2004 Jetta TDi battery light issue

frankie m · Jun 15, 2017

About 2 months ago I had the bodyshop replace 2 fenders which were starting to rust. When I got the car back, the door ajar light on the dash would stay on intermittently - I turned off the interior lights.

Still have not had a chance to return to the body shop to fix the issue- which is probably wires or a switch in the door?

Last week just as I returned home with AC on, the battery light came on and volts dropped to 12 volts. I have a Scangauge. A few hours later I started the car and volts were back up to 14.

I used my Sears multi-meter to check the battery- off its over 12 volts.
When i start the car volts are 14.

Then on 2 occasions I disconnected the negative terminal as car was on - the first time, car stayed running, the 2nd time, the car died.

Everytime I turn the AC on. after a few minutes the battery light will come on.

Im afraid to drive the car.

Soon I will use my mechanic/brother-in-laws meter to check if the alternator is charging.

Do u think the 2 problems - 1 fenders changed causing door ajar light to stay on intermittently AND 2 battery light on are related?

Thanks
Frank

pruzink · Jun 15, 2017

I was having some very strange electrical issues with my 2004 Jetta. It turned out to be the fuseable link for the alternator. To the eye, the link looked fine & it tested fine checking it for resistance with a multimeter. You need to remove the securing nuts & look at the link under those nuts. The link is aluminum, the nut is steel, the wire is copper & this sets up a galvanic cell where all 3 dissimilar metals meet. If it looks like it has cracks in the aluminum, replace it.

pruzink · Jun 15, 2017

Generally, the battery light coming on means that the alternator isn't charging the battery. The fenders being changed probably didn't have anything to do with the issue. I would not recommend disconnecting the negative terminal to the battery with the car running; their are enough electronics that are sensitive to voltage spikes so why risk creating problems. When I had my fuseable link problem, I had some very erratic voltages.

Tdijarhead · Jun 15, 2017

If your scan gauge indicated the voltage dropped while the car was running then your alternator pulley is starting to fail, or the alternator itself is going on vacation.

You can actually drive these cars quite a distance on just battery power. Pull the hand brake up one click to disable the daytime driving lights, and shut off the heat or ac and anything else that uses electricity.

frankie m · Jun 15, 2017

Thanks for all the replies- I learn something new everyday.
I installed a Euro headlight switch so the DRLs are not always on.

I will check that fuseable link and test the alternator.

If it is the alternator, where do u suggest buying one other than IDParts- sorry if this is against forum rules.

benIV · Jun 15, 2017

I usually buy from idparts but have bought from kermatdi and cascadegerman (formerly boraparts). I'm sure there are plenty of others too. I've even bought from napa and had an alternator that wasn't awful.

Tdijarhead · Jun 16, 2017

I like idparts or metalmanparts, but I recently needed a starter and didn't have time to wait the 2-3 days needed for shippping, so I bought it from autozone. If you go that route make sure you keep your receipt so if it fails you can get a replacement. The alternator is mounted on top so at least it's fairly easy to change.

wonneber · Jun 16, 2017

frankie m said:
About 2 months ago I had the bodyshop replace 2 fenders which were starting to rust. When I got the car back, the door ajar light on the dash would stay on intermittently - I turned off the interior lights.

Still have not had a chance to return to the body shop to fix the issue- which is probably wires or a switch in the door?

Last week just as I returned home with AC on, the battery light came on and volts dropped to 12 volts. I have a Scangauge. A few hours later I started the car and volts were back up to 14.

I used my Sears multi-meter to check the battery- off its over 12 volts.
When i start the car volts are 14.

Then on 2 occasions I disconnected the negative terminal as car was on - the first time, car stayed running, the 2nd time, the car died.

Everytime I turn the AC on. after a few minutes the battery light will come on.

Im afraid to drive the car.

Soon I will use my mechanic/brother-in-laws meter to check if the alternator is charging.

Do u think the 2 problems - 1 fenders changed causing door ajar light to stay on intermittently AND 2 battery light on are related?

Thanks
Frank

I can't rule out the body shop, they may have disconnected or messed with a ground wire somewhere.
I would ask the question first before pointing the finger.

The 12 volts off seems a bit low, I think 12.6 is full charge.
14 volts running is a bit low also.
Do you have access to another car to compare the voltages?

With the car running, head lights on, fan on high measure the voltage from the battery negative post to the case of the alternator.
I would not want to see more then a few tenths of a volt.
Same test from the positive post to the alternator stud.

