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VW MKIV-A4 TDIs (VE and PD) This is a general discussion about A4/MkIV Jetta (99.5-~2005), Golf(99.5-2006), and New Beetle(98-2006). Both VE and PD engines are covered here.

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Old May 30th, 2020, 11:08   #1
Join Date: Jan 2014
Location: NS, Canada
Default Alternator output 11.9V, battery light flickering when bringing RPM above 2,000?

2002 Jetta TDI

I was driving along last week and the instrument cluster died (no speedometer, tachometer, etc). After a few more minutes of driving (I was almost home) they all snapped back on with the addition of the ABS light, airbag light, and battery light remaining even after turning the car off and on again.

I cleared the airbag code, and the ABS light went off itself after a little while. The battery light remained on. I replaced the battery and alternator last July.

I was headed back into town so I yanked the battery out just in case (I didn't have time to do a proper check until later that evening) and dropped it off at the VW dealer in my town. They checked it, told me it was faulty, and gave me a new one by the time I returned from my other errands.

I installed that, the car started fine, and the lights were out. That's when I decided to get the multi meter and test some connections. With the car off, the battery is at 12.8V. When I touch the positive lead to the alternator casing, and the negative lead to the positive battery terminal, it's also 12.8V. I tested the alternator fuse like this as well, and while I was doing it decided to test all of the connections in the tray. They were all fine.

With the car running, it drops to 11.9V. When I give the car fuel and bring it beyond 2,000 RPM the battery light comes on and begins to flicker, and goes out when I let off the fuel.

Would this be a bad voltage regulator or is the entire alternator shot? It's a refurbished Bosch 120A alternator (car is manual). I am going to pick another one up on Monday, but I can't do anything right now as everything is closed, so I figured I'd pick your guys' brains while I wait.

Thanks a lot for the help!

Long-time lurker. ;]
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Old May 30th, 2020, 12:53   #2
Nero Morg
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Definitely looks like the alternator is on its way out. Get it rebuilt or replaced soon.
Whatever. Just use your darned blinker. And stay off your phone.
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Old May 30th, 2020, 14:25   #3
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Location: US
TDI(s): 2001 Jetta

By no means absolute, but classically overcharging is the regulator, undercharging is the alternator. If you can rig up a separate little harness, you could momentarily full-field the alternator and see if voltage goes up. That would tell you if it is the regulator.
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Old May 30th, 2020, 15:07   #4
Vince Waldon
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Location: Edmonton Alberta Canada

Could also be the clutch on the alternator pulley... slipping until the revs are up a bit.

Search here for "one-way clutch" or thereabouts and you should find lots of discussion.

Worn brushes and/or slip rings sometimes have a RPM-dependant failure mode as well.
Vince Waldon Edmonton AB Canada
Canadian Grizzly's excellent low-power/limp mode troubleshooting guide
VW's TSB for scientifically tracking down parasitic current drains
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Old May 31st, 2020, 16:04   #5
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Join Date: Feb 2003
Location: Niceville, FL
Fuel Economy: 53/48/42

Can definitely be the clutch; it'll produce EXACTLY this symptom. Seen it personally.

That's cheaper than an alternator.
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Old June 4th, 2020, 05:48   #6
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The regulator/rectifier/brushes should also be a suspect. They are comparable to an alternator clutch cost( ~$50 ). and replaceable by itself, though on an ALH it is slightly more difficult than a BEW.
1990 D250, NV5600, Luk 05-101, 16cm housing, TST Kit KDP tab, 366 spring, Powr-Lok'd 3.07, HX35...IC soon
She wasn't built to travel at the speed a rumour flies

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Old June 4th, 2020, 06:01   #7
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Where did you get the “replacement alternator “ that was replaced last August, and what brand was it?
If it wasn't a Bosch reman I would be suspect.
I too have seen the alt clutch slipping as Genesis says, my daughters car did the exactly the same thing.
2001 golf auto to 5 speed manual swap,G60/VR6 clutch setup,coolant migration mod.
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Old June 4th, 2020, 12:58   #8
Join Date: May 2004
Location: central Ohio
Fuel Economy: 50/45/35

Our alternators are finicky, exacting. A lot of bad product out there. If that rebuild was like a third the price of a new one, it's likely a hit or miss rebuild job.
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