Main Relay (Computer/Fuel System): Description and Operation

PURPOSE
The control relay supplies positive current to purge solenoid, air flow sensor, Engine Control Module (ECM), and injectors.

MFI Relay Circuit:

OPERATION
The operation of the fuel pump is controlled by the ECM through the Control Relay. While cranking, current (supplied by the ignition switch "START" position) flows through coil L2 to close switch S1. As a result, the fuel pump is energized. Additionally, current (supplied by the ignition switch "ON" position) flows through coil L3 to close switch S2. As a result, current is supplied to the ECM, Air Flow Sensor (AFS), injectors, EGR solenoid, Purge Control solenoid and the L1 coil of the S1 switch.

After the engine starts and the key is returned to the "ON" position, the ECM provides the ground to the L1 coil, allowing current to close switch S1. As a result the fuel pump continues to be supplied power, but is now under control of the ECM.

In the event of engine stall or other abnormal conditions, the ECM interrupts the ground, opening the L1 coil and interrupting the fuel pump power circuit.

NOTE: Failure of the control relay may interrupt power supply to the fuel pump, injectors and/or the ECM, resulting in start failure.