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Alternator tested good, battery and brake light on, ideas?

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jm1080

15+ Year Contributor
640
3
Jun 13, 2006
Boca, Florida
Hey everyone, been having an issue with the car, like always.. Car was fine, then out of no where while driving on the highway, the car stumbles and CEL comes on for a second, then car goes normal again. This happened a few times and I noticed battery voltage real low (have battery voltage set by dsmlink to show on stock boost gauge). I pulled over and the battery voltage went back up, I decided to drive to my destination and I made it after an hour of driving like this. On my way back home, I took it to Advance Auto Parts and battery tested at 13 volts with 838 CCA out of 850 stated recharge needed. My battery is relocated to the trunk by the way. Alternator tested at low voltage of 12 volts on load and without load.

The next day I had to drive to work, so I risked it since battery was still good and the battery light and brake light went on after a minute of driving. So I turned back and figured it was alternator. I then took out the alternator and took it to Advance Auto and it tested good! I then took it for second opinion to Autozone and they tested it 3 times and it was good! I'm stumped, I haven't modified anything in years, just DD.

Couple days before this, I tested with voltmeter the alternator and it was at 14.5 volts with lights on, 13.8 volts at distribution block, and 13.2 at battery. I'm unsure if this is normal as battery is in trunk. Any more help appreciated!
 
Auto part stores test the alternators at higher voltage than a car would...so most of the time...they'll check out good. Is it on warranty?
 
Check your battery terminals if you haven't already. Clean em, clamp em tight as heck. Then check your wiring to them or anything connected to terminals such as amp wiring etc. Check ground wiring as well.
 
I had an alternator go bad on me (under warranty from Auto Zone) and it tested good even tho I knew it was bad. I took it out in the parking lot and f***ed it up then brought it back in and said, "test this one, I brought the wrong one in." They retested it and gave me a new one for free under warranty. When I installed it, fixed my problems.

I believe the voltage regulator starts to go and it will cause charging issues on our DSM's but until it gets bad enough their tester won't red light it.
 
Hmm, well since I thought alternator was good, I reinstalled alternator and it ran good for 20 minutes, I check voltage at alternator and it 14.5, at distribution block it was 13.1, and battery at 11.75, this was after 20 minutes of idle. In the beginning when warming up, alternator was at 15, distribution block at 14.5, and battery around 13.8 or so.

Then with accessories, high beams, and ac on high, voltage at alternator was 12.4, distribution block was 11.92, and battery 11.96.

Finally, I decided to pull the negative on the battery terminal and car died....So does this mean alternator is 100% cuplrit? I should of done this in the beginning! So pissed, I believed in the alternator testing at advance and autozone, both tested fine...
 
It's the alternator dude, I promise. I've been dealing with Auto Zone's lifetime warranty crap for 8yrs and how it's never their alternator that's bad when they test it. Sometimes they can test it on the car (under load) which could pop a bad test too?
 
Replace the alternator it's crap now. Don't buy from autozone. I have better luck with alternators from the wrecking yards LOL
 
yeah im going with the voltage regulator also, its inside the alternator and can work then not then work again and will cause the same problems your having
 
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