Battery or Alternator

Old 04-14-08, 04:19 PM
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Battery or Alternator

How do I check to see if the battery or alternator is bad?
Something is pulling the battery down when I drive anywhere.
Old 04-14-08, 04:27 PM
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Originally Posted by tatebrown View Post
How do I check to see if the battery or alternator is bad?
Something is pulling the battery down when I drive anywhere.
check the battery voltage with the engine off, and then with the engine running. voltage should be higher with the engine running.
a good battery will check out at around 12.5 volts and you should have around 13.5 to 14 with the engine running.
advance or napa should be able to load test the battery for you.
Old 04-14-08, 04:32 PM
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If you have basic voltmeter that can read the D.C. voltage with well charged system and lights off engine running it should be reading about 13.2~14.5 volt range depending on the tempture itself.

if reading too low check the alternator belt to make sure it is not slipping [ that one of pretty common failure ] if slipping you should hear squeal when you hit the throttle or right after start up.

Most modern alternator do have interal voltage regulater inside the unit so it will get little tricky to troubleshooting and i rather not shoot my foot in dark without knowing what brand alternator you have on your vechile there is few diffrent style.

now for the batteryif the post stud or side post connectors is very clean no sign of corroded or overheated connetion the only way you can do is load test the battery but however with battery i just do simple voltage test if reading too low take the battery out of that vechile and take that battery to any repair shop or battery shop one or auto parts store and ask them to run the load test on the battery.[ they have proper tester to verify if the battery is go or no go ]

hope that will help ya some details here

Old 04-14-08, 08:00 PM
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"pulling batterie down" meaning - battery light comes up?
guys are probably right:
1. lose alternator belt, or slipping belt. sometimes oil gets on the wrong parts, like pullies. try Beltease maybe? some spray WD40
2. corroded batterie posts. oh many have i seen! most neglected area, and will kill battery slowly but steadily. but! it'll kill it, not cause battery light to come up. battery light comes up when alternator is not producing enough power to the engine, and relays have to switch to draw some from battery.
i vote for lose alternator belt. if it's easy to remove, might be a good idea to bring aqlternator to a parts store to check out. on some cars it's very easy to do.
from personal experience, load testing battery at a parts store most likely will result in "oh, you got a bad one here" result for obvious reason - they always have "just the right one for your vehicle, and it's a much better one" sitting brand new on the shelf.
Old 04-15-08, 03:14 PM
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The battery only gets used when the ignition is off. Any problem when the car is running is almost always the alternator.
Old 04-15-08, 07:16 PM
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do yourself a favor also when you change out the alternator, change out the belt tensioner too unless it is very high dollar the springs do loose strength over time.
remember Murphy was an optimist.

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