When the ignition switch is turned to the "ON" position (before
engaging starter), the energizes the fuel pump relay for two seconds
causing the fuel pump to pressurize the fuel system. If the Powertrain Control Module (PCM)
does not receive ignition reference pulses (engine cranking or running)
within two seconds, it shuts "OFF" the fuel pump relay, causing the
fuel pump to stop.
As a backup system to the fuel pump relay the fuel pump also can be
energized by the fuel pump switch and engine oil pressure sensor. The
sensor has two internal circuits. One operates the oil pressure
indicator or gauge on the instrument cluster, and the other is a
normally open switch which closes when oil pressure reaches about 28
kPa (4 psi). If the fuel pump relay fails, the fuel pump and engine oil
pressure indicator switch runs the fuel pump.
An inoperative fuel pump relay can result in long cranking times,
particularly if the engine is cold. The fuel pump switch and engine oil
pressure indicator switch energizes the fuel pump as soon as oil
pressure reaches about 28 kPa (4 psi).