The door light switch is part of the door latch mech. Not in the door jam area where they were working.

Genesis · Jun 16, 2017

Then on 2 occasions I disconnected the negative terminal as car was on - the first time, car stayed running, the 2nd time, the car died.

You probably destroyed the alternator diode bridge doing that the first time and you now have a trashed alternator, which is why it died the second time.

NEVER disconnect a car's battery while the engine is running; the battery is the low-resistance shunt that keeps both regulator overshoot and back-emf spikes from various things in the vehicle (including the alternator and any running DC motor SUCH AS THE FANS BEHIND THE RADIATOR!) from destroying various semiconductor junctions -- including but not limited to the rectifier bridge in the alternator itself.

frankie m · Jun 18, 2017

Thanks for all the tips- thats the last time Ill disconnect a battery when car is running but I think the alternator was already out.

Going to test it again today and then order a new VW alternator. Its funny that VW sells a new alternator and that Bosch sells a new alternator. You would think they are the same.

I got a quote from a local repair place and they can rebuild mine for $140 with a 6 month warranty. I think I will just buy a new VW one.

Thanks again.

Genesis · Jun 18, 2017

Not if it kept running the first time it wasn't.

But it is now

frankie m · Jun 18, 2017

I used a Craftsman tester-

With car off, volts are at 12.31
With car on, volts are at 14.1

Then I turned on the headlights & AC on high fan and the Volts were at 13.73.
The battery light then came on and volts dropped to 11.8-12.1 range.

Then when I tested from the negative battery to alternator body, volts were .013 and quickly dropped to 0

When I tested from the positive battery to alternator bolt, volts were all over the place.

My Scanguage was reading about 11.6 with battery light on.
Should i order that new alternator? Or should I do a test with my brother-in-laws better gauge?

wonneber · Jun 20, 2017

frankie m said:
I used a Craftsman tester-

With car off, volts are at 12.31
With car on, volts are at 14.1

Then I turned on the headlights & AC on high fan and the Volts were at 13.73.
The battery light then came on and volts dropped to 11.8-12.1 range.

Then when I tested from the negative battery to alternator body, volts were .013 and quickly dropped to 0

When I tested from the positive battery to alternator bolt, volts were all over the place.

My Scanguage was reading about 11.6 with battery light on.
Should i order that new alternator? Or should I do a test with my brother-in-laws better gauge?

There is a bad connection between the batters and the alternator.
There should be a fuse in the box over the battery.
Those are known to get a bit of corrosion which add resistance.
Measure the drop from the battery to each side of the of the wire.
I'm sure your on the right track.

Genesis · Jun 20, 2017

Concur.

A VOM (on volts mode) between the battery positive and big stud on the alternator should read ZERO, ALWAYS, period. A few millivolts during heavy charging (cable drop) is ok, but any material reading and particularly one that fluctuates is NOT. That's a direct battery link (through the high-current fuse) so if you see VOLTAGE between those two points you have a bad connection between those two points SOMEWHERE.

Find it and the problem will likely go away, assuming the rectifier bridge in the alternator hasn't been damaged by the bad connection.

frankie m · Jun 21, 2017

wonneber said:
There is a bad connection between the batters and the alternator.
There should be a fuse in the box over the battery.
Those are known to get a bit of corrosion which add resistance.
Measure the drop from the battery to each side of the of the wire.
I'm sure your on the right track.

Would i put negative or positive on battery?
Thanks

flee · Jun 21, 2017

frankie m said:
Would i put negative or positive on battery?
Thanks

If it is an analog tester the negative should go the lowest potential, in this case the battery when the engine is running.
It will be obvious if the needle pins at 0, then it is backward.
If it is a digital tester it won't matter.

frankie m · Jun 25, 2017

I tested from the battery to the fuse posts on top of battery today.

With negative on battery and positive on each fuse post was about 0.4 volts.
With positive on battery, and negative on each fuse post was about 12.4 volts.

Going to have my brother-in-law test it with his better meter this week.

wonneber · Jun 26, 2017

To test the ground you put 1 lead on the negative terminal of the battery and the other lead to the case of alternator. (car running, fan and lights on for a load).

For the power side, 1 lead on the positive terminal of the battery and the other lead to the large stud on the alternator.

Either test you only want a few tenths of a volt drop.

frankie m · Jun 28, 2017

Changed the Alternator and all is well!

Knock on wood

2004 Jetta TDi battery light issue

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