Battery light on after alternator replacement new belt and 2 yr old battery (1 Viewer)

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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
Messages
155
Location
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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Joined
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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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Sorry back down to around 12.4. I guess it's possible I got a bad alternator. I just installed it on Thurday
 

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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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