Circuit Description

The EVAP vacuum switch is a normally closed switch, mounted in vacuum line between EVAP canister and EVAP solenoid. Battery voltage is supplied to switch through a pull-up resistor in the PCM. When PCM commands EVAP canister purge solenoid valve open (purge on), engine vacuum draws the evaporative emissions from the canister and EVAP switch opens (signal voltage high) providing feedback to PCM, indicating EVAP system is operating. When PCM commands purge off, the EVAP vacuum switch closes (signal voltage low), confirming that purging has ceased.