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High side vs low side current

EFI Wiring Fundamentals

Discussion and questions related to the course EFI Wiring Fundamentals


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When wiring up higher current draw items that are driven from the ecu for example injectors or coil drivers, do you need to spec the high side wiring the same as the low side wiring? I.e. when injectors are driven from the ecu on the low side / by switching to ground rather than high side / swithching positive is the current on the positive side higher than the negative side as the injector has "converted energy" resulting in the negative side being required to carry a lower current than the positive side and hence use a smaller guage wire for the negative side back to the ecu compare to the positive side from the power supply?

And if so, how do you calculate this? Is it simply an application of ohms law?

G'day Ben.

The same current flowing into the injector from the power supply relay will be leaving the injector, through the wire to the ECU, and then from the ECU to ground. The energy that gets converted from electrical to mechanical (opening the injector) is seen as the voltage drop across the injector; at one side of the injector you'll measure 12V, and when the ECU has the injector open on the other side you'll measure 0V, giving a 12V drop across the injector :-).

You've made a good point though, in that I should have gone into it in more detail in the current section of the course. In any closed electrical circuit, the same level of current is flowing in the wire at any point. Think of it like the water pipe example, the same amount of water is flowing into the tap as is leaving the tap, but the pressure is higher at the entrance of the tap than it is at the exit.

So, yes, you should run the same size wire from the power supply to the injector as you do back to the ECU, for high impedance injectors (usually a peak draw of around 1A) I find 22AWG wire is completely adequate :-).

Thanks for clarifying that Zac!

So on ECU's like the M880 with Autosport connectors that only accept 22AWG, you could potentially have current issues with low impedance injectors - both with the wire current capabilities and the ability of the ECU to sink the current to ground? I know the M880 is rated to support low impedance injectors but would think you could run into issues with the limitations of the 22AWG wire and connector?

The current passed through the low impedance injectors is controlled by the ECU, typically being either a 4-1, or an 8-2 setup. That is, they pass the injector 4A (or 8A) of current for a very short time initially to open it quickly, and then control the current down to 1A (or 2A) to hold it open for the rest of the injection event. Then when the current supply is stopped, the injector can also close faster from this less energised state. You can average the total current draw over the injection period, and come out at something closer to 1.7A. Additional to this, the injector circuit is not being continuously driven, it will spend a good portion of its time with no current passing through it at all, so you can average this 1.7A value down even further.

I've wired up a few Pectel SQ6M's that were driving low impedance injectors, and after doing the math was more than happy the 22AWG wire I was using would be up to the task, it never gave any issues :-).

Once again, thanks for your valuable feedback Zac!

All part of the service mate :-).