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View Full Version : how do i know if its my battery or alternator?



Eurotuned_A4
10-05-2008, 05:25 PM
Basically like the title states. I had to jump the car and either my alternator isnt working properly or i have a dying battery.

oldblu
10-05-2008, 05:39 PM
How old is the battery? I recently replaced mine. 140K miles on the factory battery.

Eurotuned_A4
10-05-2008, 05:42 PM
not the original battery. its an optima and i put it in about 4 years ago.

TH85
10-05-2008, 05:49 PM
same thing happend to me on Friday...its waiting at the dealership so they can look at it on monday...i will let you know what they say

i turned on my car after driving it for a little bit and no clicking, just one little sound right when i turned the key...but after that nothing...

had to have the car jumped and got it over to the nearest dealership...pretty sure its the alternator cause my battery is a few years old like yours

Eurotuned_A4
10-05-2008, 05:52 PM
hmm, i woudlnt think the battery would go bad that fast. only be on for about 40-50k miles. I just want to diagnose it without taking it to a shop. i have more imprtant things that need fixing

blmlozz
10-05-2008, 06:13 PM
start the car, and then disconect the battery, if the car runs it's the battery, if not it's the alternator. the battery just turns the starter motor, after the car is acutaly started the alternator provides all the power the car uses. it could still be the alternator if the car runs, since it might be able to run enough to power the car but not charge the battery, in this case it will need to be removed to be tested.

oldblu
10-05-2008, 06:13 PM
Hope it's not the alternator... Though if you're in the states and there is an Autozone near by, they'll test your alternator for free. But I'm willing to bet it's the battery

Eurotuned_A4
10-05-2008, 06:22 PM
hmm, ill have to mess with it tomorrow. battery is easier to replace so im hoping its the battery.

LowSantorin
10-05-2008, 07:35 PM
you can also get your battery tested. Most of the time places can check your battery without removing it from the car. If evergreen state you mean wa i have a tester where you located?

manufan
10-05-2008, 07:49 PM
What does your voltmeter in the car read? If you're not seeing 14 volts running, it's likely your alternator.

njm23
10-05-2008, 10:20 PM
check grounds ect.. autozone tests everything from the battery so if something is missed up in between it wont give you an accurate read as to what is wrong. BTW put a voltmeter onto the alternator to see if it is working, same with the battery. very simple and a lot safer than pulling the battery with the car running like suggested..

Eurotuned_A4
10-05-2008, 11:00 PM
you can also get your battery tested. Most of the time places can check your battery without removing it from the car. If evergreen state you mean wa i have a tester where you located?

yeah im located in Washington, bonney lake to be exact.

I am seeing 14 volts after the car is running, but if i let it sit for a little(like 5-10 mins) the car wont start. So im assuming its the battery. cant i just take a battery from another car and try that out? that seems easiest, lol. dont really want to take my car anywhere to get it tested as it will most likely not start if it sits.

Eurotuned_A4
10-06-2008, 10:45 AM
tried a different battery from another car. Car ran fine and started prefectly time and time again. So im thinking it is the battery. Even when the car is running it sa constant 14 volts so im gonna go get another battery today.

Seerlah
10-06-2008, 10:51 AM
Check out the link (http://www.audiworld.com/tech/elec129.shtml).

390wheel
10-06-2008, 11:00 AM
may be something is draining the battery??? disconnect the negative side from the battery when the car is off turn everything off, take the key out, connect a regular bulb tester to the negative terminal on the battery and the other side to the ground wire. then start taking the fuses out and see which one will make the bulb go dimmer. your radio supposed to take a little, and make sure to check that you have to close the doors every time you take out a different fuse.

onemoremile
10-06-2008, 11:23 AM
If the car runs after jump starting and electrical load doesn't hurt it then it is the battery.

If the car starts from a fully charged battery but will sputter or have lights dim after driving for a while then it is the alternator. If the voltage meter isn't past the 12 mark with the engine running it is the alternator.

It could also just be the wiring or connections.

Eurotuned_A4
10-06-2008, 11:28 AM
If the car runs after jump starting and electrical load doesn't hurt it then it is the battery.

If the car starts from a fully charged battery but will sputter or have lights dim after driving for a while then it is the alternator. If the voltage meter isn't past the 12 mark with the engine running it is the alternator.

It could also just be the wiring or connections.

sounds like its the battery then. thank for all the information everyone.

onemoremile
10-06-2008, 04:45 PM
I had a bad battery a year or two ago. I put the charger on the battery and used it to start the car. Then drove 3 hours to Chicago with the lights, radio, and seat heaters on. No electrical problems whatsoever. When we arrived I turned it off and it was dead flat. Did the same thing with the charger for the drive home. Replaced the battery with an Optima 34R and all is well.

VMRWheels
10-06-2008, 04:53 PM
check voltage on teh battery after you turn off the car, if it's not showing around 14.4v (fully charged voltage) or if it show's 14.4, and 10 min later, it shows 9v.... then it's the battery. you can always test the output of the alternator as well.

jS4
10-14-2009, 09:07 AM
I have 25k miles on a 2006 S4 that has an alternator problem. Five days ago my car wouldn't start because my battery was dead. I got a jump, bought a battery and replaced it. I was on the freeway last night and the power started to cut out while in traffic. I quickly turned off my lights and radio and revved the engine to 3k to keep the power on until I got home. As soon as I turned the car off the batter was dead again. I'm working from home today waiting on the car to be fixed. Hopefully you don't lose power on the freeway like I almost did.