Hi,

The signal is a output voltage form a sensor and it is connected to the ECU (engine control unit) that will pulsing the fuel injector. The pic is always monitoring the signal (on portA.4) and under some situation it will overwrite the signal that the sensor send to ECU by switching on the relay; the pic will PWM on PortB.3 to send the voltage to ECU.

Hope this will make the thing clearer.

Thanks!