i jump started the car with a 3 in one jump starter. the car starts up fine. drive it around for a bit, cut off the ignition and the car won't start again. the battery seems to be healthy because i measured it with a voltmeter and the output when the car is off is 12.9 volts and 13.7 volts when the car is on.

what could be the problem?

 

am i right to assume a possible alternator problem?

 

Check the plugs around the starter, see if they are pluged in all the way.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Hatch4urSnatch &raquo;</TD></TR><TR><TD CLASS="quote">... the battery seems to be healthy because i measured it with a voltmeter and the output when the car is off is 12.9 volts and 13.7 volts when the car is on...</TD></TR></TABLE>You just kinda tested the alternator. If you want to test the battery under load, turn on your headlights & ventilation fan, with the engine off. Then see how far below 12.9v it goes.

 

Could possibly be the battery. It can read the correct voltage but still not be able to carry the load. You can take it to someplace like Advance auto and they will usual test it for free. If it does not die while you are driving it that usualy means the altenator is supplying the car with power while you are driving. I just wouldnt rule out the battery totally

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JimBlake &raquo;</TD></TR><TR><TD CLASS="quote">You just kinda tested the alternator. If you want to test the battery under load, turn on your headlights & ventilation fan, with the engine off. Then see how far below 12.9v it goes.
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how am i testing the alternator when the car is off. voltage would be directly from the battery, correct? when i test it under load, what would be the correct voltage?

 

When the car is off & you measured battery voltage, you didn't have any loads turned on. Often a bad battery will still read 12.5v or so when there's no load. But once you started the engine, you were no longer testing the BATTERY.

There's more than one way for a battery to fail...

Wait overnight & check voltage. Sometimes the battery won't hold it's charge, it can discharge itself internally.

Turn on some stuff without the engine running. I don't really know numbers, maybe I'll try it at home. I suppose a good battery will stay above 12v with the headlights turned on. A bad battery might go down below 10v??

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JimBlake &raquo;</TD></TR><TR><TD CLASS="quote">When the car is off & you measured battery voltage, you didn't have any loads turned on. Often a bad battery will still read 12.5v or so when there's no load. But once you started the engine, you were no longer testing the BATTERY.

There's more than one way for a battery to fail...

Wait overnight & check voltage. Sometimes the battery won't hold it's charge, it can discharge itself internally.

Turn on some stuff without the engine running. I don't really know numbers, maybe I'll try it at home. I suppose a good battery will stay above 12v with the headlights turned on. A bad battery might go down below 10v??
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yes, i understand that when the car is running, i'm not testing the battery alone. i just thought that when testing the battery under load, the output of the voltage wouldn't be an accurate measurement because the energy from the battery is being extracted.

thanks for your help.

 

I'd personally remove the alternator and battery and have 'em tested at a local auto store (AutoZone, Checker, etc) to make sure they hold voltage.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Hatch4urSnatch &raquo;</TD></TR><TR><TD CLASS="quote">... i just thought that when testing the battery under load, the output of the voltage wouldn't be an accurate measurement because the energy from the battery is being extracted.</TD></TR></TABLE>That's exactly why its a good measurement. It's easy for the battery to read 12.5 volts when there's no power being drawn. The real test is what voltage can it hold while there IS a load. That's how they test your battery at the parts store.

I checked 2 of my cars this morning after being parked all night. The new one read 12.5v without load, and about 12.1 with the headlights on. The '98 (5 yr-old battery) read 12.5 no-load, and about 12.0 with the headlights on. It only takes a minute to check.