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 control relay pin #9 to the coil to close one of the two switches. As a result, the fuel pump is supplied power. Additionally current (supplied by pins #10 & 3 of the ignition switch "ignition" position) flows through the other coil to close the other switch. Power is then supplied to the ECU, Air Flow Sensor (AFS), igniter, injectors and battery power is available to the 1st coil of the 1st switch. After engine start, (power no longer supplied by the ignition switch "start" position) the ECU provides a ground to the 1st coil, allowing the battery supplied current to close the switch. 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 1st 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.