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Old 09-04-2012, 09:03 AM   #1
dans04wrx
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Join Date: May 2007
Default Battery/Alternator

Hey guys, I am having some battery/alternator trouble. I dont want to pay to bring it somewhere to have it checked out. What is the best way to check both so i know for sure it is one and not the other?

Symptoms are as follows, if i leave the car off for about 3 days, the car doesnt start. The engine doesnt even turn. There are lights though. If i start it everyday, it starts, but now 100% normal start. This morning, i tried starting it and nothing. Then i boosted it, left it on for a few minutes, turned it off, then tried to turn it on again and it did not start.

The battery does look to have some kind of corrosion or whatever on the positive terminal.

Essentially, what is the best way to check the alternator/battery to narrow it down?

Thanks
04rex
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Old 09-04-2012, 09:35 AM   #2
tebriel
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2004 JBP WRX

Default

Autozone, or other parts store should check it for you for free. Otherwise you can get a digital multimeter and check out how many volts the alternator is putting out when the car is running. Should be around 14v iirc. When the car is off, the battery should be putting out 12v.

A way to check if it's the alt is to start the car, let it run for a couple minutes and then disconnect the battery. If the car stays running the alternator is fine.
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Old 09-04-2012, 10:50 AM   #3
dans04wrx
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I heard taking out the battery terminal while the car is running could damage the computer on the car.
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Old 09-04-2012, 10:55 AM   #4
quazimoto
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Quote:
Originally Posted by dans04wrx View Post
I heard taking out the battery terminal while the car is running could damage the computer on the car.
Thats correct. That method was used for old ass non OBD cars.
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Old 09-04-2012, 12:35 PM   #5
philly15
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Get new battery if you leave it for a couple days and it doesn't start means its not holding the charge if the alternator was bad car would shut off while driving for a while but as stated autozone should check it alternator should put out 14-16 volts best way would to use an amp clamp and see if it's putting out the recommended amperage on the alternator
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Old 09-04-2012, 02:38 PM   #6
tebriel
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I didn't know that about obd cars. GTK. Best bet is to just take it to a parts shop and have them test your charging system for free.
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Old 09-04-2012, 02:48 PM   #7
rkramer
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Quote:
Originally Posted by quazimoto View Post
Thats correct. That method was used for old ass non OBD cars.
pre-dating OBD even... Pretty much any vehicle that had any electronic circuit could be fried by taking out the battery.
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