How do I determine if the alternator is bad on my 2003 Jetta?
With a voltmeter, check voltage at battery when car is running. Should be 13.5 - 14.5 volts DC.
Call your local Auto Zone if you are in the US and see if they will check it . If it is bad then you are running on the battery only and you cannot go far that way.
Yah.
That is what I am going to do.
It’s at 12.66 with the engine NOT running, which is just right.
Thx for that.
I hope that that is what it is …
Thanks for the quick reply tcmichnorth!
Yah.
I already did. They ran a test and it read that the voltage regulator was failing.
They are not always correct so I am going to do the voltmeter test.
The thing is that I live in a small town and don’t put many miles on the car, so the battery
may just need a good charge.
Thanks for the quick reply VOLVO_V70.
Not if it reading 12.66 volts, it doesn’t. That is a charged battery.
Thx.
I tested with a 12V meter, although I read it supposed to be a 20V?
Anyway it came out at 12.06 not running and 13.89 running.
12.06V means the battery needs a charge. 13.89V running is OK, and low-normal, probably because the alternator is working hard charging the battery.
Nothing should be 20V.
The 20V came from a post on Advance Auto Parts site. AAP is where the alternator was purchased in 2013.
First read at AutoZone was “failing voltage regulator.”
So I went to local NAPA to see what it would read and it was 12.66.
Last nite at home with a 12V it was 12.06
So it looks hopeful that the alternator is ok and that I need to charge the battery because
it’s short local driving and the battery does not get fully charged?
20V?! 
Maybe they meant to set your voltmeter to read up to 20V? That would make sense if you have one that could read either 10 or 20 Volts…
Yes, I would agree with that.
Yes, it read to set it to 20V on AAP site. Sorry for not being clear on that. 
Last nite we set it to 12V, which was the highest voltage and we got the 12.06.
So I am happy that I just may not need a new alternator.
Thx
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