battery or alternator which is dead

tony02gt · 05-08-2009, 12:24 AM

hey my wife driving my car an 02 gt home tonight when lights go dim and car dies after about 5 miles. i jumped it off with her car and it runs then dies when i turn the light on or just let it idle for about a minute. so which is it bad alternator or battery

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01GTDropTop · 05-08-2009, 12:03 PM

Check the battery to see if is pushing the correct amount of volts, if it is, check the wire that connects the battery to the alternator.

devongarver · 05-08-2009, 12:55 PM

jump it again, pull the positive cables(keep them together though) off the positive battery terminal. if the motor dies, ur alternator is dead. If you have a multimeter/voltmeter, jump the car to get it started, and check for about 14v across the battery terminals, if you are seeing more like 10-11v, alternator is dead. however, the more you let the battery die completely and charge it up, the more likely you will need a new battery too, one will take out the other if it is bad eventually.

sidenote:autozone will test both for free

mustangman281 · 05-08-2009, 01:53 PM

1. do NOT pull the battery cables off the battery to test the altenator you risk killing your computer. That is an old school trick that doesn't work on efi cars.

2. From the sound of it you altenator is finished. Take it off and take it to autozone.

devongarver · 05-08-2009, 03:32 PM

how will pulling the positive cables off kill the computer? its hooked up parallel to the circuit...i dont understand what damage could be done? Can you explain for me plz?

tony02gt · 05-08-2009, 03:34 PM

thanks for the help i took battery back to store still underfull warr. then took it to autozone to get alternator check and everything is fine now

mustangman281 · 05-08-2009, 03:35 PM

It sends a power surge to the computer and can short it out.

devongarver · 05-08-2009, 03:41 PM

1. so was it just the battery then? i thought for sure it was gonna be the alternator..
2. ox, im not tying to argue with you at all, i just dont understand how disconnecting with cause any spike in voltage or current at all,...besides that, i thought the computer was ttl and a 5v setup knocked down by a step down before it even enters the computer..im totally lost here i guess.

wolverine8490 · 05-08-2009, 06:30 PM

I would have put my money on the Alt as well. 95% of the time the alt can keep the car charged enough to keep it running even if the battery is toast.

Also, as far as frying the computer, I agree. You would be surprised how little it takes to fry them. I fried and OBDI ecc (pontiac) just by pulling the test key out with the ignition key on not running. It is better to be safe than sorry.

vanquish · 05-08-2009, 08:14 PM

+1 same exact thing happened to me on my old car and it was only the alternator that was bad.