Component Functional Description - Evaporative System Integrity Module (ESIM) And Switch
FUNCTIONAL OPERATION OF THE ESIM: The ESIM is configured with a normally open vacuum switch and normally closed vacuum relief and pressure relief valves (look at the illustration). The PCM uses the ESIM for all system leak diagnostics (P0440, P0455 and P0456). The relief valves regulate the system pressure between approximately 1" H2O and -2" H2O (250Pa and -500Pa).
- When the system vacuum reaches between (-187Pa) and (-250Pa) it acts on the diaphragm and closes the vacuum switch. When the switch closes it allows continuity between the terminals. This allows the ESIM Signal to pass through the switch, pulling the signal low to ground. The PCM monitors the signal to determine the state of the switch (open or closed). The ESIM Signal will typically be a 12.0 volt or 5.0 volt signal depending on the vehicle. The voltage signals can vary between vehicles and may be somewhere between 5.0 and 12.0 volts. Basically, when the PCM reads voltage on the signal circuit it determines the switch is open and when the signal is pulled low it determines the switch is closed.
- The vacuum relief valve is intended to maintain the seal on the system during engine off events. However if the vacuum exceeds the regulation point the valve will pull off the seat to protect the system from excessive vacuum conditions.
- The pressure relief valve limits any pressure build up in the Fuel Tank during refueling and allows the tank pressure to normalize during increasing temperature events. This is additionally beneficial because it achieves a switch closed (pass threshold) sooner than if the tank has to decay from a much higher pressure.