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

[rlbarranco]

Hi All,
Thanks in advance for the commentary:
My BDR starts great when cold but not so much when hot. Just to eliminate the basics I'll have the battery load and alternator tested then double check that the major grounding points are solid but, with the ignition on (engine not running), I get 12.4v at the battery but only 9.9 to 11 volts displayed on both the voltage gauge and the FAST EFI controller.
That makes me think there may be an impedance issue that possibly gets worse as the car heats up (understood that the voltage will be lower at the gauges due to expected load from things like fuel pumps, etc but, 2+ volts down seems odd).

Thoughts?

[Ron61]

You have most likely already checked this but if all the things you listed don't prove out try looking at the starter. I had a similar problem years ago and checked everything including having the starter tested at a so called auto repair shop and they said it was good. I had another starter that fit that engine so just for trying something I stuck it in and I never had any problem after that. I put the original started back in to prove it and ass soon as the engine got hot and I stopped it wouldn't turn over, so I bought a new starter and never had another problem. Also check that your starter isn't close to a header as that can heat them up.

Ron

[rlbarranco]

Hi Ron,

Thanks for the reply. The old (mini) starter was starting to fail so I proactively replaced it 2 weeks ago.
I read somewhere that over time, an electrical problem will adversely affect the starter solenoid. As we all know, heat soak will also hurt the starter so that probably contributed to the original starter issue.

What does your voltage gauge read with the ignition on but not running?
And, are you carb or EFI?

[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.

[rlbarranco]

Agreed Randal, I've had the car nearly 9 years now so, it's probably time for a good main wire(s) connection point cleaning and wire ought check.

Thanks again for the replies everyone.

[Alfa02]

I don't know if this would be the same issue, but a good friend has a FFR Daytona, with org. (Early Model, 8-Stack FI), every time he would turns on the headlamps, or AC, something that drew amps, or sometimes just starting the car, the volts would drop to 10volts, and the FI would reset to factory setting, and the car would run rich, turns out it was too small of gauge wire from battery to starter, (Long Run), they also installed a larger Amp alt.