Signal Waveform
The three Waveforms below are taken from the #1 and #4 Injectors. The top Waveform is from the high side (Feed Side) of the #1 and #4 Injectors, while the middle Waveform is from the low side (Ground Side) of the #1 Injector and the bottom Waveform is from the low side of the #4 Injector.
The middle Waveform is the same as the top Waveform because there is no ground for the circuit. With no current flowing in the circuit, the #1 injector is not energized and fuel does not flow.
The bottom Waveform shows that ground is supplied and there is a voltage drop across the #4 Injector. With current flowing in the circuit, the #4 Injector is energized and fuel flows.