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Help understanding Alternator Voltage
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NewGasket
New User
Nov 10, 2022, 7:07 PM
Post #1 of 7
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Help understanding Alternator Voltage
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1996 Lexus ES300, ~200,000 miles, 3.0 V6.
While engine is running, with lights on or off, I get 14.9 Volts at the battery. I've read that 15 Volts should be the max, but am concerned mine is right at the max. I heard to test the rectifier, put the DMM on AC. I did. I get 33.4 Volts AC with my leads one way on the battery, then 0 Volts AC with my leads the other way. This puzzled me, I thought polarity shouldn't matter on AC. I tried it with two different DMM's and got the same readings. Are these voltages normal or is my alternator bad?
(This post was edited by NewGasket on Nov 10, 2022, 7:08 PM)
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Hammer Time
Ultimate Carjunky
/ Moderator
Nov 10, 2022, 8:30 PM
Post #2 of 7
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Re: Help understanding Alternator Voltage
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Why are you digging into this. Do you have some sort of symptom you are trying to diagnose?
Is your battery cooking or dieing?
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Nov 10, 2022, 8:41 PM
Post #3 of 7
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Re: Help understanding Alternator Voltage
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You get the idea of an alternator converting AC to DC however regulated shouldn't put out over 15V.
!!> DO YOU HAVE A PROBLEM OR JUST TESTING IT OUT?
> For practicality you won't easily find a "rectifier" or much if there's a problem and you wanted to fix it there would need the whole thing.
>Check you multimeter with other things - household batteries, low volt batteries etc., if you want to learn using your device know it's correct!
I'm surprised it stays at that voltage if you turned more items on, blower fan, rear defroster/defogger are high loads watch and raise idle speed it will cover that normally and stay within a range. If accurate never drop too low, just volts below 12.6 know that is the baseline for your battery +/- .01 if out of the car fully charged.
Being in a car with various RPMs all deal with a range other factors, temperatures if extreme for vehicles. So it shouldn't be staying at your results if YOU change the load and RPMs. That's not normal,
T
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NewGasket
New User
Nov 10, 2022, 8:52 PM
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Re: Help understanding Alternator Voltage
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The car battery has a manufacture date of 2/20. It failed to start the car. I put a battery charger on it, it took a charge well. After charging the battery over night, the car started fine.
I then wanted to figure out why the car didn't start earlier, so I attempted to diagnose the system with the DMM, checking battery, and read a voltage of 12.6, which is normal.
Next, to check the alternator function, with the car running, the voltage ran lower than it should, which lead me to believe the alternator was bad.
Believing the alternator to be bad, I ordered a new alternator. In the process of changing the alternator, I noticed some corrosion on the negative terminal. I cleaned the terminals. Then the thought came to me that maybe it was the terminal and not the alternator.
I disconnected the ground from the battery why changing the alternator. When I went to attach the ground to the battery once the new alternator was installed, I noticed a lot of amps going through the ground terminal. That blew the 100 amp alternator fuse.
It turns out the new alternator (bought on Amazon) turned out to be bad, it had an internal short in it that blew my 100 amp alternator fuse.
While I had the original alternator out, I decided to take the opportunity to have it tested at AutoZone. They tested it twice and it passed all tests both times.
Now, believing the original alternator to be good, and figured maybe it was just the corroded terminal all along keeping the battery from charging, I put the original alternator back in the car.
The terminals now thoroughly clean. However, not having a lot of faith in the Autzone test I wanted to check the alternator myself the best I could, and proceeded to do the voltage checks I earlier described.
The battery voltage at rest was about 12.39 V, so the 14.9 V while charging could be the alternator trying to charge back up the battery at max rate. However I can't understand the 33 V AC reading. Everything I see online says it should be less than 0.5 V AC.
Thank you for your time and expertise.
(This post was edited by NewGasket on Nov 10, 2022, 9:02 PM)
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Hammer Time
Ultimate Carjunky
/ Moderator
Nov 10, 2022, 9:00 PM
Post #5 of 7
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Re: Help understanding Alternator Voltage
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If your battery had a sticker on it of 2/20, it would likely be at least 6 months older than that. They put those stickers on them when they get stale on the shelf. They recharge them and put them back with a new sticker. New batteries don't use stickers.
You may be looking for a problem that doesn't exist.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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NewGasket
New User
Nov 10, 2022, 9:08 PM
Post #6 of 7
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Re: Help understanding Alternator Voltage
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OK, that is good to know. This battery came from Walmart, I think it is made by Johnson Controls or Interstate. Batteries from there seem to only last 2-1/2 years from that sticker date, which like you said, is likely many months older, and could be dying at 3 years.
Thanks for your help, I'll trust the Autozone test and see if it keeps working.
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Tom Greenleaf
Ultimate Carjunky
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Nov 10, 2022, 9:25 PM
Post #7 of 7
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Re: Help understanding Alternator Voltage
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OK. Yes it could be going bad or is bad @ 3 years.
Be warned about lead/acid batteries of the sort from a "Department Store" vs automotive parts if just how they are handled. Can't be dropped, tipped over isn't good or a disaster. Just that, these places don't necessarily know how to handle a dangerous item IMO it's a bad mix with places that also sell food and who knows what also!
You are really getting the shell made by XYZ the plates may be made anywhere IDK it's how packaged for use is the brand name you see.
That store's best feature is price and don't care will honor the warranty of what you bought know it will go by date embossed in the case if you can't prove you bought it there for months could be up to a year old when you got it new - BAD - look for another - learn that.
Just FYI,
T
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