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Old 10-01-2012, 02:28 AM   #1
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Default Alternator and battery question

My car now has trouble starting. It has just clocked 203k miles. I thought I may have a parasitic drain(I have an amp and capacitor installed). But after unplugging the battery overnight, it still won't start.

When the car is off, the battery reads 12.3V.
When the car is on, the battery reads 15.8V.
When I attempt to start the car, the battery drops to 6V.

Do these readings indicate the battery or alternator is bad? I replaced the battery a month ago, when I first started to have this problem. After installing the new battery(pro rated warranty replacement) the car worked fine. The battery brand is "Super Start" from Oreillys.

Thanks in advanced.
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Old 10-01-2012, 04:52 AM   #2
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The voltage with th engine running is too high.
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Old 10-01-2012, 09:16 AM   #3
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The voltage with th engine running is too high.
By about 1 volt. It would have to do that for a while to kill a liquid cell battery but would kill a stock AGM quickly. From those reading it sounds like the voltage control in the alternator went bad and killed the battery. I bet if you put the battery on a cheap tester it would check out as ok, but the expensive load type tester would show it as junk.

My persona course of action would be replace the alternator and replace the battery.
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Old 10-01-2012, 09:19 AM   #4
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and voltage when off is .3v too low!

dropping to 6v during cranking is a bit too low too. I think i see around 9-10v
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Old 10-01-2012, 09:23 AM   #5
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and voltage when off is .3v too low!

dropping to 6v during cranking is a bit too low too. I think i see around 9-10v


Dropping to 6v during cranking but having normal voltage with no load is indicative of a battery with a high internal resistance.
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Old 10-01-2012, 11:22 AM   #6
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I just noticed the harness that plugs in the back of the alternator has a broken tab and is loose when I push it in the alternator. Could this be the source of the problem?

I will replace the battery this morning, but I'm concerned that if I replace the alternator and the harness is the problem. Is it possible to replace the connector?
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Old 10-01-2012, 11:36 AM   #7
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I will replace the battery this morning, but I'm concerned that if I replace the alternator and the harness is the problem. Is it possible to replace the connector?
is it possible to fly by pushing down on air?
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Old 10-01-2012, 11:37 AM   #8
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Originally Posted by Leafy View Post


Dropping to 6v during cranking but having normal voltage with no load is indicative of a battery with a high internal resistance.
It can also indicate high resistance anywhere between the battery and the load (e.g. corrosion, badly tightened leads etc).

Dropping to 6V would mean no ECU action at all.
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Old 10-01-2012, 12:21 PM   #9
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Well I pushed in the harness, push started the car, and the battery/alternator light is on. I'll be replacing both tonight after work.
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Old 10-02-2012, 03:12 PM   #10
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Replaced both battery and alternator. Car now shows 14.7V when turned on.

Thank you.

Does anyone know where I could buy a replacement connector for the alternator harness?
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Old 10-02-2012, 03:16 PM   #11
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Junkyard, panic motorsports, ballenger motorsports, eastern beaver.
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