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power loss to 10volts

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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hi, i have recently wired a dual fan system on my carb vehicle. while the engine is running the battery gauge shows around 10.5 volts, as soon as i increase the RPM the voltage goes up to 12. My alternator is new and is high amperage. can anyone explain why this is happening ?

If the alternator can't supply the current required (at the RPM it is turning), then the needed current has to come from the battery, as the battery discharges (or the current requirement is very high), the voltage will drop. When you speed up the alternator by increasing the engine speed, it produces more current, so the voltage increased. Eventually, you will have enough excess current that the battery will actually charge. You should see the voltage rise to somewhere in the 13.5 - 14v range.

So you probably need to spin your alternator faster at idle (smaller alternator pulley, and/or larger drive pulley).

Note: Some self-energizing alternators don't actually start working until the field current has been energized, and you need to rev the engine to about 2000 - 2500 RPM before it will actually work at idle.

I suspect it may also have something to do with how the fans have been wired in. If they take some of the draw, such as for relay(s), from the same feed as the voltmeter, it will increase the voltage draw.

Exactly how did you wire them in, seems like the wrong wire/fuse may have been used to switch the relays.

If you're concerned, I would suggest checking the voltage at the battery at idle and increased rpm against the voltmeter reading.

I don't think it is directly related to the alternator, as even without the vehicle running the battery should maintain >12V even witht he fans running, if it is charged.

Depending on your specific vehicle, my preference is to take a fused feed directly from the battery, through a relay for each fan, and then have them grounded to a good ground/earth. Some of these electric fans can draw as much as 20A each, so you also need to make sure the gauge of wiring, AND earth, is adaquate for the draw.