ESIM Switch Functional Description
FUNCTIONAL OPERATION OF THE ESIM: The ESIM is configured with a normally open vacuum switch and normally closed vacuum relief and pressure relief valves (see figure). 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 12 volt signal to pass through the switch, pulling the signal low to ground. When the PCM sees the signal pulled low it determines the switch to be 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 ay 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.
It is best to analyze the entire Evap System components and wiring regardless of the DTC present. A sealed system will create vacuum as it cools or pressure as it heats up. Even a slight temperature change will cause the system to build pressure or create vacuum if the system is sealed and not leaking. We will use this principle in the test steps below to diagnose the system by monitoring the ESIM signal, Fuel Tank Pressure (FTP) Sensor signal and Purge Solenoid duty cycle with the scan tool while performing a few simple checks to help in determining if in fact the system is failing, and to possibly pin point the cause of the issue before disassembling any components.
The Fuel Tank Pressure Sensor is not used by the PCM in determining a leak but can be used to help in diagnosing the system.