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Low voltage when Idling

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Hey Ratsun,

I went to start up my 521 yesterday and the car very nearly didn't turn over in time. Thankfully it started but then 20 minutes down the road I noticed the car starting to stutter and loose power. Eventually the car stalled and left me stranded at the side of the road. I jumped started the truck from my girlfriends car and managed to get it home. The multimeter was reading low on the battery when static (7.17). It was around 12.0 after jumpstarting it but then could see it quickly drain even with the car idling. I tried turning the headlights on and it stalled instantly. 

I've since replaced that battery. I'm now getting 12.7 when stationary, 12.41 when idling and 12.07 when the headlights on full beam and turn signals on. This sounds low to me. Am i right in thinking it needs to be above 13.5 to be operational and able to drive?

I'm not sure if this is related but i've noticed the left side low beam and right side high beam is out. There is also a bluetooth amplifier in the cab that the previous owner installed. The previous owner replaced the alternator with more powerful one from a 94 Maxima. 

Appreciate any help here. Is 12.41 (when idling) too low to drive? Is it this way because it's a new battery? The bluetooth amp? or does it sound like a faulty alternator or ground issue? 


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It's the alternator, the regulator, or the associated wiring. You probably didn't need a new battery, but that's part of the price of learning 😛


The regulator is on the passenger fender side of the engine bay and has a 6 pin plug. Is it still there & connected, or has it been modified for an internally regulated alternator?

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When running, briefly remove the ground cable from the battery. This forces the engine to run on the alternator. If it quits then it's not charging. Could be alternator or as said the regulator.


Inspect the two wire plastic plug in the back of the alternator. Is it securely tight and pushed in?

Is the red charge light on when you turn the ignition on but don't start it yet???

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So I started the car and disconnected the negative cable from the battery and it died instantly. I also reconnected, started the engine and read the multimeter from the negative to the alternator casing and bracket and both gave me zero. I guess this means it's time for a new alternator. I think @thisismatt might be right in saying oil has gotten inside. It looks to be pretty filthy on the top side closest to the engine and leaking down the side of the engine.



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Reading negative post to alternator case should read zero as they are connected. Read across the battery posts running and not running. It will read what ever is in the battery OR charge rate if charging.

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Thanks @thisismattIs it essential I remove the alternator? Can I just disconnect the battery and blast it with brakekleen or carb cleaner while still on the car or do I need to be careful to get it on the belt or other parts of the alternator? 

@datzenmikeYeah that was my original reading. Its like 12.4 stationary and 12.2 idling : |


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Just for fun take a reading when running right on the terminal on the alternator.


Make sure those two wires are clean and making good contact.


How does the large bottom one connect to the positive battery post?



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Hey @datzenmikeThe readings fluctuated a little. The first time i did it but only had the battery terminals hand tight where I had been messing around with it and it I tested it over the course of 5 mins. It started at 1.68 and finished around 2.05.

The second I did with the battery heads snug tight and it gave me this but again it was dancing around a little...


The red and black cable goes here


The two point plug at the bottom goes back up to the external regulator but bypasses it and I guess into the main fusebox?


@thisismatt 😂

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  • 2 weeks later...

Hey @thisismatt / @datzenmike So I followed the positive cable from the B+ stud and battery positive and figured out the 100amp fuse that sits between the starter and alternator was blown.


Since replacing the fuse i'm now getting around 14.7v with the car idling but I noticed it jumps up to 15.67 with around 25k rpm and was hanging around 15.58 after even when idling. Does this suggest its over charging or is that normal with some revs on the car?




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IIRC if you look at the plug on the alt with the clip at the "top", the left wire is the sense wire, which is what the alternator uses to adjust its output. If sense isn't connected or has a poor connection the alt will push it's output too high trying to raise the sense voltage.

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I'm not aware of a fuse or fusible link between the alternator and the battery not the battery and the fuse box. The fuse pictured does not belong on a 521. And certainly not 100 amps!!!!! What alternator do you have on there now. Obviously an internal regulated one but what?

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I put a 2002 100 amp alternator from an Altima in my 710. The output cable was larger than my stock battery cable!!! The easiest way to connect with the positive cable was to connect to the starter lug. There's no fuse needed. Naturally I replaced the two battery cables with something larger from a Chevy Lumina. The negative fits nicely to the starter mounting bolt.






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10 hours ago, Matt H said:

 The previous owner upgraded the alternator to a 90a from a 94 Maxima.


Has to be the internal regulator.


Make sure the two wire plastic plug is secure. Somehow the stock plug has been removed and the Maxima plug installed. Maybe wires were not soldered? Bad connection? 



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