Battery voltage fluctuates

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Battery voltage fluctuates

Post by django » Sun Aug 13, 2017 7:10 pm


I've got a VL with a s1 rb25 with rb20 ecu (FP). I'm nearly ready to start a tune. My afrs are't holding steady at all on idle and nistune shows battery voltage fluctuating all over the place, 12v to 16v in spikes from 14 causing a miss (I assume from this). I checked my voltage at the battery and it's holding a steady 14.2 on idle and 12.4 engine off. Battery is good, exedy HP 1 yr old. I'm wondering, how does nistune detect the battery voltage? I might have an earth issue in my loom somewhere in that circuit?

exciter and dash light are wired correctly.

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Re: Battery voltage fluctuates

Post by PL » Mon Aug 21, 2017 2:21 pm

Yeah this is pretty common. I see it more and more in logs from RB's these days. I suspect it's an age related thing - from old alternators. New RB's (most R34's) seem to be pretty solid. But older ones (particularly track cars) with RB20's or 25's from R33 often have large fluctuations/noise. You won't see it at the battery because the battery is a giant capacitor so it'll smooth things out. And you'd need to use a scope anyway.

The other thing is that the voltage will drop (and often get noisier) across a dyno pull. It's not unusual to see voltage drop by over a volt from low rpm to high rpm.

I'm yet to do logs on a car before/after alternator replacement so this is purely speculation. But I take a log at the end of each tune and it's definitely something that I'm seeing more of as these cars age.


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