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

Ausm

Lifer
Oct 9, 1999
25,215
14
81
Put a meter on the Altenator to see if it has any output can also be checked by your guages in the vehicle unless you have idiot lights..


Ausm
 

robphelan

Diamond Member
Aug 28, 2003
4,085
17
81
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.
 

daveymark

Lifer
Sep 15, 2003
10,576
1
0
anyone have an idea if will autozone test it for free? I know they do the diagnostic code checker for free
 

Titan

Golden Member
Oct 15, 1999
1,819
0
0
if your alternator is dead, your battery won't charge and your car will prolly die in short order. If it runs for hours after jumping, it's prolly the battery.

I had the alternator go in my old volvo at school. I had to have a friend jump me 3 times to get me to the garage, at night. Boy that was fun. I remember being suprised at how quickly the car died, especially with lights on.
 

Aimster

Lifer
Jan 5, 2003
16,129
2
0
If your car needs a jump then it is your battery

If it was your alternater your car would die in the middle of driving.

Peace
 

daveymark

Lifer
Sep 15, 2003
10,576
1
0
I drove 50 miles after I jumped it. It didn't die, I arrived at my destination. Then this morning it wouldn't start again. Does that mean it's probably the battery?
 

nakedfrog

No Lifer
Apr 3, 2001
58,124
12,312
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Originally posted by: daveymark
I drove 50 miles after I jumped it. It didn't die, I arrived at my destination. Then this morning it wouldn't start again. Does that mean it's probably the battery?

Yep. 50 miles should have put a bit of a charge into it, so it's probably not holding it any more.
 

Rumpltzer

Diamond Member
Jun 7, 2003
4,815
33
91
Originally posted by: daveymark
anyone have an idea if will autozone test it for free? I know they do the diagnostic code checker for free
Yes, AutoZone will do a load test on the battery and let you know if it's dead. They do it for free. It takes just a few minutes.

They'll also test your alternator for free if you want to take it out. Same deal with the starter.
 

Rumpltzer

Diamond Member
Jun 7, 2003
4,815
33
91
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".

 

mrphones

Golden Member
Jul 2, 2001
1,431
0
71
Battery.....Or depeding on what type of vehicle, it could be something in between that's not charging it. I would start with the battery first...
 
Nov 5, 2001
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Originally posted by: Aimster
If it was your alternater your car would die in the middle of driving.

Not necessarily true. My Honda had a bad alternator, and it would drain the battery , and I would have to jump it.
 

OutHouse

Lifer
Jun 5, 2000
36,413
616
126
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.

yip, i have used this method a few times and has always worked and was correct.
 
Aug 16, 2001
22,529
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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.

That must be before stuff like Fuel Injection, ECU and other electronics.
The car will die no matter what if you do that.
 

homercles337

Diamond Member
Dec 29, 2004
6,345
3
71
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]
 

Captante

Lifer
Oct 20, 2003
30,269
10,774
136
Alternators don't always fail completely right away, often they stop charging gradually, so the only way to be sure is to test the charging output, however if your battery is more the 3-4 years old it may be dead too... be really careful if you go to Sears or somplace like that though because they'll rip you right off and have you replacing the battery & the alternator wether you need it or not.. best bet is to test yourself or remove the alternator and take it to Autozone as has been suggested.
 

DaShen

Lifer
Dec 1, 2000
10,710
1
0
Take out the battery and take it to AutoZone.

If not battery,

Take out the alternator and take it to AutoZone.
 

Scarpozzi

Lifer
Jun 13, 2000
26,389
1,778
126
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.....
 

Phoenix86

Lifer
May 21, 2003
14,643
9
81
Originally posted by: Aimster
If your car needs a jump then it is your battery

If it was your alternater your car would die in the middle of driving.

Peace

Unless a bad alternator cause the battery to die.

edit: You need to have the alternator tested, not the battery. You know the battery's dead, it's a matter of why. The fact it isn't charging doesn't mean jack. It could mean the alternator isn't charging, or the battery isn't holding a charge.

edit2: Chances are if the alternator is bad, the battery needs to be replaced, but not always.
 

sao123

Lifer
May 27, 2002
12,648
201
106
Take a drive...
Advanced auto will test your battery, starter, ignition, and alternator for free...