car battery or alternator?

Question

i've had to "jump start" my car two or three times within the last couple of weeks. the car is 4 years old , 40K miles, and i've never had to change the battery or the alternator. i am assuming either the battery or the alternator is bad. what can i do to determine which is (battery or alternator) is bad? thank you

Answer 1

Yeah, pull the battery cable off the battery and find out which is bad, then you can look ate replacing the computer and anything else you just fried with that high voltage spike.

Most places with check your charging system either for free or for a nominal fee. Personally I would say it is the battery, if you want to know for sure. Usually if the alternator was bad, the light would come on, and the lights and accessories would die and then the car would shut down as the battery voltage declines.

Answer 2

You don't have what is any hard problem here as long as this condition was not present before the battery was replaced. If there was no problem before, the PCM has lost its memory related to fuel trim and idle data. What you need to do sounds stupid, but it was what we had to do whenever we unhooked power for any reason. You are going to have to unhook the battery again and leave the power off for at least 30 seconds. After the time has passed, reconnect the power. Now without touching the gas pedal or attempting to keep the motor running, start the motor this way. Turn the ignition on and let the PCM set its parameters so give it about 5 seconds. Start the motor, it might stall. If it does stall, just recrank and start the motor. When the motor starts, it will probably race but idle down by itself. You now have to let the motor run by itself without touching the gas pedal or doing anything else while the PCM "learns" the settings. They say you have to wait until the motor gets hot so let it run until it hits about 90 degrees C or around 200 degrees F. Then shut it off for a couple of seconds and it should be good to go. I know this sounds dumb, but I did it thousands of times.

Answer 3

Start your car, first of all. Now, disconnect the battery while the car is running. If the car dies, the alternator is bad. And if you're still not for sure, take your car to the auto parts store and they have a tester for both the alternator and the battery.

Answer 4

first run engine until fully warmed up [cooling fan cycles then shut off.
second turn ign switch to on position, then turn on head lamps [main beams] ,blower to high position. now go to fnt of car and watch headlamp brightness [note after at least 5 minutes] they will have dimmed but you should still be able to start the car.
if not battery is bad.
third when restarted and headlamps turned on they should be much brighter than they were at the end of the load test, if still dim; alternator is suspect. if both tests pass you may have a parasitic draw [trunk light,glove box light,failing clock, or audio equipment] one last note if an alternator has a bad diode it will make a whining type of noise especially under load and have low output and kill the battery overnight make sure the battery terminals are clean and the drive belt is tight and keep hands away from moving parts.

Answer 5

Get a cheap voltmeter and measure the battery with the car running. A good alternator will keep it at 14 volts or so.
Do Not disconnect the battery with the car running. The battery smooths out the alternator voltage, and disconnecting it can cause things to be fried that you really don't want fried.

Answer 6

Sears automotive has test equipment for just that. They can test the condition of the battery as well as the charging ability of your alternator. Just don't let them con you into a bunch of work you don't need.

I ran a 'charging station' in their auto depart back in the 70s and it was a free service if you bought an alternator or battery and only a modest charge for the tests.

Answer 7

Seems like it's the battery. You got at least four years out of it. Have it load tested to make sure. At the same time it wouldn't hurt to test the alternator too. Good luck!

Answer 8

Do NOT disconnect the battery cable for a test. This was OK on old cars but not today. Doing so could cost you hundreds if not thousands. Yes, some people have done this and not experienced any problems, but computers are expensive. Just do not do it.

Answer 9

just about any parts store can test the battery and alternator for you. whatever you do don't disconnect your battery while it's running, that can cause major damage to the computers and other systems in your car, bad bad idea. very costly repairs.

Answer 10

it is possible that it is either...but I'd lean towards the battery. Go to your local auto store and they can test the battery for you for free. It is unlikely that the alternator is bad after only 4 yrs.

is it possible that you are leaving somethin on and don't know it?