Main Relay

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One half of the main relay is used by the ECU to power the electric fuel pump. ECU provides a ground for this part of relay when ignition is first turned on (for 2 seconds) or when an RPM signal is received by the ECU (cranking or running). The second half of the main relay provides power from the ignition for the ECU, injectors (through a resistor) and power for the fuel pump part of the relay.