Confirm bad alternator?

Hey Folks,

So my 2001 golf with 210k miles on it seems to have a new problem. I did check the forums, I just want to be sure. I was driving yesterday, and the battery light popped on while driving. When I tested the battery with the engine off, it read roughly 12 volts. When I tested it while running, it said roughly 10 volts, no lights or radio on. From what I read, it should read about 14 volts while it's charging. So am I right, has my alternator finally bit the dust, or is there something (cheaper) I should check first? Thanks y'all.

Sounds like an alternator as you should have 13-14 voltages at idle.

Keep in mind when you say the battery reads 12V, that means its dead. You should get a more accurate volt meter as your looking at the voltage after the decimal to indicate state of charge. As in 12.6V is 100% charged battery.

Can also be a bad clutch pulley on alternator.

Seems I read on here about changing just the regulator.

disconnect the battery and see if the car dies, if so the alternator is shot.

If the pulley is still good, then there is an excellent chance that the alternator can be repaired by changing the voltage regulator. This also depends on the condition of the slip rings. If they are badly worn, then changing the voltage regulator will not last long as the new brushes will wear through the slip rings, and you will then again be without a working alternator.

Wow. Thanks y'all for all the responses. So it sounds like regardless it's the alternator. My voltmeter does read at least one more digit, but I was silly and didn't write down the exact voltage. I just figured voltage down == no good. There are no unusual noises coming from the engine bay, and the accessory belt is tight, so I'm assuming the pulley is where it's supposed to be. Thanks so much for your insight, I'll just have to order a new alternator when the next paycheck comes in. If any of you are around the SE PA area, feel like drinking beer and watching me wrench on my car? You can just grunt every once in a while to let me know I'm doing OK!

No, the serpentine belt being tight tells you nada. On a TDI, the alt pulley has a clutch mechanism in it that free wheels in one direction and locks up in the other direction. if it free wheels in both directions, the alt will not turn when the engine is running. The other failure mode for the pulley is when the pulley seizes up and that failure mode usually destroys the tensioner.

To check, take off the serpentine belt, block the internal fan of the alternator with a screwdriver and try to turn the pulley. it should turn smoothly in on direction and not turn in the other direction.

Do not buy a garbage reman from AutoZone unless you like changing your alternator time after time after time. If the pulley is the problem, it is replaceable for a lot less cash that even a garbage AutoZone alternator.

It's important to know what the issue is before you start buying parts.
There are a couple of common failure modes that can be fixed for a lot less than an entirely new alternator.

Here is a pretty comprehensive write-up of the whole process:
http://www.myturbodiesel.com/1000q/multi/alternator-troubleshooting-voltage-regulator-repair.htm

It's worth mentioning that at least some of the possible failure types could get worse if not addressed. If the alternator pulley siezes or falls off completely then it can take the belt and/or tensioner with it. So you should try to diagnose the issue sooner rather than later.
Also important to note that your car can run without the alternator (limited by the amount of time it takes for battery to die). So if you needed to remove the accessory belt entirely you could do that.

You can check the one way pulley without removing the belt. Just use a mechanics pick to push on the fan fins inside the front of the alternator in both directions. If it won't turn either direction the pulley is frozen, if it freewheels more than one direction the pulley is slipping.

Probe the fat red wire on the engine side of the fuse box on top of the battery, poke the probe lead into the insulation while engine running and measure voltage. If voltage is lower at the battery than it is here you have a corroded terminal connection in that wire.

OK, thanks for all of the input, folks. I used the tdiclub how-to to get the alternator out of the car. If I stop the internal fan, the pulley does not move in either direction, but if I don't, it spins fine. Also, I took off the back of the alternator, and the brushes were worn down to nubs. Good thing I ordered a new alternator from boraparts.

In an unrelated note, took the 2011 jsw in for 20k service, and they replaced the rear brakes - pads and rotors. Said the rotors were "out-of-round" and pitted. We asked them to look at some weird vibrations and shimmying when braking at speed. All replaced under warranty, but odd, no?