[Bristol603]

I have read the instructions for where to pick off the signal for the injection pulse duration, namely an injector feed wire that is low when the ignition is on but the engine not running.

The electronic injection system on my car is a very early one. Looking at the wiring diagram for my car, the injectors all have one feed wire directly connected to earth with the other being fed from the ECU. I believe the wire from the ECU will go high to open the injector and generate a pulse of fuel. If I follow the standard instructions for sensing the injector pulse, it seems to me this will be tied to earth and not show anything.

Do I need to do something different? Would sensing the positive feed wire work? Would I need to invert the sensed signal, e.g. through an op-amp, as my pulse will be injecting fuel when the sensed signal is high.

I apologize if this is a naive question.