2006 Jetta Alternator Issues

Hi guys, ive read lots on this issue but i figured I'd start a thread to try to get more ideas. Any help is appreciated!
So 2 weeks ago I got back to a park n fly to find my battery stone dead (left a light on ) so I ended up calling a tow truck for a boost. let the car charge awhile before starting it but then it only ran for about 5 min before dying so I figure alternator and had it towed home. Charged the battery overnight and did some tests the next day and determined the alternator was shot so I went and got a reman from Canadian Tire and threw it in to discover the alternator fuse had blown, likely from boosting it. I replaced the fuse and the reman was working nicely so i decide to return my old one for the core and figured the reman would be fine.
Earlier this week my battery light came on while driving home, got home and tested, fuse was fine but alternator was not charging. I did some more tests before taking the alternator back and swapping it for another reman. threw that one in started the car and tested running voltage. it was around 14.3 then 2 minutes after running I watched the voltage drop down to 12.8 and continue to drop.
From what I've read I have screwed myself by returning my core for these junk remans but I find it hard to believe I got 2 duds back to back like that. I am wondering if there is anything I can test on the 2 pin connector that plugs into the alternator that might be causing it to cut out?

Does the alternator light come on when you turn the key to "on" ?

It does when I first start the car after putting the replacement alternator in but goes out after a few minutes and does not come on again, even after I lose the charging voltage or restart the car and the alternator not charging

But it does come on when you turn the key to "on" but before you start the car?

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I do not think so but I cant remember for sure at hand, will have to check when I get home tonight

So when I turn the key on but dont start the car, the alternator light comes on with the lamp test but then goes out as normal and only the battery and check engine lights are on. When I start the car only battery light is on

Have you checked the battery for a dead cell... would cause undue stress on the altenator?

Yes, I ran a battery recondition from my charger overnight the other night the left it on trickle charge last night. battery is sitting at 13.1 volts steady. Also the night the first alternator went on me I had an hour drive home, made it the whole way and was still able to start the car a few times when I was doing tests before I put the charger on. I'm certain it is not the battery

What are the voltage values I should read at the 2 pin connector that plugs into the alternator? Im wondering if its not getting the proper excitation signal?

2 Bad Alternators?

Jettario said:

What are the voltage values I should read at the 2 pin connector that plugs into the alternator? Im wondering if its not getting the proper excitation signal?

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Yes, me, too. Exact same problem. Battery died while driving. New alternator. New battery. Starts fine but battery light stays on. Battery is 12 v, then after a start 11v, then after a start, lower, etc.. I can recharge battery and starting is no trouble. But the alternator should put about 14 volts on the battery while the engine is running and it's not.

Can the car computer in any way disable the alternator? What is that 2-wire plug at the top? I think an alternator should put out 14volts as long as it's spinning because it doesn't need an excitation voltage, right?

It takes me hours to pull this alternator. I sure would love to find an alternative. All fuses checked (3 green ones and the metal bars above the battery).

The alternator has no permanent magnets in it, so needs an excitation voltage to get started. The excitation voltage is provided thru the same circuit that activates the ALT light, which is why having a working ALT light is relevant.

So, one of the two thin wires at the alternator is used by the cluster to provide that excitation voltage.

The other is used by the ECU to gauge alternator load, which the ECU then uses to decide if it should shed optional loads like the coolant glow plugs during periods of high overall current draw.

Vince Waldon said:

The alternator has no permanent magnets in it, so needs an excitation voltage to get started. The excitation voltage is provided thru the same circuit that activates the ALT light, which is why having a working ALT light is relevant.

So, one of the two thin wires at the alternator is used by the cluster to provide that excitation voltage.

The other is used by the ECU to gauge alternator load, which the ECU then uses to decide if it should shed optional loads like the coolant glow plugs during periods of high overall current draw.

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Thank you! Swapping alternators again later today. I don't know what else to do! Any guidance appreciated. Advanced Auto says the alternator is good. Took it down the street to Auto Zone and they say it's bad (only l11.85V out). Took it to O'Reilly's and they said it's bad.

I have an ALT light. How can I confirm my cluster (what's that?) is putting out excitation voltage? Second little wire in the plug seems to be n/a because my current probe meter is telling me no current coming from alternator to battery terminal on the big fat wire (so ECU certainly wouldn't think it's too high of current).

Basic question: How do I tell if the alternator is bad or my car is bad? If the exciter wire is not up to snuff, how do I fix it? I see no physical damage to wiring.

So bit of an update, I did more research and more tests on the car on the alternator plug. With the car running I had 12.6V on 1 pin which I believe to be the voltage feedback and the 12.6V was what my battery voltage was. Should have been in the 14V range if my alternator was working. The other pin had 10.88V which is the alternator excitation/load control voltage. acceptable ranges on this pin are 6-11/12V so I know my car was providing proper signals to run the alternator. Recheck actual alternator wiring/fuses etc. and everything appeared to be in perfect order.

So I said enough is enough, I went to a scrap yard and pulled an alternator out of a 2.0L passat that had a beautiful engine in it still (rear end collision) cost me $48(CAD ). Went home and through that in the car and its charging at 14.3V beautifully. no more battery light, no alternator light, no more issues so far.. going to keep an eye it for awhile and if it goes just going to service it instead of trying to put a reman back in cuz they are clearly junk.

Like most other threads on this topic conclude, DO NOT buy a junky reman and DO NOT return your core. Service OEM alternator with voltage regulator/pulley kit or go to a scrap yard and try your luck pulling a used one off a car with an engine thats still in decent shape, save yourself $300 and a lot of headaches.