12.4v at battery but 10v showing on v-gauge and FAST EFI controller

[Randall Thomas]

If your test meter is accurate, 12.4v at the battery equates to only a 70% charged battery. A fully charged battery should be a 12.7v at 70-80F degrees.

Your voltage drop (likely due to cable diameter and/or connections) is pretty extreme. With a relatively small 4 gauge cable from the trunk 10' long you'd expect to see 12.7v drop to 11.39v. You would gain one more volt with #000 cable (.409" wire dia) to 12.39v at the ECU. Before doing that remove each power and ground connections, wire brush clean and free of paint, apply a film of Copper Anti-seize and refasten. Wipe any excess off and cover with a coating of liquid electrical tape.

With your present voltage drop, it is likely the voltage will significantly drop during cranking, and likely the ignition or ECU is temporary shutting down during cranking as it dives below it's internal minimum operating threshold. When this happens, typically you experience no fire during cranking, but when you let go of the key the engine wants to fire for just that split second as power is restored to the ECU/ignition, but at the same time the cranking ceases.