Battery light on after alternator replacement new belt and 2 yr old battery

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May 25, 2005
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I just replaced my alternator and a new belt two days ago and the FJ was fine until this afternoon when the battery light came back on. No corrosion on the battery terminals and no loose wires. I had the battery on charge while I changed the alternator and it was at full charge when reinstalled. I'm at a loss for what could be the issue. Any help would be appreciated
 
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Joined
May 25, 2005
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Atlanta
I drove the truck home from work and when I started it up the battery light was out. It kept going off and on the whole way home. It didn’t really have a pattern to it. It would do it while driving and while stopped at stop lights.

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1911

chupacabra
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Measure the voltage of the battery before you start it, and again when it is running, at idle and maybe 2,000 rpm.
 
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check your voltage first but it could always be a brand new alt, unfortunately ive had that happen a few times
 
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So I checked the voltage with engine off and it was 12.54. I checked it while at idle and it would jump from 12.6 to 13.98 and then back down. The belt is nice and tight and it looks like everything is turning that's supposed to be turning.
 
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back down to what? if its getting back down to the 13v range then it sounds like the alt
 
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Sorry back down to around 12.4. I guess it's possible I got a bad alternator. I just installed it on Thurday
 

1911

chupacabra
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So I checked the voltage with engine off and it was 12.54. I checked it while at idle and it would jump from 12.6 to 13.98 and then back down. The belt is nice and tight and it looks like everything is turning that's supposed to be turning.

The battery should be good with that resting voltage.

The alternator should put out more than 13 volts even with all accessories, heat or A/c, lights, stereo, etc. on. At 2000 rpm it should be putting out at least 14 volts, almost 15. Either you have some corroded or loose connections to the battery or alternator, or the new alternator is bad. Where did you buy the new alternator?
 
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Montreal
I bought a rebuilt Denso from RockAuto and the voltage drops when stopping/idling. No lights and no audible effect. But I don't like seeing the voltmeter drop like a speedometer. It goes right up when driving so it's charging. Long story short - Sometimes you get a dud. I bought a rebuild kit from Maniac Electric Motors. Glad I kept my core to rebuild it right.
 
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Alternators require rpm's to make power. Lower rpms - as in idle speeds - will not make the power needed to run everything loaded to max and charge the battery at the same time. This is perfectly normal operation. So the voltage at idle being below 13.8 volts is not necessarily an indication of a bad alternator.

But anything below 12.8v when idling is a sign of some problem. Anything over 14.5v when at driving speeds is also a sign of a problem. But tools are needed to understand just where the problem is.

And, a loose serpentine is always suspect when voltages are low.

ps: If the idle voltage is jumping up and down (between 12.5v and 13.9v), and it isn't coincident with the AC or some other load turning off and on, then the regulator is suspect.
 
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Joined
Feb 14, 2013
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Wake Forest, NC
Resurrected this old thread as I am having the exact same issues. Rebuilt alt from Denso put in about 3 months ago (via Rock Auto). Battery is about a year old.

@adidasprof - did you get this issue resolved? What did it end up being?

@Showroom - I also bought a rebuilt alternator from Rock Auto - did your rebuild kit from Maniac fix this issue?

Thanks for any help
 
Joined
Feb 14, 2013
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Just measured the voltage

Battery (engine off): 13 volts
Battery (engine on): started at 14 volts and dropped to 13.8 volts

The battery light tends to come on in the morning when it is cold. Belt slipping? Maybe I forgot to tighten a bolt?
 
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Just an update. Battery light came on full time. Measured the battery voltage with engine running. It was 12.4 v. Definitely needs a new alternator.
 

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