Leak Detection Sensor: Description and Operation
Tank Leakage Detection (NAS Only)
New Range Rover is equipped with a tank leakage detection system. the system is controlled by the ECM.
the tank leakage detection module is located inside the wheel arch at the top of the wheel arch next to the fuel filler neck on a metal bracket.
Leak Detection Normal Operation
In its inactive state, filtered fresh air enters the evaporative system through the sprung open valve of the tank leakage detection module. When the ECM activates the tank detection module for leak testing, it first activates only the pump motor. This pumps air through a reference orifice (0.5 mm) which causes the electric motor to draw a specific amperage value. This value is then equivalent to the size of the reference orifice. the solenoid valve is then energized which seals the EVAP system and directs the pump output to pressurize the EVAP system.
the system is detected as having a large leak if the amperage value is not realized, a small leak if the same reference amperage is realized or no leak if the amperage value is higher than the reference leak value.
the leak detection module ensures accurate fuel system leak detection for leaks as small as 0.5mm. the pump contains an integral DC motor which is activated directly by the engine control module. the ECM monitors the pump motor operating current as the measurement for detecting leaks.
In its inactive state the pump motor and the change over valve of the DC Motor LDP are not energized. When purge valve operation occurs filtered air enters the fuel system compensating for engine vacuum drawing on the hydrocarbon vapors stored in the charcoal canister.
the ECM only initiates a leak diagnosis test every second time the criteria are met. the criteria are as follows:
- Engine OFF with ignition switched OFF
- Engine Control Module still in active state or what is known as "follow up mode"(Main Relay energized, control module and ECM components on-line for extended period after key off)
- Prior to Engine/Ignition switch OFF condition, vehicle must have been driven for a minimum of 20 minutes.
- Prior to minimum 20 minute drive, the vehicle must have been OFF for a minimum of 5 hours.
- Fuel Tank Capacity must be between 15 and 85% (safe approximation between 1/4 - 3/4 of a tank)
- Ambient Air Temperature between 7°C & 35°C (20°F & 95°F)
- Altitude < 2500m (8,202 feet)
- Battery Voltage between 11.5 and 14.5 Volts.
When these criteria are satisfied every second time, the ECM will start the Fuel System Leak Diagnosis Test. the test will typically be carried out once a day i.e. once after driving to work in the morning, when driving home in the evening the criteria are once again met but the test is not initiated. the following morning the test will run again.
Phase 1-Reference Measurement
the ECM activates the pump motor. the pump pulls air from the filtered air inlet and passes it through a precise 0.5mm reference orifice in the pump assembly.
the ECM simultaneously monitors the pump motor current flow. the motor current raises quickly and levels off (stabilizes) due to the orifice restriction. the ECM stores the stabilized amperage value in memory. the stored amperage value is the electrical equivalent of a 0.5 mm (0.020 inch) leak.
Phase 2-Leak Detection
the ECM energizes the Change Over Valve allowing the pressurized air to enter the fuel system through the Charcoal Canister. the ECM monitors the current flow and compares it with the stored reference measurement over a duration of time.
Once the test is concluded the ECM stops the pump motor and immediately de-energizes the change over valve. This allows the stored pressure to vent thorough the charcoal canister trapping hydrocarbon vapor and venting air to atmosphere through the filter.
Test Results
the time duration varies between 45 & 270 seconds depending on the resulting leak diagnosis test results (developed tank pressure "amperage" within a specific time period). However the chart above depicts the logic used to determine fuel system leaks.