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

PURPOSE
The MFI relay supplies positive current to the engine control module, purge solenoid, air flow sensor, cam and crank sensors, idle air control motor, throttle position sensor, oxygen sensor, variable induction control sensor, engine coolant temperature sensor, injectors, etc.

OPERATION
When the key is in the START position, current flows from the ignition switch through the control relay coil C2 to ground. This turns on control relay switch S2, supplying battery voltage to the fuel pump. Once the engine starts and the key is in the ON position, the engine control module supplies the ground circuit to the control relay coil C3, keeping control relay switch S2 closed, energizing the fuel pump.

When switch S2 is closed, battery voltage is supplied to the engine control module so the engine control module detects the power supply to the fuel pump. Additionally, whenever the engine control module detects this power supply (switch S2 closed), it will supply the ground circuit to control relay coil C1. This closes the control relay switch S1, supplying power to the engine control module and various other components (injectors, relays and sensors).

In the event of engine stall or other abnormal conditions, the engine control module will interrupt the ground supply to control relay C1, opening control relay switch S2. This will turn the fuel pump OFF.

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