Not enough output from brand new alternator?

2002 LEXUS ES 300
200,000 MILES
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CHRISTA DICKSON
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I was losing power going down road so order another alternator, but it still loses power under load it seems to start and idle good at first but then if you turn on enough things started losing power. I'm pretty sure I got the manual tensioner right there is good tension on the belt I'm not sure what to check next.
Feb 25, 2025 at 7:44 PM
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STRAILER
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Is the battery warning light on while you drive? When the engine is running let's check the alternator voltage output, here is how:

https://www.2carpros.com/articles/how-to-check-a-car-alternator

If the alternator fails the test, then it needs to be replaced, here is how. Make sure the belt is tight as well. Check out the images (below). Please let us know what happens.

Feb 27, 2025 at 11:43 AM
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CHRISTA DICKSON
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No I actually noticed it was not on until individually, it may have come on when all the warning lights came on right before it shut off. But when I start it no it's not on, but everything bogs down and gets weak and then it starts losing power. And I just put a brand-new alternator on it literally got it out of the box yesterday. Does that make a difference?
Feb 27, 2025 at 12:05 PM
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CHRISTA DICKSON
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Okay, so it starts out 11.43 then gradually goes down and no battery light.
Feb 27, 2025 at 1:36 PM
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CHRISTA DICKSON
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As long as I have it on a battery charger it seems fine but when I disconnected the voltage just steady drops unless I hit the accelerator it spikes up a little bit no battery light does that mean I need a new battery?
Feb 27, 2025 at 1:51 PM
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STRAILER
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If the engine cranks okay chances are the battery is okay, but with the engine running and you get 11.43 volts this a problem, I would check the fuses: Gauge 1, ALT, AM2, ALT-s and the IG2.

This guide can help:

https://www.2carpros.com/articles/how-to-check-a-car-fuse

Here are the wiring diagrams for the charging system so you can see how it works. Check out the images (below). Please let us know what happens.
Feb 28, 2025 at 5:25 PM
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CHRISTA DICKSON
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I threw a larger battery in there and it seems to be good now it was acting like alternator so threw me off but with other battery in it it's been a few days with no issues I don't think the last cheap one was enough CCA.
Mar 2, 2025 at 3:52 PM
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STRAILER
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Is the alternator charging now?
Mar 3, 2025 at 10:28 AM
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CHRISTA DICKSON
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Ugh, I thought it was because I've been driving it for the past couple days with no issues but then tonight all of a sudden late at night in the middle of a thunderstorm it decides it's going to start losing power again, I made it home and parked it. So, I'm back apparently with the same issue again if you could help me diagnose, I'd appreciate it. thanks
Mar 4, 2025 at 3:00 AM
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CHRISTA DICKSON
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Fuses all look good.
Mar 4, 2025 at 5:54 AM
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STRAILER
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I am sorry to hear that, glad you are okay. Does the battery warning light come on when the key is on, engine not running? The system sounds like it is still not charging. If the fuses are good and the battery warning light comes on it sounds like you have a defective alternator. Once you swap out the alt, start the engine and use the voltmeter to check the charging voltage, (engine running).
Mar 4, 2025 at 10:35 AM