Is my alternator dead or not???

[maxchao]

OK as usual, here is the story.
-I drove back home from school with headlight on last night, nothing wrong happened.
-This morning it wouldn't crank, it wouldn't even try to crank.
-My roommate jump-started my P5, as I was backing out of the driveway, I turned the headlight on and the engine stalled.
-Tried again, jump-started, let sit 5 minutes, turn the headlight on, engine died. And it wouldn't even try to crank at all.
-Bought new battery from walmart, put it in, everything is fine. I drove to school.
-I was told it could be the alternator, so after I came back, I unplug the battery while the engine is running, the engine keeps running, I can turn everything on but the headlight. If I turn the emergency light on, my dome light would dim with this.

So the question is: If I can unplug the battery and the engine won't die unless I turn the headlight on, is my alternator dead??

Josh

 

[Tom03es]

Was it the original battery? Batteries do go bad and you are in the right window of time for the original battery to go bad.

 

[maxchao]

I don't know if that's the original one, I bought the car last November and not sure what the previous owner had done. I traded it in when I bought the new one from walmart.
Now I think the battery must had died, but if alternator can't supply enough power to power the headlights alone, does it mean it's on the verge of dying?

Josh

 

[bucksatan]

If everything but the headlights work with a new battery, to me it would indicate that there might be some sort of issue with the lights. If you try the lighter, or turn the stereo on and off, or play with the dome lights does it do anything?

What happens if you try and turn on the lights when the car isn't running?

 

[maxchao]

Sorry if I was confusing. Everything works with the new battery in there, but if i unplug the battery while the engine is on (so everything is running on the alternator alone.), I wouldn't be able to turn the headlight on without killing the engine.

Josh

 

[Benji]

If there is a problem with your charging system you should see a red light of a battery on your gauge cluster light up, also if the engine is running with the battery connected they your alternator is working. That being said you could still have a problem with it, i.e. bad terminals on the alternator, etc. that only act up under certain conditions

Also the alternator should be able to supply all the power you need, when the engine is running

 

[maxchao]

Oh ok I totally forgot about the battery light, what does it do? Light up when battery/alternator voltage drop below a certain point?
After driving today, I feel like the battery should be being charged. Maybe the worst thing is that the alternator won't last long, but my P5 already has 96k on it. I will probably change out the alternator after I graduate.
Hopefully it will keep on running.

Josh

 

[Don Juan]

Take it to AutoZone, they will check this prob for free.
If you can't do that start the car and stick a screwdriver to the side of the alt.
it should have a magnetic pull. It's bad otherwise.

 

[pr5owner]

isnt the easiest way to tell if the alt is dead is a volt meter?

i have a volt gauge in my car and under optimum conditions the alt should give out 14.4v while the car is running, now mine after 7 years has dropped to 14.1v, however the battery is 12v, so anything above 12.5-13v should be enough to recharge the battery.

 

[Tom03es]

Voltage fluctuates all over the place with the engine running. I see it go all over the place. 14.4 to 12.8 or so. It just depends on the current loading on the electrical system. Mine will be at 14.4ish and then drop to 12.8ish when I turn on the headlights. And then after awhile, the voltage creeps back up toward the 14ish range.

 

[maxchao]

That's a good idea, I'm thinking about investing on a multimeter too. You guys measure across the battery terminal right?

Also, my car is still running like a champ, so I guess the alternator is ok.

Josh

 

[pr5owner]

uhh no i have a volt meter by autometer, but a multimeter will work fine as well i guess..

 

[berrywise]

Mind if I piggyback onto this thread. How does one test to see if their alternator is bad?

My wife told me her car was dead so I gave her a jump, figuring she might have left the lights on inside the car or something, but later she called me to say the car had just stalled while in rush hour traffic on her way home.

I tested the battery using my multimeter and it's still got life in it. It looks like my alternator is pretty far down inside the engine. Anyone have any tips on how to test it? Is it easier to get at if I bring it over to a friend's house who has a jack and stands?

Thanks!

 

[Tom03es]

I think you need to get it to an autoparts store that has a tester. With the engine running, they can then test the alternator right on the car. I believe (not 100%) that our alternators CANNOT be bench tested for some reason.

And yes, the alternator is a huge PITA to get to. Parts have to come off one way or another- either the intake manifold or the driveshaft. Some have said they have removed the cruise control and have brought it up that way.

 

[berrywise]

I think you need to get it to an autoparts store that has a tester. With the engine running, they can then test the alternator right on the car. I believe (not 100%) that our alternators CANNOT be bench tested for some reason.

And yes, the alternator is a huge PITA to get to. Parts have to come off one way or another- either the intake manifold or the driveshaft. Some have said they have removed the cruise control and have brought it up that way.

Thanks Tom03es. There is a Checker Auto nearby and I rang them up and they said they could check it for me. I'll have to see how willing my buddy is to help me fix my car if that is the problem as just changing my brake pads and rotors is enough of a challenge for me.

 

[maxchao]

What I did is just crank it up. (jumpstart if you have to), and while the engine is running unplug one of the battery terminal. If the car die then your alternator probably is no hope, if it doesn't die, I don't know, maybe that means the problem is the battery. Mine would die only if I turn the headlight on with battery unplugged. Well, the best way is still get checked by those machine though.

Josh

 

[flash75]

You should never test an alternator by unplugging the battery. You can ruin a good alternator using that method for testing. As mentioned check it with a voltmeter. You don't have to crawl under the car, it can be checked at the positive battery terminal with the engine running. If you have around 14-14.5 volts the alternator is charging, if about 12.8 volts or less the alternator isn't charging.

Clifton

 

[maxchao]

Oh ok yes listen to him not me. It's just that I didn't have access to a voltmeter.

Josh