Switch, Evaporative Emissions System Monitor: Operation
The ESIM (5) mounts directly to the evaporative canister (6), eliminating the need for a mounting bracket. It is critical that the ESIM is mounted vertically. On vehicles where the evaporative canister is mounted on an angle, the ESIM requires an adaptor to maintain a vertical position. When the ESIM is installed vertically, the electrical connector is in the 3 o'clock position.
The ESIM assembly consists of a housing (1), a small weight (5) and a large weight (6) that serve as check valves, a diaphragm (2), a switch (3) and a cover (4). There is one large weight and one small weight check valve in the ESIM assembly. A seal is attached at the end of each weighted check valve. The large weight check valve seals for pressure. The small weight check valve seals for vacuum. The weighted check valves are contained within the ESIM housing.
During refueling, pressure is built up in the evaporative system. When pressure reaches approximately.5 inches of water, the large check valve (1) unseats and pressure vents to the fresh air filter.
Conversely when the system cools, the resulting vacuum lifts the small check valve (4) from its seat and allows fresh air to enter the system and relieve the vacuum condition. When a calibrated amount of vacuum is achieved in the evaporative system, the diaphragm (3) is pulled inward, pushing on the spring and closing the switch contacts.
The ESIM conducts tests on the evaporative system as follows: An engine off, non-intrusive test for small leaks and an engine running, intrusive test for medium/large leaks.
The ESIM weights seal the evaporative system during engine off conditions. If the evaporative system is sealed, it will be pulled into a vacuum, either due to the cool down from operating temperature or during ambient temperature cycling. When the vacuum in the system exceeds about 1 inch of water, the vacuum switch closes. The switch closure sends a signal to the Powertrain Control Module (PCM). In order to pass the non-intrusive small leak test, the ESIM switch must close within a calculated amount of time and within a specified amount of key-off events.
If the ESIM switch does not close as specified, the test is considered inconclusive and the intrusive engine running test will be run during the next key-on cycle. This intrusive test will run on the next cold engine running condition.
Conditions for running the intrusive test are:
- After the vehicle is started, the engine coolant temperature must be within 10° C (50° F) of ambient to indicate a cold start.
- The fuel level must be between 12% and 88%.
- The engine must be in closed loop.
- Manifold vacuum must be greater than a minimum specified value.
- Ambient temperature must be between 4° C and 37° C (39° F and 98° F) and the elevation level must be below 8500 feet.
The test is accomplished by the PCM activating the purge solenoid to create a vacuum in the evaporative system. The PCM then measures the amount of time it takes for the vacuum to dissipate. This is known as the vacuum decay method. If the switch opens quickly a large leak is recorded. If the switch opens after a predetermined amount of time, then the small leak matures. If the switch does not close, then a general evaporative failure is recorded.
Even when all of the thresholds are met, a small leak won't be recorded until after the medium/large leak monitor has been run. If the medium/large leak test runs and the ESIM switch doesn't close, a general evaporative test is run. The purge solenoid is activated for approximately 10 seconds, increasing the amount of vacuum in the system. If the ESIM switch closes after the extended purge activation, a large leak fault is generated. If the switch doesn't close, a general evaporative system fault is generated.
The purge monitor tests the integrity of the hose attached between the purge valve and throttle body/intake manifold. The purge monitor is a two stage test and it runs only after the evaporative system passes the small leak test.
Stage one of the purge monitor is non-intrusive. The PCM monitors the purge vapor ratio. If the ratio is above a calibrated specification, the monitor passes. Stage two is an intrusive test and it runs only if stage one fails. During the stage two test, the PCM commands the purge solenoid to flow at a specified rate to force the purge vapor ratio to update. The vapor ratio is compared to a calibrated specification and if it is less than specified, a failure is recorded.
The ESIM switch stuck closed monitor checks to see if the switch is stuck closed. This is a power down test that runs at key-off; when the PCM sees 0 RPM, the purge solenoid is energized for a maximum of 30 seconds, venting any vacuum trapped in the evaporative system. If the switch opens or was open before the test began, the monitor passes. If the switch doesn't open, the monitor fails.
The PCM also uses the ESIM to detect a loose or missing gas cap when a medium/large leak is detected following a change in the fuel level (25% minimum).