Car won't start unless jumped. How to tell if it's the battery or alternator?

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mike2fix

Diamond Member
Feb 1, 2001
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Originally posted by: robphelan
the old method of testing the alternator was to disconnect the neg terminal of the battery while the care is running. If it dies, the alt is the problem.



This holds true for Generators,not alternators.
 

lightpants

Platinum Member
Aug 13, 2001
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Originally posted by: homercles337
Originally posted by: robphelan
the old method of testing the alternator was to disconnect the neg terminal of the battery while the care is running. If it dies, the alt is the problem.

Exaclty what i was going to say. Just make sure the car hasnt been runnign for a while. Some batteries are just weak, but will take some charge. So, just jump it, then disconnect the battery immediately.

[edit] meant battery ... [/edit]



DO NOT DO THIS!!!!
If you do, on newer cars the alternator will go to full charge and you have a very real chance of ruining your ecu or other electronic components with the spike.
Take to car to autozone or someplace else and have it checked.



 

Cattlegod

Diamond Member
May 22, 2001
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i can tell you it is the battery, it is common sense really.

i mean think about it, the car would stall if the alternator was bad...

 

royaldank

Diamond Member
Apr 19, 2001
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Is it out of water? You could get you some more time out of the battery if you fill it up.
 

B00ne

Platinum Member
May 21, 2001
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Originally posted by: Rumpltzer
Originally posted by: Heisenberg
Put a DMM on the battery while it's running and see what the voltage is.
To be a little more clear... you have a 12V battery in your car. If you disconnect the battery and measure the voltage, it might read something like 11.5V.

If you turn on the engine, the alternator will charge the battery and you'll read something higher... maybe 14V. If the alternator is dead, you won't read "something higher".

However if the voltage regulator of the alternator is broken he will read something alot higher (15-17V) leading to a quick death of the battery. So he can have a dead battery and a "broken" alternator...
 

Eli

Super Moderator | Elite Member
Oct 9, 1999
50,422
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Originally posted by: Scarpozzi
What year is the car? Have you ever changed your battery? Usually batteries have 5 year warranties on them. You can expect a battery to last at least that long....some will last 3-4 years....it's a mixed bag.

AZ will check your battery. The alternator can't charge a bad battery. :p

If the alternator goes bad, it will sometimes draw current from the battery even when the car is off... (bad diode can make the alternator (generator) turn into a motor) So start with the battery and don't listen to any mechanics trying to talk you into replacing the alternator yet.....
An alternator is not the same thing as a generator; it will not turn into a motor when the rectifier diodes fail. The diodes are simply to convert the AC the alternator produces into DC to charge the battery.
 

YoshiSato

Banned
Jul 31, 2005
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Originally posted by: Eli
Originally posted by: Scarpozzi
What year is the car? Have you ever changed your battery? Usually batteries have 5 year warranties on them. You can expect a battery to last at least that long....some will last 3-4 years....it's a mixed bag.

AZ will check your battery. The alternator can't charge a bad battery. :p

If the alternator goes bad, it will sometimes draw current from the battery even when the car is off... (bad diode can make the alternator (generator) turn into a motor) So start with the battery and don't listen to any mechanics trying to talk you into replacing the alternator yet.....
An alternator is not the same thing as a generator; it will not turn into a motor when the rectifier diodes fail. The diodes are simply to convert the AC the alternator produces into DC to charge the battery.

Technically Alternators create AC power while generators create DC power
 

AbusedMk47

Junior Member
Jan 1, 2021
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Hey guys, I Have a Mk4 Gti I have a Similar Problem to this. My car would be completely dead but after I jump it it will turn on, crank, start, & Run.... it doesn’t cut off actually if I turn it off I can start it back up again after a while then it’ll eventually die again & need a jump
 

pcgeek11

Lifer
Jun 12, 2005
21,408
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Hey guys, I Have a Mk4 Gti I have a Similar Problem to this. My car would be completely dead but after I jump it it will turn on, crank, start, & Run.... it doesn’t cut off actually if I turn it off I can start it back up again after a while then it’ll eventually die again & need a jump


Battery.
 
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Papagayo

Platinum Member
Jul 28, 2003
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Take out the battery and take it to AutoZone.

If not battery,

Take out the alternator and take it to AutoZone.

If you can get the car started.. don't take anything out..

Take the car to Autozone and have them test both..

When the car is running, the alternator should put out around 15-16 volts to charge the battery and enough to power the car..