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Old 07-19-2020, 06:59 PM #1
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Alternator Voltage Question

I had to jumpstart my 4Runner so I drove it to OíReillyís and they said my battery was good and the voltage regulator failed.

I drove home, charged my battery to full, cleaned the terminals that were filthy. This morning it struggled to start but I was able to get back to OíReillyís. A different person told me the battery was bad. Itís over 3 years old so I put in a new one.

Then they tested the alternator again and said the voltage regulator still failed. I went home and used a multimeter and got the following numbers.

With 4Runner off 12.78V
@idle No lights 13.46V
@2000 rpm no lights 13.55V
@idle with lights 13.42V
@idle with headlights, radio, and AC on high 13.36V

Iím supposed to drive to MT tomorrow and I donít really want to put in a cheap remanned Ultima alternator from OíReillyís. Based on those numbers the alternator seems to be working. And I donít hear a whine.

What are your thoughts? Should it be closer to 14V?
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Old 07-19-2020, 07:46 PM #2
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Those numbers sound good to me. 13.5 - 13.25 is perfect.

If alternator was bad, cranking the car and using voltage it would go down not up.

Your alternator holds close to 13.5, which is what a voltage regulator is supposed to due.

Battery (car off) could be higher though, over 13, just my 2 cents. If it is brand new it probably needs to charge more.
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Old 07-20-2020, 08:29 AM #3
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Those numbers look normal to me also.
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Old 07-20-2020, 11:27 AM #4
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Get a GM diode to replace the 7.5A Alt-S fuse. You'll gain about 1 volt. Cold start up I see anywhere around 14.8 then settles to about 14.2 - 14.4.

I've done this to my 07 & 19.

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Old 07-21-2020, 01:37 AM #5
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The diode idea is interesting. Any idea why you get an extra volt?
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Old 07-21-2020, 07:10 AM #6
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Is the new battery an AGM? Was the old battery an AGM? They require higher voltage to fully charge, but it shouldn't be an issue in a brand new battery. It becomes an issue after several months of not getting a full charge. A standard "Wet Cell" battery should be charging fine with 13.5v to 14.0v.
The GM diode is an alternative but takes a fuse out of the equation and in an extremely unlikely circumstance it could start a fire. There are fused options, but you have probably already left for your trip, so it won't do you much good now.
Sounds like you might have a parasitic draw or your starter might be having trouble, judging by your volt readings. You could have dirty connections at the starter. A battery voltage of 12.78 at rest for 6 hours or more should be plenty of juice to start your truck. Your lower voltage readings with all accessories on look to be in an acceptable range, but under load while moving they could be dropping lower. I have an Ultima in my 5th gen and it's been doing pretty good, but your right, you never know with remans.
Here is a video I did involving the "Dirty Parts Voltage Booster". It does the same thing the GM diode does, but has a fuse inline for circuit protection. If you have a wet cell battery, the booster might over charge it and cause problems that way. There are a few variables involved with this so more research might be needed. A quality lithium jump booster is a good thing to have as well. Hope it's something simple and your trip goes well with no troubles. Good Luck!
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Old 07-25-2020, 09:36 AM #7
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Last time I checked mine it was 12.2V with car off and 14V with car running. But that was March and it was cold. Iím pretty sure the battery voltage will read higher when cold
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