# Alternator Voltage Issue



## ChevyGuy (Dec 13, 2014)

How is the battery ground connected? The stock battery ground cable has a small wire that goes off to the computer for voltage sensing. There's also a "donut" around the big cable to ground. The computer uses that to measure the battery charging current. Either could cause the computer to command a different voltage.


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## Williaby (Apr 11, 2015)

When I did the big 3, I added the bolt to the engine and ran a 1/0 from the engine. Then added a 1/0 from the negative terminal. So there are three wires coming off of the negative terminal.


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## ChevyGuy (Dec 13, 2014)

So, it looks like you've bypassed the current sensor. I'd think it would cause an overvoltage instead of under, but perhaps the computer knows something is wrong.


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## Williaby (Apr 11, 2015)

Is this the donut sensor? I was looking online and this looks like what was being referenced by others for the voltage sensor. It's just father down.


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## ChevyGuy (Dec 13, 2014)

Yes. That's measuring the current in that wire. In a stock setup, that would be measuring all the current going to/from the battery with the exception of a small wire going to the computer. But your upgrade is bypassing that.


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## Mochenmat14 (May 22, 2015)

Williaby said:


> When I did the big 3, I added the bolt to the engine and ran a 1/0 from the engine. Then added a 1/0 from the negative terminal. So there are three wires coming off of the negative terminal.
> 
> View attachment 269543
> View attachment 269545



why no heatshrink!?


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## Williaby (Apr 11, 2015)

I used heatshrink when I first installed but apparently it was cheap as it all melted under the heat. I’m ordering new heatshrink from illcustomz to replace it.


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## bsumpter (Dec 18, 2013)

I got the 390 from CES, bypassed PCM, and I rest around 15-15.2. I ran my 1/0 through the donut.


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## Williaby (Apr 11, 2015)

Did you replace the stock wires with 1/0 or just add the 1/0? I added the extra wire but now am wondering if that’s part of the issue. I’ll try running it through the donut.


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## bsumpter (Dec 18, 2013)

I left all the factory wires and added the 1/0. I have 2 runs going to the alternator, additional ground on the core support, additional ground going to the trans mount. then 4 runs of positive and negative going to the back.


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## grtpumpkin (Nov 25, 2016)

I would ditch the stock negative battery cable altogether and replace it with a 1/0 cable routed through the sensor ring. Should operate as designed after that.


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## Blazekm805 (9 mo ago)

bsumpter said:


> I got the 390 from CES, bypassed PCM, and I rest around 15-15.2. I ran my 1/0 through the donut.


Brilliant. Did this affect any other electrical components? I have a 2nd battery in the trunk that is not getting charged properly either. My 390amp alt does not have a regulator so I believe is being run by the computer...i am looking into a voltage control which from what i understand will allow me to set the voltage I want it to run at, but I am trying to do my homework first so I do not harm my car. Do you what to do in my case? The car drops down to 12.2v after a minute or 2.


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