This is an old one but worth the try.
(Battery or Alternator)?
Loosen one of your battery cables so it can be taken off easily but it will make enough contact to start the engine.
Make sure all the running lights are "OFF" (Headlights, Taillights, etc.) WHY? > If this proves your alternator is o.K. You will blow out the bulbs. Guaranteed <<.
Start the engine and briefly remove the loose battery cable.
If the engine dies out. It is your alternator.
If not and the engine continues to run, it's your battery.
The battery is only needed to start th engine. Once it starts the alternator kicks in and supplies all the needed power to keep the engine running.
I did that little trick years ago and it answered my question of "alternator or battery" every time.
AGAIN, MAKE SURE ALL THE LIGHTS ARE OFF, BECAUSE OF THE REASON STATED ABOVE.
I blew out my headlights and taillights once doing this little trick.
I do have to say 9 times out of 10 it was the battery.
From what you described it seems to me it is your battery.
Although the alternator may look dirty and old, they are sometimes like a "Timex Watch".
It takes a lickin' but keeps on tickin'.
Thursday, November 12th, 2009 AT 7:32 PM