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Common Paths question

EFI Wiring Fundamentals

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Good morning!

I was looking over the diagram that shows the ECU grounded at both the Battery and the Engine. It was explained that if the engine to chassis ground fails that the starter circuit could complete itself through the ECU. This idea makes sense to me.

What I'm not grasping is that the diagram shows two ground connections at the ECU. While thinking about this, I thought that it would be odd to run two separate ground pins to two separate locations. I can see the thought behind (but would not practice), splicing the ground wire to reach both engine and battery grounds, not ground pin 1 and 2 going to separate locations.

Now let me play Devils advocate for a moment. If Pins 1 and 2 are going to separate grounds (Pin 1 battery and Pin 2 engine), I can imagine how the starter current could go through the ECU given a bad ground strap. Now, if Pins 1 and 2 are spliced, then connected to battery and engine grounds I cannot picture how it could feed through the ECU to get to ground, as the path of least resistance should be from the engine, to the splice(s), then the battery. In this scenario, could the current make its' way through the ECU? If so, how?


G'day Jerry.

You're right in that if you spliced the two ground wires from the ECU together, then ran one to the engine, and one to the chassis, in the event of a ground strap failure the large starter current wouldn't enter the ecu. But those ground wires would be sized for their expected current load coming out of the ECU, not the large starter current, so they'd most likely smoke up and could be pretty dangerous.