Originally Posted by wstar

If it were really that simple, there wouldn't bee any need for excess ground wiring at all. You'd just ground the block with one big fat cable somewhere between the block and the body, run another big fat line between the battery negative terminal and the body, and be done with it as far as ground reference is concerned. Then you'd run the grounding for various sensors and components to the body or block wherever convenient. As long as all wires are rated for the correct amperage and all connections are of good quality, that should be the end of it in the simple view.

However, take a look at the stock wiring Nissan puts in the car. There's basically already a "grounding-kit"-like layout going on, with extra grounding bus wires attaching at multiple points around the block and the body of the engine bay. They're doing that for a reason, and my suspicion (not being a car electronics genius or anything) is that they are trying to ensure that all ground references are equal to a certain precision.

Think back to basic ohm's law. Any time you have resistance between two points, you have a voltage difference for current flowing between those two points. Since the body and the engine block are not superconducters, they do have resistances which are non-trivial, and probably not nearly as low as a length of fat copper wire running the same distance.

Without the extra grounding, therefore, if you had a pair of sensors, one on each side of the engine, which are grounded to the block and receive battery voltage over copper, one might be receiving +14.5V and the other +14.4V, because the grounding points they're using on the block are different (there's resistance in the block between them), and one of them is grounded "closer" to the battery resistance-wise.

The more low-resistance ground paths you can add in close proximity to the grounding points of various sensors at the body/block in the engine bay, the more you can reduce this source of ground reference error. That would be a good reason for Nissan to add all the extra wrap-around grounding-wire paths they do at the factory, and more of the same certainly can't hurt.