battery not holding charge

[tDUBjetta]

Hi everyone, I have here a 2002 Jetta 2.0l, and the battery is not keeping charge. Now I know this is usually an alternator problem. However I have replaced the alternator and the the new one tested fine, (90 amp alternator) and passed all tests at auto-zone, and placing the new alternator in, still didn't help. The battery should be reading about 14.2V on the multi-meter, and it started off at about 12.89 or so and would slowly decrease while the car was running. The battery is also brand new and tested okay at auto-zone as well.

Now I have checked all of the connections and they're clean and solid as well as secure.

Not sure if there is some wiring issues or fuses that could blow in between, that could cause the problem.

And no there is nothing turned on in the car that would lead to a parasitic drain. So that option is out the window.

If you have any solutions you can think of. That'd be great

[Anony00GT]

At 12.x volts with the car running and a verified good battery, the alternator is not charging. Plain and simple.

Obviously, the alternator itself was not the reason. Firstly, check the big alternator fuse, in the fuse box on top of the battery. If that's good, then look at the small wire connected to the alternator. You should have 12v there with the key on.

If you have no power on the small wire with the key on, figure out where that circuit is broken, starting with fuses. If you do have power at the small wire, ohm out the big wire between the alternator and battery.

A Bentley manual would be huge here.

[rommeldawg]

well for clarity are you saying that the battery/car dies while driving or that when left say over night the battery is dead. one would indicate a bad alternator even if its new it can still be bad (sure its a reman) and the later would perhaps indicate an electrical draw on the system that continues to drain the system even with the key off.

[tDUBjetta]

It is just when the car is on and running, if you leave it off overnight the battery charge will basically pick up where it left off the last time driving it. And the alternator tested OK on three levels, that is why I'm doubting it being the alternator problem.

[Anony00GT]

It is just when the car is on and running, if you leave it off overnight the battery charge will basically pick up where it left off the last time driving it. And the alternator tested OK on three levels, that is why I'm doubting it being the alternator problem.

Did you read my post above at all?

With your voltage readings, the alternator is not charging. That doesn't necessarily mean the alternator itself is faulty. Proper diagnosis must be performed first. You simply threw a new alternator at the problem without checking anything, and now the car has a brand new shiny $200+ alternator in it, yet still isn't fixed.

Diagnose twice, fix it right, fix it once.

Driving this car is killing your battery. Definitely replace the battery immediately once you get this issue resolved, a weak battery is the archenemy of a new alternator. The alternator probably came with a big orange warning label advising you of this as well.

[tbaeastcoast]

sounds like you need an alternator.

[Anony00GT]

sounds like you need an alternator.

He already replaced the alternator. It didn't fix the problem.

[VW MAFIA]

Alternator bro, dnt trust auto zone, u kno betta

[Anony00GT]

Alternator bro, dnt trust auto zone, u kno betta

[germancarnut51]

The small (usually blue color) wire on the alternator is the exciter wire. When you start the car, it supplies a 12V reference voltage to the alternator. The alternator needs the 12V voltage to start making voltage. Simply speaking, no 12V on the blue wire, and the alternator won't work. The 12V on the blue wire comes via switched power from the ignition switch.

So, if you meter the small blue wire at the alternator with the ignition switch on, there should be 12V present. No 12V means there's a problem between the ignition switch and the blue wire terminal on the alternator, AND the alternator will not charge the battery.

If you are not getting 12V on the blue wire at the alternator, you can still test the alternator by running a jumper wire from the battery positive terminal to the blue wire, and touching the jumper onto the alternator blue wire terminal momentarily with the engine running. The alternator should start charging the battery, measuring about 14V across the battery terminals.

If you don't get the 14V across the battery terminals, the alternator is BAD.

If you do get the 14V across the battery terminal, then you probably have a problem in the wiring.

[jettatech]

test the alt output at the stud on the alt. if it tests good, continue onto doing your voltage drop tests. dont forget your ground cable can cause very similar problems. ive seen headlight flicker because of a voltage drop at the chassis ground on an a4 vr hth.

[jettatech]

oh, and the a4 chassis alt harness has been known to creat a voltage drop/resistance to the point the fuse box melts....