Not enough output from brand new alternator?

Tiny
CHRISTA DICKSON
  • MEMBER
  • 2002 LEXUS ES 300
  • 200,000 MILES
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.
Tuesday, February 25th, 2025 AT 7:44 PM

10 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 51,500 POSTS
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.

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Thursday, February 27th, 2025 AT 11:43 AM
Tiny
CHRISTA DICKSON
  • MEMBER
  • 148 POSTS
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?
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Thursday, February 27th, 2025 AT 12:05 PM
Tiny
CHRISTA DICKSON
  • MEMBER
  • 148 POSTS
Okay, so it starts out 11.43 then gradually goes down and no battery light.
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Thursday, February 27th, 2025 AT 1:36 PM
Tiny
CHRISTA DICKSON
  • MEMBER
  • 148 POSTS
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?
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Thursday, February 27th, 2025 AT 1:51 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 51,500 POSTS
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.
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Friday, February 28th, 2025 AT 5:25 PM
Tiny
CHRISTA DICKSON
  • MEMBER
  • 148 POSTS
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.
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Sunday, March 2nd, 2025 AT 3:52 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 51,500 POSTS
Is the alternator charging now?
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Monday, March 3rd, 2025 AT 10:28 AM
Tiny
CHRISTA DICKSON
  • MEMBER
  • 148 POSTS
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
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Tuesday, March 4th, 2025 AT 3:00 AM
Tiny
CHRISTA DICKSON
  • MEMBER
  • 148 POSTS
Fuses all look good.
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Tuesday, March 4th, 2025 AT 5:54 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 51,500 POSTS
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).
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Tuesday, March 4th, 2025 AT 10:35 AM

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