power problem: slightly bad alternator???

[e304lyfe]

here's my problem.

car works for a week or 3, then the battery goes dead.

everything usually goes well... then i hit the car one time, 'n it feels weak... then after drivin it around even for an hour or so, its dead on the next crank.

once the car's started its always fine, never dies while i'm going.

i know the battery is fine, because i had it checked out at pep boys (load tested, 'n whatever.. took 'n hour 'n the computer spit out it was fine), and just to check i swapped batteries w/ my dad's m3 'n my car's still doing this and his is fine w/ my old battery.

the car will not die just from sitting...

it seams like driving it at night (with the lights on) speeds up the problem

i changed the brushes in the alternator, and looked at the ground wire on the alternator and it looked fine.

any ideas what's goin on?

the voltage across the battery is a low 12 volts, while my dad's e30 m3 always comes up as a high 13 volts. as far as i can remember my car has always read a low 12 volts across the battery... can anyone else check a 325i and verify? or is this a problem right here?

i disconnected the ground wire of the battery while the car is running, and it still runs fine.

my next idea is the alternator is weak but still working, if this makes any sense. plausable? or am i wasting my time on this idea? some sort of drain? ......YOUR HELP IS MUCH APPRECIATED, THANKS IN ADVANCE!!!!

[Tricked 323i]

Have them check your alternator as well. Sounds like your alternator is going bad......

[e304lyfe]

Have them check your alternator as well. Sounds like your alternator is going bad......

how do i go about doing this? so an alternator can be somewhat bad?

i replaced the brushes in it... and i disconnected the negative end of the battery when the car is running and it still runs, which i have "heard" is an indication that the alternator is good...

the car also never dies while driving... so it seams to me the alternator is atleast doing a little bit of work to keep the car running.

[Tricked 323i]

Take it to any auto parts store, I think you had mentioned pep boys they can test the voltage while your alternator is still in the car. Good luck

[bmwguru007]

If you only have 12 volts at the battery terminals while your car is running there is definently a problem. Normal charging voltage on these cars is about 14 volts. To me it sounds like you have barely enough voltage to keep the battery charged under normal conditions and any additional load causes the battery to discharge. The voltage regulator commonly needs replacement but the alternator itself fails much less often. I had a problem with flickering lights on my car and had my alternator tested at three different places and all of them said it was fine. I replaced the voltage regulator and the problem was gone. You probably want to get it tested, but don't rely solely on those tests.

[e304lyfe]

If you only have 12 volts at the battery terminals while your car is running there is definently a problem. Normal charging voltage on these cars is about 14 volts. To me it sounds like you have barely enough voltage to keep the battery charged under normal conditions and any additional load causes the battery to discharge. The voltage regulator commonly needs replacement but the alternator itself fails much less often. I had a problem with flickering lights on my car and had my alternator tested at three different places and all of them said it was fine. I replaced the voltage regulator and the problem was gone. You probably want to get it tested, but don't rely solely on those tests.

ok... well, i replaced the voltage regulator and brushes on the alternator already.

so does this narrow it down to the alternator being bad? ...as i know the battery is perfectly fine. or maybe still a drain somewhere? ...but the battery never read near 14 volts ever on this car. so i take it its not being charged properly, and that means the alternator is probably at fault (???).

[bmwguru007]

It doesn't rule out other stuff as electical problems can be tricky but it's a good starting point. Take the alternator somewhere and get it tested. It should be obvious if it isn't putting out enough voltage. Assuming it fails the test and the voltage regulator is good and installed correctly you can go two directions. Get a replacement alternator or take yours somewhere to be inspected and fixed or maybe rebuilt if possible. A replacement (rebuilt most likely) will cost in the neighborhood of $150-200.