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The car starts fine in the morning. Since the battery has signs of leakage, I remove it in the afternoon, and put a new one, with a factory sticker indicating it was made two months ago.

There is no indication any power is coming in, not the lightest flicker of lights. I reinstall the old battery and see the same outcome. The circuit seems open.

The old battery gives a reading of 12.4V and the new one 12.5V. These are not loaded voltages, but I assume that for at least the old battery (which was used a few hours prior) there would be enough power for the lights, if not for the starter engine.

My first suspicion is that one of the cables is loose or broken. I had used a battery brush on both sides of the contacts, and so an electric interruption there is unlikely.

I can measure by an Ohm-meter to confirm that the ground (black) cable is fine to any point on the chassis.

What is a point of contact to determine whether the live (red) cable is fine?

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  • Make, Model of vehicle? Do you have a simple wiring diagram for your vehicle (perhaps out of the back of your Haynes/chilton manual? er... why not?) For a modern car, I'd place the positive test on the positive bus bar in the engine mounted high amperage fusebox. Of course, the quick answer is... at the other end of the red cable, where ever it goes. – zipzit Apr 9 at 15:55
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The problem turns out to be that a secondary thin cable, which runs along the positive thick cable, has a tiny connector whose ends had disconnected.

The reason for the existence of this secondary cable shall remain a mystery, but it's certainly not going directly to the starter motor. It's much too thin for that. It appears to be a design feature, to make it possible to disconnect the battery without actually having the wrench on hand to disconnect it.

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