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Why is my alternator light flickering and sometimes staying on?

Tiny
UHSNOEBIRD
  • MEMBER
  • 2005 TOYOTA CAMRY
  • 2.2L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 333,000 MILES
My light flickers and even stays on like the alternator is not charging but I know better than that because I have a voltmeter inside my car, and it shows 14 volts and charging.
My friend has a 2003 and his does the same thing.
Tuesday, November 2nd, 2021 AT 3:42 AM

34 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Good morning,

It sounds like a diode has failed inside the alternator causing the light to flicker as you see.

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

https://www.2carpros.com/articles/how-to-replace-an-alternator

It can still charge but send AC voltage to the battery.

Check the actual voltage at the battery with a meter. It should be 14 to 14.5 volts DC. Then change to AC volts scale and see what the reading is.

Roy
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Tuesday, November 2nd, 2021 AT 3:50 AM
Tiny
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My dc voltmeter shows 14 volts, but I haven't put it on AC voltage and tested it like that.
Will it mess my meter up by testing on AC Voltage? Thanks
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Tuesday, November 2nd, 2021 AT 4:06 AM
Tiny
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Not at all. It is a standard test all technicians perform.

Roy
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Tuesday, November 2nd, 2021 AT 4:32 AM
Tiny
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I will try to get that done today.
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Tuesday, November 2nd, 2021 AT 4:36 AM
Tiny
ASEMASTER6371
  • MECHANIC
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Sounds like a plan.

Let us know if you have any other questions.

Roy
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Tuesday, November 2nd, 2021 AT 4:47 AM
Tiny
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Will let you know the results. I have another question on this same car and getting ready to post it on a gas pedal position sensor.
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Tuesday, November 2nd, 2021 AT 5:51 AM
Tiny
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Okay, I will be looking for it.

Roy
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Tuesday, November 2nd, 2021 AT 6:29 AM
Tiny
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Already posted it.
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Tuesday, November 2nd, 2021 AT 6:47 AM
Tiny
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  • MEMBER
  • 342 POSTS
I put the voltmeter in the ac voltage range, and it started off in the 160 volt and started climbing one volt at a time and when it got to 170 ac volts, I took the probes off, but the voltage was still climbing higher. I wasn't sure how long you wanted me to keep the probes on the battery in the ac volt range.
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Tuesday, November 2nd, 2021 AT 11:14 AM
Tiny
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The ac voltage would go higher and drop back down to like 50 volts or less then back up high then back down again.
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Tuesday, November 2nd, 2021 AT 11:16 AM
Tiny
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Replace the alternator.

You have a bad alternator.

Roy
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Tuesday, November 2nd, 2021 AT 11:20 AM
Tiny
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What should the ac voltage looked like when I tested it?
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Tuesday, November 2nd, 2021 AT 11:22 AM
Tiny
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My DC voltmeter in the car shows 14.0 volts.
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Tuesday, November 2nd, 2021 AT 11:28 AM
Tiny
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Ac voltage should be 0. Anything above that indicates bad diodes.

Roy
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Tuesday, November 2nd, 2021 AT 11:32 AM
Tiny
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Does that apply to all alternators on vehicles no matter what the make is?
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Tuesday, November 2nd, 2021 AT 11:39 AM
Tiny
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Correct. There should be 0 AC volts coming from the alternator.

Roy
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Tuesday, November 2nd, 2021 AT 11:44 AM
Tiny
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You taught me something there and I appreciate it.
Could I buy the diodes and put them in myself, I have some electronic knowledge.
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Tuesday, November 2nd, 2021 AT 11:48 AM
Tiny
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They are pressed into the diode trio assembly.

If you could find them, yes, you could replace them.

Roy
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Tuesday, November 2nd, 2021 AT 11:58 AM
Tiny
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Doesn't Ebay or Amazon sell alternator rebuild kits?
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Tuesday, November 2nd, 2021 AT 12:16 PM
Tiny
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No clue. I would never consider rebuilding one. I replace them and then I have a warranty.

Roy
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Tuesday, November 2nd, 2021 AT 12:25 PM

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