be selected on the transfer box.
NOTE: A leak test can be performed using the Land Rover recommended diagnostic tool. This overrides the above conditions and is useful for checking correct system and component operation.
Phase 2 - Leak Detection
The ECM performs a check for minor leaks after every 2nd major leak check or after refueling is detected.
When the leak check is complete, the ECM stops the DMTL pump and opens (de-energizes) the normally open change over valve.
If the fuel filler cap is opened or refueling is detected during the leak check, by a sudden drop in the current draw or a rise in the fuel level, the ECM aborts the leak check.
If a leak is detected during the check, the ECM stores an appropriate fault code in its memory. If a leak is detected on two consecutive checks, the ECM illuminates the Malfunction Indicator Lamp (MIL) in the instrument cluster on the next drive cycle.
The duration of a leak check can be between 60 and 600 seconds depending on the test results (developed tank pressure amperage within a specific time period) and fuel tank level.
Test Results
The chart above depicts the logic used to determine fuel system leaks.