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

Control Relay:

The operation of the Fuel Pump is controlled by the ECU through the Control Relay. While cranking, current (supplied by the ignition switch "start" position) flows through coil L2 to close one of the two switches (S1). As a result, the fuel pump is supplied power. Additionally current (supplied by the ignition switch "ignition" position) flows through coil L3 to close the other switch (S2). Power is then supplied to the ECU, Air Flow Sensor (AFS), igniter, injectors and battery power is available to the L1 coil of the S1 switch. After engine start, (power no longer supplied by the ignition switch "start" position) the ECU provides a ground to the L1 coil, allowing the battery supplied current to close switch S1. As a result the fuel pump continues to be supplied power, but is now under the full control of the ECU. In the event of engine stall or other abnormal conditions, the ECU 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 ECU, resulting in start failure.